diff --git a/.gitignore b/.gitignore
index 3b1cc7d10dcae..ffcf73067f0f0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,7 +3,7 @@
 /.dev_profile*
 /.local-chromium/
 /.local-firefox/
-/test/test-user-data-dir*
+/test/output-*/
 build/
 coverage
 coverage/
diff --git a/package.json b/package.json
index a7dc920c8eef7..2a6bc74df94a1 100644
--- a/package.json
+++ b/package.json
@@ -86,9 +86,12 @@
     "@microsoft/api-documenter": "7.17.16",
     "@microsoft/api-extractor": "7.24.2",
     "@types/debug": "4.1.7",
+    "@types/diff": "5.0.2",
     "@types/mime": "2.0.3",
     "@types/mocha": "9.1.1",
     "@types/node": "17.0.38",
+    "@types/pixelmatch": "5.2.4",
+    "@types/pngjs": "6.0.1",
     "@types/progress": "2.0.5",
     "@types/proxy-from-env": "1.0.1",
     "@types/rimraf": "3.0.2",
@@ -101,6 +104,7 @@
     "c8": "7.11.3",
     "commonmark": "0.30.0",
     "cross-env": "7.0.3",
+    "diff": "5.1.0",
     "eslint": "8.16.0",
     "eslint-config-prettier": "8.5.0",
     "eslint-formatter-codeframe": "7.32.1",
diff --git a/src/common/AriaQueryHandler.ts b/src/common/AriaQueryHandler.ts
index 0fb9878e9df82..c0a23274e9ce8 100644
--- a/src/common/AriaQueryHandler.ts
+++ b/src/common/AriaQueryHandler.ts
@@ -111,7 +111,9 @@ const waitFor = async (
   return domWorld._waitForSelectorInPage(
     (_: Element, selector: string) =>
       (
-        globalThis as unknown as { ariaQuerySelector(selector: string): void }
+        globalThis as any as unknown as {
+          ariaQuerySelector(selector: string): void;
+        }
       ).ariaQuerySelector(selector),
     selector,
     options,
diff --git a/src/common/DOMWorld.ts b/src/common/DOMWorld.ts
index 11b83f8d20667..4ed27b8183ff4 100644
--- a/src/common/DOMWorld.ts
+++ b/src/common/DOMWorld.ts
@@ -719,10 +719,10 @@ export class DOMWorld {
     function deliverResult(name: string, seq: number, result: unknown): void {
       // eslint-disable-next-line @typescript-eslint/ban-ts-comment
       // @ts-ignore Code is evaluated in a different context.
-      globalThis[name].callbacks.get(seq).resolve(result);
+      (globalThis as any)[name].callbacks.get(seq).resolve(result);
       // eslint-disable-next-line @typescript-eslint/ban-ts-comment
       // @ts-ignore Code is evaluated in a different context.
-      globalThis[name].callbacks.delete(seq);
+      (globalThis as any)[name].callbacks.delete(seq);
     }
   };
 
diff --git a/src/common/Debug.ts b/src/common/Debug.ts
index 6e2f96baa575b..f768b4792fc58 100644
--- a/src/common/Debug.ts
+++ b/src/common/Debug.ts
@@ -65,7 +65,7 @@ export const debug = (prefix: string): ((...args: unknown[]) => void) => {
   }
 
   return (...logArgs: unknown[]): void => {
-    const debugLevel = globalThis.__PUPPETEER_DEBUG;
+    const debugLevel = (globalThis as any).__PUPPETEER_DEBUG;
     if (!debugLevel) {
       return;
     }
diff --git a/src/common/DeviceDescriptors.ts b/src/common/DeviceDescriptors.ts
index 75a4a65d312f3..fc2cfc20b85f3 100644
--- a/src/common/DeviceDescriptors.ts
+++ b/src/common/DeviceDescriptors.ts
@@ -1527,6 +1527,7 @@ const devices: Device[] = [
     },
   },
 ];
+
 /**
  * @public
  */
diff --git a/src/common/Errors.ts b/src/common/Errors.ts
index 2676caa893f6d..fbd23c3abea1f 100644
--- a/src/common/Errors.ts
+++ b/src/common/Errors.ts
@@ -36,6 +36,7 @@ export class CustomError extends Error {
  * @public
  */
 export class TimeoutError extends CustomError {}
+
 /**
  * ProtocolError is emitted whenever there is an error from the protocol.
  *
@@ -45,13 +46,19 @@ export class ProtocolError extends CustomError {
   public code?: number;
   public originalMessage = '';
 }
+
 /**
  * @public
  */
-export type PuppeteerErrors = Record<string, typeof CustomError>;
+export interface PuppeteerErrors {
+  TimeoutError: typeof TimeoutError;
+  ProtocolError: typeof ProtocolError;
+}
+
 /**
  * @public
  */
-export const puppeteerErrors: PuppeteerErrors = {
+export const puppeteerErrors: PuppeteerErrors = Object.freeze({
   TimeoutError,
-};
+  ProtocolError,
+});
diff --git a/src/common/Page.ts b/src/common/Page.ts
index 7bad98a876084..b22f9c993ee35 100644
--- a/src/common/Page.ts
+++ b/src/common/Page.ts
@@ -2293,7 +2293,9 @@ export class Page extends EventEmitter {
    */
   async emulateMediaType(type?: string): Promise<void> {
     assert(
-      type === 'screen' || type === 'print' || type === null,
+      type === 'screen' ||
+        type === 'print' ||
+        (type ?? undefined) === undefined,
       'Unsupported media type: ' + type
     );
     await this.#client.send('Emulation.setEmulatedMedia', {
diff --git a/src/common/fetch.ts b/src/common/fetch.ts
index c0ca7e3baf02c..218c178986c65 100644
--- a/src/common/fetch.ts
+++ b/src/common/fetch.ts
@@ -16,5 +16,5 @@
 
 /* Use the global version if we're in the browser, else load the node-fetch module. */
 export const getFetch = async (): Promise<typeof fetch> => {
-  return globalThis.fetch || (await import('cross-fetch')).fetch;
+  return (globalThis as any).fetch || (await import('cross-fetch')).fetch;
 };
diff --git a/test/.eslintrc.js b/test/.eslintrc.js
index 9d86da20e14ed..878d3896627af 100644
--- a/test/.eslintrc.js
+++ b/test/.eslintrc.js
@@ -1,5 +1,6 @@
 module.exports = {
   rules: {
+    'arrow-body-style': ['error', 'always'],
     'no-restricted-imports': [
       'error',
       {
diff --git a/test/fixtures/dumpio.js b/test/fixtures/dumpio.js
index 40b9714f6caeb..a16cf4d633d68 100644
--- a/test/fixtures/dumpio.js
+++ b/test/fixtures/dumpio.js
@@ -2,7 +2,9 @@
   const [, , puppeteerRoot, options] = process.argv;
   const browser = await require(puppeteerRoot).launch(JSON.parse(options));
   const page = await browser.newPage();
-  await page.evaluate(() => console.error('message from dumpio'));
+  await page.evaluate(() => {
+    return console.error('message from dumpio');
+  });
   await page.close();
   await browser.close();
 })();
diff --git a/test/src/CDPSession.spec.ts b/test/src/CDPSession.spec.ts
index d5999d3d70e13..34c095e01d6ef 100644
--- a/test/src/CDPSession.spec.ts
+++ b/test/src/CDPSession.spec.ts
@@ -21,7 +21,8 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeChromeOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
+import { isErrorLike } from '../../lib/cjs/puppeteer/common/util.js';
 
 describeChromeOnly('Target.createCDPSession', function () {
   setupTestBrowserHooks();
@@ -36,7 +37,9 @@ describeChromeOnly('Target.createCDPSession', function () {
       client.send('Runtime.enable'),
       client.send('Runtime.evaluate', { expression: 'window.foo = "bar"' }),
     ]);
-    const foo = await page.evaluate(() => globalThis.foo);
+    const foo = await page.evaluate(() => {
+      return (globalThis as any).foo;
+    });
     expect(foo).toBe('bar');
   });
   it('should send events', async () => {
@@ -45,7 +48,9 @@ describeChromeOnly('Target.createCDPSession', function () {
     const client = await page.target().createCDPSession();
     await client.send('Network.enable');
     const events = [];
-    client.on('Network.requestWillBeSent', (event) => events.push(event));
+    client.on('Network.requestWillBeSent', (event) => {
+      return events.push(event);
+    });
     await page.goto(server.EMPTY_PAGE);
     expect(events.length).toBe(1);
   });
@@ -77,14 +82,16 @@ describeChromeOnly('Target.createCDPSession', function () {
     });
     expect(evalResponse.result.value).toBe(3);
     await client.detach();
-    let error = null;
+    let error!: Error;
     try {
       await client.send('Runtime.evaluate', {
         expression: '3 + 1',
         returnByValue: true,
       });
     } catch (error_) {
-      error = error_;
+      if (isErrorLike(error_)) {
+        error = error_ as Error;
+      }
     }
     expect(error.message).toContain('Session closed.');
   });
@@ -92,7 +99,9 @@ describeChromeOnly('Target.createCDPSession', function () {
     const { page } = getTestState();
 
     const client = await page.target().createCDPSession();
-    const error = await theSourceOfTheProblems().catch((error) => error);
+    const error = await theSourceOfTheProblems().catch((error) => {
+      return error;
+    });
     expect(error.stack).toContain('theSourceOfTheProblems');
     expect(error.message).toContain('ThisCommand.DoesNotExist');
 
diff --git a/test/src/EventEmitter.spec.ts b/test/src/EventEmitter.spec.ts
index f15a7952ffaa3..44624f8f55829 100644
--- a/test/src/EventEmitter.spec.ts
+++ b/test/src/EventEmitter.spec.ts
@@ -19,7 +19,7 @@ import sinon from 'sinon';
 import expect from 'expect';
 
 describe('EventEmitter', () => {
-  let emitter;
+  let emitter: EventEmitter;
 
   beforeEach(() => {
     emitter = new EventEmitter();
@@ -40,7 +40,7 @@ describe('EventEmitter', () => {
         emitter[methodName]('foo', listener);
         emitter.emit('foo', data);
         expect(listener.callCount).toEqual(1);
-        expect(listener.firstCall.args[0]).toBe(data);
+        expect(listener.firstCall.args[0]!).toBe(data);
       });
 
       it(`${methodName}: supports chaining`, () => {
@@ -116,7 +116,7 @@ describe('EventEmitter', () => {
 
       emitter.emit('foo', data);
       expect(listener.callCount).toEqual(1);
-      expect(listener.firstCall.args[0]).toBe(data);
+      expect(listener.firstCall.args[0]!).toBe(data);
     });
 
     it('returns true if the event has listeners', () => {
diff --git a/test/src/NetworkManager.spec.ts b/test/src/NetworkManager.spec.ts
index b78aa8c32606c..f1d6b6d88cbdd 100644
--- a/test/src/NetworkManager.spec.ts
+++ b/test/src/NetworkManager.spec.ts
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { describeChromeOnly } from './mocha-utils'; // eslint-disable-line import/extensions
+import { describeChromeOnly } from './mocha-utils.js';
 
 import expect from 'expect';
 import {
diff --git a/test/src/accessibility.spec.ts b/test/src/accessibility.spec.ts
index 65d3cb3057bb5..e146e89566218 100644
--- a/test/src/accessibility.spec.ts
+++ b/test/src/accessibility.spec.ts
@@ -14,13 +14,15 @@
  * limitations under the License.
  */
 
+import assert from 'assert';
 import expect from 'expect';
+import { SerializedAXNode } from '../../lib/cjs/puppeteer/common/Accessibility.js';
 import {
   getTestState,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describeFailsFirefox('Accessibility', function () {
   setupTestBrowserHooks();
@@ -170,7 +172,10 @@ describeFailsFirefox('Accessibility', function () {
     );
     const snapshot = await page.accessibility.snapshot();
     // See https://chromium-review.googlesource.com/c/chromium/src/+/3088862
-    expect(snapshot.children[0].roledescription).toEqual(undefined);
+    assert(snapshot);
+    assert(snapshot.children);
+    assert(snapshot.children[0]!);
+    expect(snapshot.children[0]!.roledescription).toBeUndefined();
   });
   it('orientation', async () => {
     const { page } = getTestState();
@@ -179,14 +184,20 @@ describeFailsFirefox('Accessibility', function () {
       '<a href="" role="slider" aria-orientation="vertical">11</a>'
     );
     const snapshot = await page.accessibility.snapshot();
-    expect(snapshot.children[0].orientation).toEqual('vertical');
+    assert(snapshot);
+    assert(snapshot.children);
+    assert(snapshot.children[0]!);
+    expect(snapshot.children[0]!.orientation).toEqual('vertical');
   });
   it('autocomplete', async () => {
     const { page } = getTestState();
 
     await page.setContent('<input type="number" aria-autocomplete="list" />');
     const snapshot = await page.accessibility.snapshot();
-    expect(snapshot.children[0].autocomplete).toEqual('list');
+    assert(snapshot);
+    assert(snapshot.children);
+    assert(snapshot.children[0]!);
+    expect(snapshot.children[0]!.autocomplete).toEqual('list');
   });
   it('multiselectable', async () => {
     const { page } = getTestState();
@@ -195,7 +206,10 @@ describeFailsFirefox('Accessibility', function () {
       '<div role="grid" tabIndex=-1 aria-multiselectable=true>hey</div>'
     );
     const snapshot = await page.accessibility.snapshot();
-    expect(snapshot.children[0].multiselectable).toEqual(true);
+    assert(snapshot);
+    assert(snapshot.children);
+    assert(snapshot.children[0]!);
+    expect(snapshot.children[0]!.multiselectable).toEqual(true);
   });
   it('keyshortcuts', async () => {
     const { page } = getTestState();
@@ -204,7 +218,10 @@ describeFailsFirefox('Accessibility', function () {
       '<div role="grid" tabIndex=-1 aria-keyshortcuts="foo">hey</div>'
     );
     const snapshot = await page.accessibility.snapshot();
-    expect(snapshot.children[0].keyshortcuts).toEqual('foo');
+    assert(snapshot);
+    assert(snapshot.children);
+    assert(snapshot.children[0]!);
+    expect(snapshot.children[0]!.keyshortcuts).toEqual('foo');
   });
   describe('filtering children of leaf nodes', function () {
     it('should not report text nodes inside controls', async () => {
@@ -286,7 +303,9 @@ describeFailsFirefox('Accessibility', function () {
             ],
           };
       const snapshot = await page.accessibility.snapshot();
-      expect(snapshot.children[0]).toEqual(golden);
+      assert(snapshot);
+      assert(snapshot.children);
+      expect(snapshot.children[0]!).toEqual(golden);
     });
     it('rich text editable fields with role should have children', async () => {
       const { page, isFirefox } = getTestState();
@@ -324,7 +343,9 @@ describeFailsFirefox('Accessibility', function () {
             ],
           };
       const snapshot = await page.accessibility.snapshot();
-      expect(snapshot.children[0]).toEqual(golden);
+      assert(snapshot);
+      assert(snapshot.children);
+      expect(snapshot.children[0]!).toEqual(golden);
     });
 
     // Firefox does not support contenteditable="plaintext-only".
@@ -335,7 +356,9 @@ describeFailsFirefox('Accessibility', function () {
         await page.setContent(`
           <div contenteditable="plaintext-only" role='textbox'>Edit this image:<img src="fakeimage.png" alt="my fake image"></div>`);
         const snapshot = await page.accessibility.snapshot();
-        expect(snapshot.children[0]).toEqual({
+        assert(snapshot);
+        assert(snapshot.children);
+        expect(snapshot.children[0]!).toEqual({
           role: 'textbox',
           name: '',
           value: 'Edit this image:',
@@ -363,7 +386,9 @@ describeFailsFirefox('Accessibility', function () {
             value: 'this is the inner content ',
           };
       const snapshot = await page.accessibility.snapshot();
-      expect(snapshot.children[0]).toEqual(golden);
+      assert(snapshot);
+      assert(snapshot.children);
+      expect(snapshot.children[0]!).toEqual(golden);
     });
     it('checkbox with and tabIndex and label should not have children', async () => {
       const { page, isFirefox } = getTestState();
@@ -385,7 +410,9 @@ describeFailsFirefox('Accessibility', function () {
             checked: true,
           };
       const snapshot = await page.accessibility.snapshot();
-      expect(snapshot.children[0]).toEqual(golden);
+      assert(snapshot);
+      assert(snapshot.children);
+      expect(snapshot.children[0]!).toEqual(golden);
     });
     it('checkbox without label should not have children', async () => {
       const { page, isFirefox } = getTestState();
@@ -407,7 +434,9 @@ describeFailsFirefox('Accessibility', function () {
             checked: true,
           };
       const snapshot = await page.accessibility.snapshot();
-      expect(snapshot.children[0]).toEqual(golden);
+      assert(snapshot);
+      assert(snapshot.children);
+      expect(snapshot.children[0]!).toEqual(golden);
     });
 
     describe('root option', function () {
@@ -416,7 +445,7 @@ describeFailsFirefox('Accessibility', function () {
 
         await page.setContent(`<button>My Button</button>`);
 
-        const button = await page.$<HTMLButtonElement>('button');
+        const button = (await page.$('button'))!;
         expect(await page.accessibility.snapshot({ root: button })).toEqual({
           role: 'button',
           name: 'My Button',
@@ -427,7 +456,7 @@ describeFailsFirefox('Accessibility', function () {
 
         await page.setContent(`<input title="My Input" value="My Value">`);
 
-        const input = await page.$('input');
+        const input = (await page.$('input'))!;
         expect(await page.accessibility.snapshot({ root: input })).toEqual({
           role: 'textbox',
           name: 'My Input',
@@ -445,7 +474,7 @@ describeFailsFirefox('Accessibility', function () {
             </div>
           `);
 
-        const menu = await page.$('div[role="menu"]');
+        const menu = (await page.$('div[role="menu"]'))!;
         expect(await page.accessibility.snapshot({ root: menu })).toEqual({
           role: 'menu',
           name: 'My Menu',
@@ -461,8 +490,10 @@ describeFailsFirefox('Accessibility', function () {
         const { page } = getTestState();
 
         await page.setContent(`<button>My Button</button>`);
-        const button = await page.$('button');
-        await page.$eval('button', (button) => button.remove());
+        const button = (await page.$('button'))!;
+        await page.$eval('button', (button) => {
+          return button.remove();
+        });
         expect(await page.accessibility.snapshot({ root: button })).toEqual(
           null
         );
@@ -471,7 +502,7 @@ describeFailsFirefox('Accessibility', function () {
         const { page } = getTestState();
 
         await page.setContent(`<div><button>My Button</button></div>`);
-        const div = await page.$('div');
+        const div = (await page.$('div'))!;
         expect(await page.accessibility.snapshot({ root: div })).toEqual(null);
         expect(
           await page.accessibility.snapshot({
@@ -492,11 +523,14 @@ describeFailsFirefox('Accessibility', function () {
       });
     });
   });
-  function findFocusedNode(node) {
-    if (node.focused) {
+
+  function findFocusedNode(
+    node: SerializedAXNode | null
+  ): SerializedAXNode | null {
+    if (node?.focused) {
       return node;
     }
-    for (const child of node.children || []) {
+    for (const child of node?.children || []) {
       const focusedChild = findFocusedNode(child);
       if (focusedChild) {
         return focusedChild;
diff --git a/test/src/ariaqueryhandler.spec.ts b/test/src/ariaqueryhandler.spec.ts
index 45d5bfc6eb3da..1c6dffe0eade7 100644
--- a/test/src/ariaqueryhandler.spec.ts
+++ b/test/src/ariaqueryhandler.spec.ts
@@ -20,10 +20,11 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeChromeOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 import { ElementHandle } from '../../lib/cjs/puppeteer/common/JSHandle.js';
 import utils from './utils.js';
+import assert from 'assert';
 
 describeChromeOnly('AriaQueryHandler', () => {
   setupTestBrowserHooks();
@@ -38,8 +39,11 @@ describeChromeOnly('AriaQueryHandler', () => {
     });
     it('should find button', async () => {
       const { page } = getTestState();
-      const expectFound = async (button: ElementHandle) => {
-        const id = await button.evaluate((button: Element) => button.id);
+      const expectFound = async (button: ElementHandle | null) => {
+        assert(button);
+        const id = await button.evaluate((button: Element) => {
+          return button.id;
+        });
         expect(id).toBe('btn');
       };
       let button = await page.$(
@@ -94,8 +98,10 @@ describeChromeOnly('AriaQueryHandler', () => {
       await page.setContent(
         '<div id="div"><button id="btn" role="button">Submit</button></div>'
       );
-      const button = await page.$('aria/[role="button"]');
-      const id = await button.evaluate((button: Element) => button.id);
+      const button = (await page.$('aria/[role="button"]'))!;
+      const id = await button!.evaluate((button: Element) => {
+        return button.id;
+      });
       expect(id).toBe('btn');
     });
 
@@ -104,8 +110,10 @@ describeChromeOnly('AriaQueryHandler', () => {
       await page.setContent(
         '<div id="div"><button id="btn" role="button">Submit</button></div>'
       );
-      const button = await page.$('aria/Submit[role="button"]');
-      const id = await button.evaluate((button: Element) => button.id);
+      const button = (await page.$('aria/Submit[role="button"]'))!;
+      const id = await button!.evaluate((button: Element) => {
+        return button.id;
+      });
       expect(id).toBe('btn');
     });
 
@@ -117,8 +125,10 @@ describeChromeOnly('AriaQueryHandler', () => {
         <div role="menu" id="mnu2" aria-label="menu div"></div>
         `
       );
-      const div = await page.$('aria/menu div');
-      const id = await div.evaluate((div: Element) => div.id);
+      const div = (await page.$('aria/menu div'))!;
+      const id = await div!.evaluate((div: Element) => {
+        return div.id;
+      });
       expect(id).toBe('mnu1');
     });
 
@@ -130,8 +140,10 @@ describeChromeOnly('AriaQueryHandler', () => {
         <div role="menu" id="mnu2" aria-label="menu-label2">menu div</div>
         `
       );
-      const menu = await page.$('aria/menu-label1');
-      const id = await menu.evaluate((div: Element) => div.id);
+      const menu = (await page.$('aria/menu-label1'))!;
+      const id = await menu!.evaluate((div: Element) => {
+        return div.id;
+      });
       expect(id).toBe('mnu1');
     });
 
@@ -143,8 +155,10 @@ describeChromeOnly('AriaQueryHandler', () => {
         <div role="menu" id="mnu2" aria-label="menu-label2">menu div</div>
         `
       );
-      const menu = await page.$('aria/menu-label2');
-      const id = await menu.evaluate((div: Element) => div.id);
+      const menu = (await page.$('aria/menu-label2'))!;
+      const id = await menu!.evaluate((div: Element) => {
+        return div.id;
+      });
       expect(id).toBe('mnu2');
     });
   });
@@ -160,7 +174,11 @@ describeChromeOnly('AriaQueryHandler', () => {
       );
       const divs = await page.$$('aria/menu div');
       const ids = await Promise.all(
-        divs.map((n) => n.evaluate((div: Element) => div.id))
+        divs.map((n) => {
+          return n.evaluate((div: Element) => {
+            return div.id;
+          });
+        })
       );
       expect(ids.join(', ')).toBe('mnu1, mnu2');
     });
@@ -178,16 +196,19 @@ describeChromeOnly('AriaQueryHandler', () => {
         }
         `
       );
-      const sum = await page.$$eval('aria/[role="button"]', (buttons) =>
-        buttons.reduce((acc, button) => acc + Number(button.textContent), 0)
-      );
+      const sum = await page.$$eval('aria/[role="button"]', (buttons) => {
+        return buttons.reduce((acc, button) => {
+          return acc + Number(button.textContent);
+        }, 0);
+      });
       expect(sum).toBe(50005000);
     });
   });
 
   describe('waitForSelector (aria)', function () {
-    const addElement = (tag) =>
-      document.body.appendChild(document.createElement(tag));
+    const addElement = (tag: string) => {
+      return document.body.appendChild(document.createElement(tag));
+    };
 
     it('should immediately resolve promise if node exists', async () => {
       const { page, server } = getTestState();
@@ -199,11 +220,11 @@ describeChromeOnly('AriaQueryHandler', () => {
     it('should work for ElementHandler.waitForSelector', async () => {
       const { page, server } = getTestState();
       await page.goto(server.EMPTY_PAGE);
-      await page.evaluate(
-        () => (document.body.innerHTML = `<div><button>test</button></div>`)
-      );
-      const element = await page.$('div');
-      await element.waitForSelector('aria/test');
+      await page.evaluate(() => {
+        return (document.body.innerHTML = `<div><button>test</button></div>`);
+      });
+      const element = (await page.$('div'))!;
+      await element!.waitForSelector('aria/test');
     });
 
     it('should persist query handler bindings across reloads', async () => {
@@ -235,7 +256,9 @@ describeChromeOnly('AriaQueryHandler', () => {
     it('should work independently of `exposeFunction`', async () => {
       const { page, server } = getTestState();
       await page.goto(server.EMPTY_PAGE);
-      await page.exposeFunction('ariaQuerySelector', (a, b) => a + b);
+      await page.exposeFunction('ariaQuerySelector', (a: number, b: number) => {
+        return a + b;
+      });
       await page.evaluate(addElement, 'button');
       await page.waitForSelector('aria/[role="button"]');
       const result = await page.evaluate('globalThis.ariaQuerySelector(2,8)');
@@ -245,13 +268,18 @@ describeChromeOnly('AriaQueryHandler', () => {
     it('should work with removed MutationObserver', async () => {
       const { page } = getTestState();
 
-      await page.evaluate(() => delete window.MutationObserver);
+      await page.evaluate(() => {
+        // @ts-expect-error This is the point of the test.
+        return delete window.MutationObserver;
+      });
       const [handle] = await Promise.all([
         page.waitForSelector('aria/anything'),
         page.setContent(`<h1>anything</h1>`),
       ]);
       expect(
-        await page.evaluate((x: HTMLElement) => x.textContent, handle)
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, handle)
       ).toBe('anything');
     });
 
@@ -263,10 +291,10 @@ describeChromeOnly('AriaQueryHandler', () => {
       const watchdog = frame.waitForSelector('aria/[role="heading"]');
       await frame.evaluate(addElement, 'br');
       await frame.evaluate(addElement, 'h1');
-      const elementHandle = await watchdog;
-      const tagName = await elementHandle
-        .getProperty('tagName')
-        .then((element) => element.jsonValue());
+      const elementHandle = (await watchdog)!;
+      const tagName = await (
+        await elementHandle.getProperty('tagName')
+      ).jsonValue();
       expect(tagName).toBe('H1');
     });
 
@@ -276,11 +304,10 @@ describeChromeOnly('AriaQueryHandler', () => {
       await page.goto(server.EMPTY_PAGE);
       const watchdog = page.waitForSelector('aria/name');
       await page.evaluate(addElement, 'span');
-      await page.evaluate(
-        () =>
-          (document.querySelector('span').innerHTML =
-            '<h3><div aria-label="name"></div></h3>')
-      );
+      await page.evaluate(() => {
+        return (document.querySelector('span')!.innerHTML =
+          '<h3><div aria-label="name"></div></h3>');
+      });
       await watchdog;
     });
 
@@ -291,10 +318,10 @@ describeChromeOnly('AriaQueryHandler', () => {
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       const otherFrame = page.frames()[1];
       const watchdog = page.waitForSelector('aria/[role="button"]');
-      await otherFrame.evaluate(addElement, 'button');
+      await otherFrame!.evaluate(addElement, 'button');
       await page.evaluate(addElement, 'button');
       const elementHandle = await watchdog;
-      expect(elementHandle.executionContext().frame()).toBe(page.mainFrame());
+      expect(elementHandle!.executionContext().frame()).toBe(page.mainFrame());
     });
 
     it('should run in specified frame', async () => {
@@ -304,13 +331,13 @@ describeChromeOnly('AriaQueryHandler', () => {
       await utils.attachFrame(page, 'frame2', server.EMPTY_PAGE);
       const frame1 = page.frames()[1];
       const frame2 = page.frames()[2];
-      const waitForSelectorPromise = frame2.waitForSelector(
+      const waitForSelectorPromise = frame2!.waitForSelector(
         'aria/[role="button"]'
       );
-      await frame1.evaluate(addElement, 'button');
-      await frame2.evaluate(addElement, 'button');
+      await frame1!.evaluate(addElement, 'button');
+      await frame2!.evaluate(addElement, 'button');
       const elementHandle = await waitForSelectorPromise;
-      expect(elementHandle.executionContext().frame()).toBe(frame2);
+      expect(elementHandle!.executionContext().frame()).toBe(frame2);
     });
 
     it('should throw when frame is detached', async () => {
@@ -318,10 +345,12 @@ describeChromeOnly('AriaQueryHandler', () => {
 
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       const frame = page.frames()[1];
-      let waitError = null;
-      const waitPromise = frame
+      let waitError!: Error;
+      const waitPromise = frame!
         .waitForSelector('aria/does-not-exist')
-        .catch((error) => (waitError = error));
+        .catch((error) => {
+          return (waitError = error);
+        });
       await utils.detachFrame(page, 'frame1');
       await waitPromise;
       expect(waitError).toBeTruthy();
@@ -336,7 +365,9 @@ describeChromeOnly('AriaQueryHandler', () => {
       let imgFound = false;
       const waitForSelector = page
         .waitForSelector('aria/[role="img"]')
-        .then(() => (imgFound = true));
+        .then(() => {
+          return (imgFound = true);
+        });
       await page.goto(server.EMPTY_PAGE);
       expect(imgFound).toBe(false);
       await page.reload();
@@ -352,18 +383,22 @@ describeChromeOnly('AriaQueryHandler', () => {
       let divFound = false;
       const waitForSelector = page
         .waitForSelector('aria/name', { visible: true })
-        .then(() => (divFound = true));
+        .then(() => {
+          return (divFound = true);
+        });
       await page.setContent(
         `<div aria-label='name' style='display: none; visibility: hidden;'>1</div>`
       );
       expect(divFound).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('display')
-      );
+      await page.evaluate(() => {
+        return document.querySelector('div')!.style.removeProperty('display');
+      });
       expect(divFound).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('visibility')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')!
+          .style.removeProperty('visibility');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divFound).toBe(true);
     });
@@ -374,18 +409,22 @@ describeChromeOnly('AriaQueryHandler', () => {
       let divVisible = false;
       const waitForSelector = page
         .waitForSelector('aria/inner', { visible: true })
-        .then(() => (divVisible = true));
+        .then(() => {
+          return (divVisible = true);
+        });
       await page.setContent(
         `<div style='display: none; visibility: hidden;'><div aria-label="inner">hi</div></div>`
       );
       expect(divVisible).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('display')
-      );
+      await page.evaluate(() => {
+        return document.querySelector('div')!.style.removeProperty('display');
+      });
       expect(divVisible).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('visibility')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')!
+          .style.removeProperty('visibility');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divVisible).toBe(true);
     });
@@ -399,12 +438,16 @@ describeChromeOnly('AriaQueryHandler', () => {
       );
       const waitForSelector = page
         .waitForSelector('aria/[role="button"]', { hidden: true })
-        .then(() => (divHidden = true));
+        .then(() => {
+          return (divHidden = true);
+        });
       await page.waitForSelector('aria/[role="button"]'); // do a round trip
       expect(divHidden).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.setProperty('visibility', 'hidden')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')!
+          .style.setProperty('visibility', 'hidden');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divHidden).toBe(true);
     });
@@ -416,12 +459,16 @@ describeChromeOnly('AriaQueryHandler', () => {
       await page.setContent(`<div role='main' style='display: block;'></div>`);
       const waitForSelector = page
         .waitForSelector('aria/[role="main"]', { hidden: true })
-        .then(() => (divHidden = true));
+        .then(() => {
+          return (divHidden = true);
+        });
       await page.waitForSelector('aria/[role="main"]'); // do a round trip
       expect(divHidden).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.setProperty('display', 'none')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')!
+          .style.setProperty('display', 'none');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divHidden).toBe(true);
     });
@@ -433,10 +480,14 @@ describeChromeOnly('AriaQueryHandler', () => {
       let divRemoved = false;
       const waitForSelector = page
         .waitForSelector('aria/[role="main"]', { hidden: true })
-        .then(() => (divRemoved = true));
+        .then(() => {
+          return (divRemoved = true);
+        });
       await page.waitForSelector('aria/[role="main"]'); // do a round trip
       expect(divRemoved).toBe(false);
-      await page.evaluate(() => document.querySelector('div').remove());
+      await page.evaluate(() => {
+        return document.querySelector('div')!.remove();
+      });
       expect(await waitForSelector).toBe(true);
       expect(divRemoved).toBe(true);
     });
@@ -453,10 +504,12 @@ describeChromeOnly('AriaQueryHandler', () => {
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .waitForSelector('aria/[role="button"]', { timeout: 10 })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
       expect(error.message).toContain(
         'waiting for selector `[role="button"]` failed: timeout'
@@ -468,10 +521,12 @@ describeChromeOnly('AriaQueryHandler', () => {
       const { page } = getTestState();
 
       await page.setContent(`<div role='main'></div>`);
-      let error = null;
+      let error!: Error;
       await page
         .waitForSelector('aria/[role="main"]', { hidden: true, timeout: 10 })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
       expect(error.message).toContain(
         'waiting for selector `[role="main"]` to be hidden failed: timeout'
@@ -482,14 +537,16 @@ describeChromeOnly('AriaQueryHandler', () => {
       const { page } = getTestState();
 
       let divFound = false;
-      const waitForSelector = page
-        .waitForSelector('aria/zombo')
-        .then(() => (divFound = true));
+      const waitForSelector = page.waitForSelector('aria/zombo').then(() => {
+        return (divFound = true);
+      });
       await page.setContent(`<div aria-label='notZombo'></div>`);
       expect(divFound).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').setAttribute('aria-label', 'zombo')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')!
+          .setAttribute('aria-label', 'zombo');
+      });
       expect(await waitForSelector).toBe(true);
     });
 
@@ -499,21 +556,22 @@ describeChromeOnly('AriaQueryHandler', () => {
       const waitForSelector = page.waitForSelector('aria/zombo');
       await page.setContent(`<div aria-label='zombo'>anything</div>`);
       expect(
-        await page.evaluate(
-          (x: HTMLElement) => x.textContent,
-          await waitForSelector
-        )
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, await waitForSelector)
       ).toBe('anything');
     });
 
     it('should have correct stack trace for timeout', async () => {
       const { page } = getTestState();
 
-      let error;
+      let error!: Error;
       await page
         .waitForSelector('aria/zombo', { timeout: 10 })
-        .catch((error_) => (error = error_));
-      expect(error.stack).toContain('waiting for selector `zombo` failed');
+        .catch((error_) => {
+          return (error = error_);
+        });
+      expect(error!.stack).toContain('waiting for selector `zombo` failed');
     });
   });
 
@@ -566,12 +624,15 @@ describeChromeOnly('AriaQueryHandler', () => {
           `
       );
     });
-    const getIds = async (elements: ElementHandle[]) =>
-      Promise.all(
-        elements.map((element) =>
-          element.evaluate((element: Element) => element.id)
-        )
+    const getIds = async (elements: ElementHandle[]) => {
+      return Promise.all(
+        elements.map((element) => {
+          return element.evaluate((element: Element) => {
+            return element.id;
+          });
+        })
       );
+    };
     it('should find by name "foo"', async () => {
       const { page } = getTestState();
       const found = await page.$$('aria/foo');
diff --git a/test/src/browser.spec.ts b/test/src/browser.spec.ts
index 6676d034fe862..2589221178668 100644
--- a/test/src/browser.spec.ts
+++ b/test/src/browser.spec.ts
@@ -15,7 +15,7 @@
  */
 
 import expect from 'expect';
-import { getTestState, setupTestBrowserHooks } from './mocha-utils'; // eslint-disable-line import/extensions
+import { getTestState, setupTestBrowserHooks } from './mocha-utils.js';
 
 describe('Browser specs', function () {
   setupTestBrowserHooks();
@@ -60,7 +60,7 @@ describe('Browser specs', function () {
       const { browser } = getTestState();
 
       const process = await browser.process();
-      expect(process.pid).toBeGreaterThan(0);
+      expect(process!.pid).toBeGreaterThan(0);
     });
     it('should not return child_process for remote browser', async () => {
       const { browser, puppeteer } = getTestState();
diff --git a/test/src/browsercontext.spec.ts b/test/src/browsercontext.spec.ts
index ba4866a764c55..48466a53104e7 100644
--- a/test/src/browsercontext.spec.ts
+++ b/test/src/browsercontext.spec.ts
@@ -19,7 +19,7 @@ import {
   getTestState,
   setupTestBrowserHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import utils from './utils.js';
 
 describe('BrowserContext', function () {
@@ -27,10 +27,12 @@ describe('BrowserContext', function () {
   it('should have default context', async () => {
     const { browser } = getTestState();
     expect(browser.browserContexts().length).toEqual(1);
-    const defaultContext = browser.browserContexts()[0];
-    expect(defaultContext.isIncognito()).toBe(false);
-    let error = null;
-    await defaultContext.close().catch((error_) => (error = error_));
+    const defaultContext = browser.browserContexts()[0]!;
+    expect(defaultContext!.isIncognito()).toBe(false);
+    let error!: Error;
+    await defaultContext!.close().catch((error_) => {
+      return (error = error_);
+    });
     expect(browser.defaultBrowserContext()).toBe(defaultContext);
     expect(error.message).toContain('cannot be closed');
   });
@@ -66,10 +68,9 @@ describe('BrowserContext', function () {
     await page.goto(server.EMPTY_PAGE);
     const [popupTarget] = await Promise.all([
       utils.waitEvent(browser, 'targetcreated'),
-      page.evaluate<(url: string) => void>(
-        (url) => window.open(url),
-        server.EMPTY_PAGE
-      ),
+      page.evaluate<(url: string) => void>((url) => {
+        return window.open(url);
+      }, server.EMPTY_PAGE),
     ]);
     expect(popupTarget.browserContext()).toBe(context);
     await context.close();
@@ -78,16 +79,16 @@ describe('BrowserContext', function () {
     const { browser, server } = getTestState();
 
     const context = await browser.createIncognitoBrowserContext();
-    const events = [];
-    context.on('targetcreated', (target) =>
-      events.push('CREATED: ' + target.url())
-    );
-    context.on('targetchanged', (target) =>
-      events.push('CHANGED: ' + target.url())
-    );
-    context.on('targetdestroyed', (target) =>
-      events.push('DESTROYED: ' + target.url())
-    );
+    const events: any[] = [];
+    context.on('targetcreated', (target) => {
+      return events.push('CREATED: ' + target.url());
+    });
+    context.on('targetchanged', (target) => {
+      return events.push('CHANGED: ' + target.url());
+    });
+    context.on('targetdestroyed', (target) => {
+      return events.push('DESTROYED: ' + target.url());
+    });
     const page = await context.newPage();
     await page.goto(server.EMPTY_PAGE);
     await page.close();
@@ -104,11 +105,13 @@ describe('BrowserContext', function () {
     const context = await browser.createIncognitoBrowserContext();
     let resolved = false;
 
-    const targetPromise = context.waitForTarget(
-      (target) => target.url() === server.EMPTY_PAGE
-    );
+    const targetPromise = context.waitForTarget((target) => {
+      return target.url() === server.EMPTY_PAGE;
+    });
     targetPromise
-      .then(() => (resolved = true))
+      .then(() => {
+        return (resolved = true);
+      })
       .catch((error) => {
         resolved = true;
         if (error instanceof puppeteer.errors.TimeoutError) {
@@ -138,10 +141,17 @@ describe('BrowserContext', function () {
 
     const context = await browser.createIncognitoBrowserContext();
     const error = await context
-      .waitForTarget((target) => target.url() === server.EMPTY_PAGE, {
-        timeout: 1,
-      })
-      .catch((error_) => error_);
+      .waitForTarget(
+        (target) => {
+          return target.url() === server.EMPTY_PAGE;
+        },
+        {
+          timeout: 1,
+        }
+      )
+      .catch((error_) => {
+        return error_;
+      });
     expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     await context.close();
   });
@@ -175,19 +185,31 @@ describe('BrowserContext', function () {
     });
 
     expect(context1.targets().length).toBe(1);
-    expect(context1.targets()[0]).toBe(page1.target());
+    expect(context1.targets()[0]!).toBe(page1.target());
     expect(context2.targets().length).toBe(1);
-    expect(context2.targets()[0]).toBe(page2.target());
+    expect(context2.targets()[0]!).toBe(page2.target());
 
     // Make sure pages don't share localstorage or cookies.
-    expect(await page1.evaluate(() => localStorage.getItem('name'))).toBe(
-      'page1'
-    );
-    expect(await page1.evaluate(() => document.cookie)).toBe('name=page1');
-    expect(await page2.evaluate(() => localStorage.getItem('name'))).toBe(
-      'page2'
-    );
-    expect(await page2.evaluate(() => document.cookie)).toBe('name=page2');
+    expect(
+      await page1.evaluate(() => {
+        return localStorage.getItem('name');
+      })
+    ).toBe('page1');
+    expect(
+      await page1.evaluate(() => {
+        return document.cookie;
+      })
+    ).toBe('name=page1');
+    expect(
+      await page2.evaluate(() => {
+        return localStorage.getItem('name');
+      })
+    ).toBe('page2');
+    expect(
+      await page2.evaluate(() => {
+        return document.cookie;
+      })
+    ).toBe('name=page2');
 
     // Cleanup contexts.
     await Promise.all([context1.close(), context2.close()]);
diff --git a/test/src/chromiumonly.spec.ts b/test/src/chromiumonly.spec.ts
index 7b64a70fd396c..651339dc7bac0 100644
--- a/test/src/chromiumonly.spec.ts
+++ b/test/src/chromiumonly.spec.ts
@@ -14,12 +14,13 @@
  * limitations under the License.
  */
 import expect from 'expect';
+import { IncomingMessage } from 'http';
 import {
   getTestState,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeChromeOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describeChromeOnly('Chromium-Specific Launcher tests', function () {
   describe('Puppeteer.launch |browserURL| option', function () {
@@ -35,14 +36,22 @@ describeChromeOnly('Chromium-Specific Launcher tests', function () {
 
       const browser1 = await puppeteer.connect({ browserURL });
       const page1 = await browser1.newPage();
-      expect(await page1.evaluate(() => 7 * 8)).toBe(56);
+      expect(
+        await page1.evaluate(() => {
+          return 7 * 8;
+        })
+      ).toBe(56);
       browser1.disconnect();
 
       const browser2 = await puppeteer.connect({
         browserURL: browserURL + '/',
       });
       const page2 = await browser2.newPage();
-      expect(await page2.evaluate(() => 8 * 7)).toBe(56);
+      expect(
+        await page2.evaluate(() => {
+          return 8 * 7;
+        })
+      ).toBe(56);
       browser2.disconnect();
       originalBrowser.close();
     });
@@ -56,13 +65,15 @@ describeChromeOnly('Chromium-Specific Launcher tests', function () {
       );
       const browserURL = 'http://127.0.0.1:21222';
 
-      let error = null;
+      let error!: Error;
       await puppeteer
         .connect({
           browserURL,
           browserWSEndpoint: originalBrowser.wsEndpoint(),
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain(
         'Exactly one of browserWSEndpoint, browserURL or transport'
       );
@@ -79,10 +90,10 @@ describeChromeOnly('Chromium-Specific Launcher tests', function () {
       );
       const browserURL = 'http://127.0.0.1:32333';
 
-      let error = null;
-      await puppeteer
-        .connect({ browserURL })
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await puppeteer.connect({ browserURL }).catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain(
         'Failed to fetch browser webSocket URL from'
       );
@@ -117,11 +128,11 @@ describeChromeOnly('Chromium-Specific Launcher tests', function () {
       const { defaultBrowserOptions, puppeteer } = getTestState();
       const options = Object.assign({ pipe: true }, defaultBrowserOptions);
       const browser = await puppeteer.launch(options);
-      const disconnectedEventPromise = new Promise((resolve) =>
-        browser.once('disconnected', resolve)
-      );
+      const disconnectedEventPromise = new Promise((resolve) => {
+        return browser.once('disconnected', resolve);
+      });
       // Emulate user exiting browser.
-      browser.process().kill();
+      browser.process()!.kill();
       await disconnectedEventPromise;
     });
   });
@@ -133,26 +144,28 @@ describeChromeOnly('Chromium-Specific Page Tests', function () {
   it('Page.setRequestInterception should work with intervention headers', async () => {
     const { server, page } = getTestState();
 
-    server.setRoute('/intervention', (req, res) =>
-      res.end(`
+    server.setRoute('/intervention', (_req, res) => {
+      return res.end(`
         <script>
           document.write('<script src="${server.CROSS_PROCESS_PREFIX}/intervention.js">' + '</scr' + 'ipt>');
         </script>
-      `)
-    );
+      `);
+    });
     server.setRedirect('/intervention.js', '/redirect.js');
-    let serverRequest = null;
+    let serverRequest: IncomingMessage | undefined;
     server.setRoute('/redirect.js', (req, res) => {
       serverRequest = req;
       res.end('console.log(1);');
     });
 
     await page.setRequestInterception(true);
-    page.on('request', (request) => request.continue());
+    page.on('request', (request) => {
+      return request.continue();
+    });
     await page.goto(server.PREFIX + '/intervention');
     // Check for feature URL substring rather than https://www.chromestatus.com to
     // make it work with Edgium.
-    expect(serverRequest.headers.intervention).toContain(
+    expect(serverRequest!.headers['intervention']).toContain(
       'feature/5718547946799104'
     );
   });
diff --git a/test/src/click.spec.ts b/test/src/click.spec.ts
index c689fe79aaee2..24a725f73aae2 100644
--- a/test/src/click.spec.ts
+++ b/test/src/click.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestPageAndContextHooks,
   setupTestBrowserHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import utils from './utils.js';
 
 describe('Page.click', function () {
@@ -31,7 +31,11 @@ describe('Page.click', function () {
 
     await page.goto(server.PREFIX + '/input/button.html');
     await page.click('button');
-    expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   it('should click svg', async () => {
     const { page } = getTestState();
@@ -42,7 +46,11 @@ describe('Page.click', function () {
         </svg>
       `);
     await page.click('circle');
-    expect(await page.evaluate(() => globalThis.__CLICKED)).toBe(42);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).__CLICKED;
+      })
+    ).toBe(42);
   });
   itFailsFirefox(
     'should click the button if window.Node is removed',
@@ -50,9 +58,16 @@ describe('Page.click', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      await page.evaluate(() => delete window.Node);
+      await page.evaluate(() => {
+        // @ts-expect-error Expected.
+        return delete window.Node;
+      });
       await page.click('button');
-      expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result;
+        })
+      ).toBe('Clicked');
     }
   );
   // @see https://github.com/puppeteer/puppeteer/issues/4281
@@ -68,7 +83,11 @@ describe('Page.click', function () {
         <span onclick='javascript:window.CLICKED=42'></span>
       `);
     await page.click('span');
-    expect(await page.evaluate(() => globalThis.CLICKED)).toBe(42);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).CLICKED;
+      })
+    ).toBe(42);
   });
   it('should not throw UnhandledPromiseRejection when page closes', async () => {
     const { page } = getTestState();
@@ -85,7 +104,11 @@ describe('Page.click', function () {
     await page.click('button');
     await page.goto(server.PREFIX + '/input/button.html');
     await page.click('button');
-    expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   itFailsFirefox('should click with disabled javascript', async () => {
     const { page, server } = getTestState();
@@ -108,7 +131,11 @@ describe('Page.click', function () {
         <span onclick='javascript:window.CLICKED = 42;'><i>woof</i><b>doggo</b></span>
       `);
     await page.click('span');
-    expect(await page.evaluate(() => globalThis.CLICKED)).toBe(42);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).CLICKED;
+      })
+    ).toBe(42);
   });
   it('should select the text by triple clicking', async () => {
     const { page, server } = getTestState();
@@ -124,9 +151,9 @@ describe('Page.click', function () {
     expect(
       await page.evaluate(() => {
         const textarea = document.querySelector('textarea');
-        return textarea.value.substring(
-          textarea.selectionStart,
-          textarea.selectionEnd
+        return textarea!.value.substring(
+          textarea!.selectionStart,
+          textarea!.selectionEnd
         );
       })
     ).toBe(text);
@@ -135,11 +162,15 @@ describe('Page.click', function () {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/offscreenbuttons.html');
-    const messages = [];
-    page.on('console', (msg) => messages.push(msg.text()));
+    const messages: any[] = [];
+    page.on('console', (msg) => {
+      return messages.push(msg.text());
+    });
     for (let i = 0; i < 11; ++i) {
       // We might've scrolled to click a button - reset to (0, 0).
-      await page.evaluate(() => window.scrollTo(0, 0));
+      await page.evaluate(() => {
+        return window.scrollTo(0, 0);
+      });
       await page.click(`#btn${i}`);
     }
     expect(messages).toEqual([
@@ -162,17 +193,33 @@ describe('Page.click', function () {
 
     await page.goto(server.PREFIX + '/wrappedlink.html');
     await page.click('a');
-    expect(await page.evaluate(() => globalThis.__clicked)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).__clicked;
+      })
+    ).toBe(true);
   });
 
   it('should click on checkbox input and toggle', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/checkbox.html');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(null);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(null);
     await page.click('input#agree');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(true);
-    expect(await page.evaluate(() => globalThis.result.events)).toEqual([
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.events;
+      })
+    ).toEqual([
       'mouseover',
       'mouseenter',
       'mousemove',
@@ -183,33 +230,49 @@ describe('Page.click', function () {
       'change',
     ]);
     await page.click('input#agree');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(false);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(false);
   });
 
   itFailsFirefox('should click on checkbox label and toggle', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/checkbox.html');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(null);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(null);
     await page.click('label[for="agree"]');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(true);
-    expect(await page.evaluate(() => globalThis.result.events)).toEqual([
-      'click',
-      'input',
-      'change',
-    ]);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.events;
+      })
+    ).toEqual(['click', 'input', 'change']);
     await page.click('label[for="agree"]');
-    expect(await page.evaluate(() => globalThis.result.check)).toBe(false);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result.check;
+      })
+    ).toBe(false);
   });
 
   it('should fail to click a missing button', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/button.html');
-    let error = null;
-    await page
-      .click('button.does-not-exist')
-      .catch((error_) => (error = error_));
+    let error!: Error;
+    await page.click('button.does-not-exist').catch((error_) => {
+      return (error = error_);
+    });
     expect(error.message).toBe(
       'No element found for selector: button.does-not-exist'
     );
@@ -218,7 +281,7 @@ describe('Page.click', function () {
   it('should not hang with touch-enabled viewports', async () => {
     const { page, puppeteer } = getTestState();
 
-    await page.setViewport(puppeteer.devices['iPhone 6'].viewport);
+    await page.setViewport(puppeteer.devices['iPhone 6']!.viewport);
     await page.mouse.down();
     await page.mouse.move(100, 10);
     await page.mouse.up();
@@ -229,13 +292,15 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.click('#button-5');
     expect(
-      await page.evaluate(() => document.querySelector('#button-5').textContent)
+      await page.evaluate(() => {
+        return document.querySelector('#button-5')!.textContent;
+      })
     ).toBe('clicked');
     await page.click('#button-80');
     expect(
-      await page.evaluate(
-        () => document.querySelector('#button-80').textContent
-      )
+      await page.evaluate(() => {
+        return document.querySelector('#button-80')!.textContent;
+      })
     ).toBe('clicked');
   });
   it('should double click the button', async () => {
@@ -243,14 +308,14 @@ describe('Page.click', function () {
 
     await page.goto(server.PREFIX + '/input/button.html');
     await page.evaluate(() => {
-      globalThis.double = false;
+      (globalThis as any).double = false;
       const button = document.querySelector('button');
-      button.addEventListener('dblclick', () => {
-        globalThis.double = true;
+      button!.addEventListener('dblclick', () => {
+        (globalThis as any).double = true;
       });
     });
-    const button = await page.$('button');
-    await button.click({ clickCount: 2 });
+    const button = (await page.$('button'))!;
+    await button!.click({ clickCount: 2 });
     expect(await page.evaluate('double')).toBe(true);
     expect(await page.evaluate('result')).toBe('Clicked');
   });
@@ -260,19 +325,27 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/button.html');
     await page.evaluate(() => {
       const button = document.querySelector('button');
-      button.textContent = 'Some really long text that will go offscreen';
-      button.style.position = 'absolute';
-      button.style.left = '368px';
+      button!.textContent = 'Some really long text that will go offscreen';
+      button!.style.position = 'absolute';
+      button!.style.left = '368px';
     });
     await page.click('button');
-    expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   it('should click a rotated button', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/rotatedButton.html');
     await page.click('button');
-    expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   it('should fire contextmenu event on right click', async () => {
     const { page, server } = getTestState();
@@ -280,7 +353,9 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.click('#button-8', { button: 'right' });
     expect(
-      await page.evaluate(() => document.querySelector('#button-8').textContent)
+      await page.evaluate(() => {
+        return document.querySelector('#button-8')!.textContent;
+      })
     ).toBe('context menu');
   });
   it('should fire aux event on middle click', async () => {
@@ -289,7 +364,9 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.click('#button-8', { button: 'middle' });
     expect(
-      await page.evaluate(() => document.querySelector('#button-8').textContent)
+      await page.evaluate(() => {
+        return document.querySelector('#button-8')!.textContent;
+      })
     ).toBe('aux click');
   });
   it('should fire back click', async () => {
@@ -298,7 +375,9 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.click('#button-8', { button: 'back' });
     expect(
-      await page.evaluate(() => document.querySelector('#button-8').textContent)
+      await page.evaluate(() => {
+        return document.querySelector('#button-8')!.textContent;
+      })
     ).toBe('back click');
   });
   it('should fire forward click', async () => {
@@ -307,7 +386,9 @@ describe('Page.click', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.click('#button-8', { button: 'forward' });
     expect(
-      await page.evaluate(() => document.querySelector('#button-8').textContent)
+      await page.evaluate(() => {
+        return document.querySelector('#button-8')!.textContent;
+      })
     ).toBe('forward click');
   });
   // @see https://github.com/puppeteer/puppeteer/issues/206
@@ -329,9 +410,13 @@ describe('Page.click', function () {
       server.PREFIX + '/input/button.html'
     );
     const frame = page.frames()[1];
-    const button = await frame.$('button');
-    await button.click();
-    expect(await frame.evaluate(() => globalThis.result)).toBe('Clicked');
+    const button = await frame!.$('button');
+    await button!.click();
+    expect(
+      await frame!.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   // @see https://github.com/puppeteer/puppeteer/issues/4110
   xit('should click the button with fixed position inside an iframe', async () => {
@@ -348,17 +433,25 @@ describe('Page.click', function () {
       server.CROSS_PROCESS_PREFIX + '/input/button.html'
     );
     const frame = page.frames()[1];
-    await frame.$eval('button', (button: HTMLElement) =>
-      button.style.setProperty('position', 'fixed')
-    );
-    await frame.click('button');
-    expect(await frame.evaluate(() => globalThis.result)).toBe('Clicked');
+    await frame!.$eval('button', (button: Element) => {
+      return (button as HTMLElement).style.setProperty('position', 'fixed');
+    });
+    await frame!.click('button');
+    expect(
+      await frame!.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   it('should click the button with deviceScaleFactor set', async () => {
     const { page, server } = getTestState();
 
     await page.setViewport({ width: 400, height: 400, deviceScaleFactor: 5 });
-    expect(await page.evaluate(() => window.devicePixelRatio)).toBe(5);
+    expect(
+      await page.evaluate(() => {
+        return window.devicePixelRatio;
+      })
+    ).toBe(5);
     await page.setContent('<div style="width:100px;height:100px">spacer</div>');
     await utils.attachFrame(
       page,
@@ -366,8 +459,12 @@ describe('Page.click', function () {
       server.PREFIX + '/input/button.html'
     );
     const frame = page.frames()[1];
-    const button = await frame.$('button');
-    await button.click();
-    expect(await frame.evaluate(() => globalThis.result)).toBe('Clicked');
+    const button = await frame!.$('button');
+    await button!.click();
+    expect(
+      await frame!.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
 });
diff --git a/test/src/cookies.spec.ts b/test/src/cookies.spec.ts
index ad7339b0d390a..4a73ed9d46cd6 100644
--- a/test/src/cookies.spec.ts
+++ b/test/src/cookies.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('Cookie specs', () => {
   setupTestBrowserHooks();
@@ -58,36 +58,36 @@ describe('Cookie specs', () => {
     });
     it('should properly report httpOnly cookie', async () => {
       const { page, server } = getTestState();
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.setHeader('Set-Cookie', 'a=b; HttpOnly; Path=/');
         res.end();
       });
       await page.goto(server.EMPTY_PAGE);
       const cookies = await page.cookies();
       expect(cookies.length).toBe(1);
-      expect(cookies[0].httpOnly).toBe(true);
+      expect(cookies[0]!.httpOnly).toBe(true);
     });
     it('should properly report "Strict" sameSite cookie', async () => {
       const { page, server } = getTestState();
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.setHeader('Set-Cookie', 'a=b; SameSite=Strict');
         res.end();
       });
       await page.goto(server.EMPTY_PAGE);
       const cookies = await page.cookies();
       expect(cookies.length).toBe(1);
-      expect(cookies[0].sameSite).toBe('Strict');
+      expect(cookies[0]!.sameSite).toBe('Strict');
     });
     it('should properly report "Lax" sameSite cookie', async () => {
       const { page, server } = getTestState();
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.setHeader('Set-Cookie', 'a=b; SameSite=Lax');
         res.end();
       });
       await page.goto(server.EMPTY_PAGE);
       const cookies = await page.cookies();
       expect(cookies.length).toBe(1);
-      expect(cookies[0].sameSite).toBe('Lax');
+      expect(cookies[0]!.sameSite).toBe('Lax');
     });
     it('should get multiple cookies', async () => {
       const { page, server } = getTestState();
@@ -97,7 +97,9 @@ describe('Cookie specs', () => {
         document.cookie = 'password=1234';
       });
       const cookies = await page.cookies();
-      cookies.sort((a, b) => a.name.localeCompare(b.name));
+      cookies.sort((a, b) => {
+        return a.name.localeCompare(b.name);
+      });
       expectCookieEquals(cookies, [
         {
           name: 'password',
@@ -149,7 +151,9 @@ describe('Cookie specs', () => {
         }
       );
       const cookies = await page.cookies('https://foo.com', 'https://baz.com');
-      cookies.sort((a, b) => a.name.localeCompare(b.name));
+      cookies.sort((a, b) => {
+        return a.name.localeCompare(b.name);
+      });
       expectCookieEquals(cookies, [
         {
           name: 'birdo',
@@ -191,9 +195,11 @@ describe('Cookie specs', () => {
         name: 'password',
         value: '123456',
       });
-      expect(await page.evaluate(() => document.cookie)).toEqual(
-        'password=123456'
-      );
+      expect(
+        await page.evaluate(() => {
+          return document.cookie;
+        })
+      ).toEqual('password=123456');
     });
     itFailsFirefox('should isolate cookies in browser contexts', async () => {
       const { page, server, browser } = getTestState();
@@ -211,10 +217,10 @@ describe('Cookie specs', () => {
       const cookies2 = await anotherPage.cookies();
       expect(cookies1.length).toBe(1);
       expect(cookies2.length).toBe(1);
-      expect(cookies1[0].name).toBe('page1cookie');
-      expect(cookies1[0].value).toBe('page1value');
-      expect(cookies2[0].name).toBe('page2cookie');
-      expect(cookies2[0].value).toBe('page2value');
+      expect(cookies1[0]!.name).toBe('page1cookie');
+      expect(cookies1[0]!.value).toBe('page1value');
+      expect(cookies2[0]!.name).toBe('page2cookie');
+      expect(cookies2[0]!.value).toBe('page2value');
       await anotherContext.close();
     });
     itFailsFirefox('should set multiple cookies', async () => {
@@ -233,7 +239,11 @@ describe('Cookie specs', () => {
       );
       const cookieStrings = await page.evaluate(() => {
         const cookies = document.cookie.split(';');
-        return cookies.map((cookie) => cookie.trim()).sort();
+        return cookies
+          .map((cookie) => {
+            return cookie.trim();
+          })
+          .sort();
       });
 
       expect(cookieStrings).toEqual(['foo=bar', 'password=123456']);
@@ -247,8 +257,8 @@ describe('Cookie specs', () => {
         value: '123456',
       });
       const cookies = await page.cookies();
-      expect(cookies[0].session).toBe(true);
-      expect(cookies[0].expires).toBe(-1);
+      expect(cookies[0]!.session).toBe(true);
+      expect(cookies[0]!.expires).toBe(-1);
     });
     itFailsFirefox('should set cookie with reasonable defaults', async () => {
       const { page, server } = getTestState();
@@ -260,7 +270,9 @@ describe('Cookie specs', () => {
       });
       const cookies = await page.cookies();
       expectCookieEquals(
-        cookies.sort((a, b) => a.name.localeCompare(b.name)),
+        cookies.sort((a, b) => {
+          return a.name.localeCompare(b.name);
+        }),
         [
           {
             name: 'password',
@@ -315,11 +327,11 @@ describe('Cookie specs', () => {
       const { page } = getTestState();
 
       await page.goto('about:blank');
-      let error = null;
+      let error!: Error;
       try {
         await page.setCookie({ name: 'example-cookie', value: 'best' });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toContain(
         'At least one of the url and domain needs to be specified'
@@ -328,7 +340,7 @@ describe('Cookie specs', () => {
     it('should not set a cookie with blank page URL', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page.goto(server.EMPTY_PAGE);
       try {
         await page.setCookie(
@@ -336,7 +348,7 @@ describe('Cookie specs', () => {
           { url: 'about:blank', name: 'example-cookie-blank', value: 'best' }
         );
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toEqual(
         `Blank page can not have cookie "example-cookie-blank"`
@@ -345,12 +357,12 @@ describe('Cookie specs', () => {
     it('should not set a cookie on a data URL page', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page.goto('data:,Hello%2C%20World!');
       try {
         await page.setCookie({ name: 'example-cookie', value: 'best' });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toContain(
         'At least one of the url and domain needs to be specified'
@@ -369,7 +381,7 @@ describe('Cookie specs', () => {
           value: 'bar',
         });
         const [cookie] = await page.cookies(SECURE_URL);
-        expect(cookie.secure).toBe(true);
+        expect(cookie!.secure).toBe(true);
       }
     );
     it('should be able to set unsecure cookie for HTTP website', async () => {
@@ -383,7 +395,7 @@ describe('Cookie specs', () => {
         value: 'bar',
       });
       const [cookie] = await page.cookies(HTTP_URL);
-      expect(cookie.secure).toBe(false);
+      expect(cookie!.secure).toBe(false);
     });
     itFailsFirefox('should set a cookie on a different domain', async () => {
       const { page, server } = getTestState();
@@ -419,9 +431,11 @@ describe('Cookie specs', () => {
       await page.goto(server.PREFIX + '/grid.html');
       await page.setCookie({ name: 'localhost-cookie', value: 'best' });
       await page.evaluate<(src: string) => Promise<void>>((src) => {
-        let fulfill;
-        const promise = new Promise<void>((x) => (fulfill = x));
-        const iframe = document.createElement('iframe');
+        let fulfill!: () => void;
+        const promise = new Promise<void>((x) => {
+          return (fulfill = x);
+        });
+        const iframe = document.createElement('iframe') as HTMLIFrameElement;
         document.body.appendChild(iframe);
         iframe.onload = fulfill;
         iframe.src = src;
@@ -435,7 +449,7 @@ describe('Cookie specs', () => {
       expect(await page.evaluate('document.cookie')).toBe(
         'localhost-cookie=best'
       );
-      expect(await page.frames()[1].evaluate('document.cookie')).toBe('');
+      expect(await page.frames()[1]!.evaluate('document.cookie')).toBe('');
 
       expectCookieEquals(await page.cookies(), [
         {
@@ -487,8 +501,10 @@ describe('Cookie specs', () => {
         try {
           await page.goto(httpsServer.PREFIX + '/grid.html');
           await page.evaluate<(src: string) => Promise<void>>((src) => {
-            let fulfill;
-            const promise = new Promise<void>((x) => (fulfill = x));
+            let fulfill!: () => void;
+            const promise = new Promise<void>((x) => {
+              return (fulfill = x);
+            });
             const iframe = document.createElement('iframe');
             document.body.appendChild(iframe);
             iframe.onload = fulfill;
@@ -502,7 +518,7 @@ describe('Cookie specs', () => {
             sameSite: 'None',
           });
 
-          expect(await page.frames()[1].evaluate('document.cookie')).toBe(
+          expect(await page.frames()[1]!.evaluate('document.cookie')).toBe(
             '127-same-site-cookie=best'
           );
           expectCookieEquals(
diff --git a/test/src/coverage.spec.ts b/test/src/coverage.spec.ts
index 4801a8eed5816..0be08ae4bae95 100644
--- a/test/src/coverage.spec.ts
+++ b/test/src/coverage.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestPageAndContextHooks,
   setupTestBrowserHooks,
   describeChromeOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('Coverage specs', function () {
   describeChromeOnly('JSCoverage', function () {
@@ -35,8 +35,8 @@ describe('Coverage specs', function () {
       });
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toContain('/jscoverage/simple.html');
-      expect(coverage[0].ranges).toEqual([
+      expect(coverage[0]!.url).toContain('/jscoverage/simple.html');
+      expect(coverage[0]!.ranges).toEqual([
         { start: 0, end: 17 },
         { start: 35, end: 61 },
       ]);
@@ -48,7 +48,7 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/jscoverage/sourceurl.html');
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toBe('nicename.js');
+      expect(coverage[0]!.url).toBe('nicename.js');
     });
     it('should ignore eval() scripts by default', async () => {
       const { page, server } = getTestState();
@@ -65,7 +65,9 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/jscoverage/eval.html');
       const coverage = await page.coverage.stopJSCoverage();
       expect(
-        coverage.find((entry) => entry.url.startsWith('debugger://'))
+        coverage.find((entry) => {
+          return entry.url.startsWith('debugger://');
+        })
       ).not.toBe(null);
       expect(coverage.length).toBe(2);
     });
@@ -75,7 +77,9 @@ describe('Coverage specs', function () {
       await page.coverage.startJSCoverage({ reportAnonymousScripts: true });
       await page.goto(server.EMPTY_PAGE);
       await page.evaluate('console.log("foo")');
-      await page.evaluate(() => console.log('bar'));
+      await page.evaluate(() => {
+        return console.log('bar');
+      });
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(0);
     });
@@ -86,9 +90,11 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/jscoverage/multiple.html');
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(2);
-      coverage.sort((a, b) => a.url.localeCompare(b.url));
-      expect(coverage[0].url).toContain('/jscoverage/script1.js');
-      expect(coverage[1].url).toContain('/jscoverage/script2.js');
+      coverage.sort((a, b) => {
+        return a.url.localeCompare(b.url);
+      });
+      expect(coverage[0]!.url).toContain('/jscoverage/script1.js');
+      expect(coverage[1]!.url).toContain('/jscoverage/script2.js');
     });
     it('should report right ranges', async () => {
       const { page, server } = getTestState();
@@ -97,9 +103,9 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/jscoverage/ranges.html');
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(1);
-      const entry = coverage[0];
+      const entry = coverage[0]!;
       expect(entry.ranges.length).toBe(1);
-      const range = entry.ranges[0];
+      const range = entry.ranges[0]!;
       expect(entry.text.substring(range.start, range.end)).toBe(
         `console.log('used!');`
       );
@@ -111,7 +117,7 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/jscoverage/unused.html');
       const coverage = await page.coverage.stopJSCoverage();
       expect(coverage.length).toBe(1);
-      const entry = coverage[0];
+      const entry = coverage[0]!;
       expect(entry.url).toContain('unused.html');
       expect(entry.ranges.length).toBe(0);
     });
@@ -166,7 +172,7 @@ describe('Coverage specs', function () {
         });
         const coverage = await page.coverage.stopJSCoverage();
         expect(coverage.length).toBe(1);
-        expect(coverage[0].rawScriptCoverage).toBeUndefined();
+        expect(coverage[0]!.rawScriptCoverage).toBeUndefined();
       });
       it('should include rawScriptCoverage field when enabled', async () => {
         const { page, server } = getTestState();
@@ -178,7 +184,7 @@ describe('Coverage specs', function () {
         });
         const coverage = await page.coverage.stopJSCoverage();
         expect(coverage.length).toBe(1);
-        expect(coverage[0].rawScriptCoverage).toBeTruthy();
+        expect(coverage[0]!.rawScriptCoverage).toBeTruthy();
       });
     });
     // @see https://crbug.com/990945
@@ -205,10 +211,10 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/csscoverage/simple.html');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toContain('/csscoverage/simple.html');
-      expect(coverage[0].ranges).toEqual([{ start: 1, end: 22 }]);
-      const range = coverage[0].ranges[0];
-      expect(coverage[0].text.substring(range.start, range.end)).toBe(
+      expect(coverage[0]!.url).toContain('/csscoverage/simple.html');
+      expect(coverage[0]!.ranges).toEqual([{ start: 1, end: 22 }]);
+      const range = coverage[0]!.ranges[0]!;
+      expect(coverage[0]!.text.substring(range.start, range.end)).toBe(
         'div { color: green; }'
       );
     });
@@ -219,7 +225,7 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/csscoverage/sourceurl.html');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toBe('nicename.css');
+      expect(coverage[0]!.url).toBe('nicename.css');
     });
     it('should report multiple stylesheets', async () => {
       const { page, server } = getTestState();
@@ -228,9 +234,11 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/csscoverage/multiple.html');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(2);
-      coverage.sort((a, b) => a.url.localeCompare(b.url));
-      expect(coverage[0].url).toContain('/csscoverage/stylesheet1.css');
-      expect(coverage[1].url).toContain('/csscoverage/stylesheet2.css');
+      coverage.sort((a, b) => {
+        return a.url.localeCompare(b.url);
+      });
+      expect(coverage[0]!.url).toContain('/csscoverage/stylesheet1.css');
+      expect(coverage[1]!.url).toContain('/csscoverage/stylesheet2.css');
     });
     it('should report stylesheets that have no coverage', async () => {
       const { page, server } = getTestState();
@@ -239,8 +247,8 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/csscoverage/unused.html');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toBe('unused.css');
-      expect(coverage[0].ranges.length).toBe(0);
+      expect(coverage[0]!.url).toBe('unused.css');
+      expect(coverage[0]!.ranges.length).toBe(0);
     });
     it('should work with media queries', async () => {
       const { page, server } = getTestState();
@@ -249,8 +257,8 @@ describe('Coverage specs', function () {
       await page.goto(server.PREFIX + '/csscoverage/media.html');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(1);
-      expect(coverage[0].url).toContain('/csscoverage/media.html');
-      expect(coverage[0].ranges).toEqual([{ start: 17, end: 38 }]);
+      expect(coverage[0]!.url).toContain('/csscoverage/media.html');
+      expect(coverage[0]!.ranges).toEqual([{ start: 17, end: 38 }]);
     });
     it('should work with complicated usecases', async () => {
       const { page, server } = getTestState();
@@ -268,9 +276,9 @@ describe('Coverage specs', function () {
       await page.coverage.startCSSCoverage();
       await page.addStyleTag({ content: 'body { margin: 10px;}' });
       // trigger style recalc
-      const margin = await page.evaluate(
-        () => window.getComputedStyle(document.body).margin
-      );
+      const margin = await page.evaluate(() => {
+        return window.getComputedStyle(document.body).margin;
+      });
       expect(margin).toBe('10px');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(0);
@@ -286,7 +294,9 @@ describe('Coverage specs', function () {
         link.rel = 'stylesheet';
         link.href = url;
         document.head.appendChild(link);
-        await new Promise((x) => (link.onload = x));
+        await new Promise((x) => {
+          return (link.onload = x);
+        });
       }, server.PREFIX + '/csscoverage/stylesheet1.css');
       const coverage = await page.coverage.stopCSSCoverage();
       expect(coverage.length).toBe(1);
diff --git a/test/src/defaultbrowsercontext.spec.ts b/test/src/defaultbrowsercontext.spec.ts
index b84428de0660d..e60186cdeeb20 100644
--- a/test/src/defaultbrowsercontext.spec.ts
+++ b/test/src/defaultbrowsercontext.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('DefaultBrowserContext', function () {
   setupTestBrowserHooks();
@@ -57,9 +57,11 @@ describe('DefaultBrowserContext', function () {
       name: 'username',
       value: 'John Doe',
     });
-    expect(await page.evaluate(() => document.cookie)).toBe(
-      'username=John Doe'
-    );
+    expect(
+      await page.evaluate(() => {
+        return document.cookie;
+      })
+    ).toBe('username=John Doe');
     expectCookieEquals(await page.cookies(), [
       {
         name: 'username',
diff --git a/test/src/dialog.spec.ts b/test/src/dialog.spec.ts
index 39339f1fa2f0b..e5cd34dec5ade 100644
--- a/test/src/dialog.spec.ts
+++ b/test/src/dialog.spec.ts
@@ -21,7 +21,7 @@ import {
   setupTestPageAndContextHooks,
   setupTestBrowserHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('Page.Events.Dialog', function () {
   setupTestBrowserHooks();
@@ -35,10 +35,12 @@ describe('Page.Events.Dialog', function () {
     });
     page.on('dialog', onDialog);
 
-    await page.evaluate(() => alert('yo'));
+    await page.evaluate(() => {
+      return alert('yo');
+    });
 
     expect(onDialog.callCount).toEqual(1);
-    const dialog = onDialog.firstCall.args[0];
+    const dialog = onDialog.firstCall.args[0]!;
     expect(dialog.type()).toBe('alert');
     expect(dialog.defaultValue()).toBe('');
     expect(dialog.message()).toBe('yo');
@@ -52,10 +54,12 @@ describe('Page.Events.Dialog', function () {
     });
     page.on('dialog', onDialog);
 
-    const result = await page.evaluate(() => prompt('question?', 'yes.'));
+    const result = await page.evaluate(() => {
+      return prompt('question?', 'yes.');
+    });
 
     expect(onDialog.callCount).toEqual(1);
-    const dialog = onDialog.firstCall.args[0];
+    const dialog = onDialog.firstCall.args[0]!;
     expect(dialog.type()).toBe('prompt');
     expect(dialog.defaultValue()).toBe('yes.');
     expect(dialog.message()).toBe('question?');
@@ -68,7 +72,9 @@ describe('Page.Events.Dialog', function () {
     page.on('dialog', (dialog) => {
       dialog.dismiss();
     });
-    const result = await page.evaluate(() => prompt('question?'));
+    const result = await page.evaluate(() => {
+      return prompt('question?');
+    });
     expect(result).toBe(null);
   });
 });
diff --git a/test/src/drag-and-drop.spec.ts b/test/src/drag-and-drop.spec.ts
index 01f2257f43c33..d4178ba1f6573 100644
--- a/test/src/drag-and-drop.spec.ts
+++ b/test/src/drag-and-drop.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestPageAndContextHooks,
   setupTestBrowserHooks,
   describeChromeOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describeChromeOnly('Input.drag', function () {
   setupTestBrowserHooks();
@@ -29,12 +29,14 @@ describeChromeOnly('Input.drag', function () {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/drag-and-drop.html');
-    const draggable = await page.$('#drag');
+    const draggable = (await page.$('#drag'))!;
 
     try {
-      await draggable.drag({ x: 1, y: 1 });
+      await draggable!.drag({ x: 1, y: 1 });
     } catch (error) {
-      expect(error.message).toContain('Drag Interception is not enabled!');
+      expect((error as Error).message).toContain(
+        'Drag Interception is not enabled!'
+      );
     }
   });
   it('should emit a dragIntercepted event when dragged', async () => {
@@ -44,11 +46,15 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
+    const draggable = (await page.$('#drag'))!;
     const data = await draggable.drag({ x: 1, y: 1 });
 
     expect(data.items.length).toBe(1);
-    expect(await page.evaluate(() => globalThis.didDragStart)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragStart;
+      })
+    ).toBe(true);
   });
   it('should emit a dragEnter', async () => {
     const { page, server } = getTestState();
@@ -57,13 +63,21 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
+    const draggable = (await page.$('#drag'))!;
     const data = await draggable.drag({ x: 1, y: 1 });
-    const dropzone = await page.$('#drop');
+    const dropzone = (await page.$('#drop'))!;
     await dropzone.dragEnter(data);
 
-    expect(await page.evaluate(() => globalThis.didDragStart)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragEnter)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragStart;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragEnter;
+      })
+    ).toBe(true);
   });
   it('should emit a dragOver event', async () => {
     const { page, server } = getTestState();
@@ -72,15 +86,27 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
+    const draggable = (await page.$('#drag'))!;
     const data = await draggable.drag({ x: 1, y: 1 });
-    const dropzone = await page.$('#drop');
+    const dropzone = (await page.$('#drop'))!;
     await dropzone.dragEnter(data);
     await dropzone.dragOver(data);
 
-    expect(await page.evaluate(() => globalThis.didDragStart)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragEnter)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragOver)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragStart;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragEnter;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragOver;
+      })
+    ).toBe(true);
   });
   it('can be dropped', async () => {
     const { page, server } = getTestState();
@@ -89,17 +115,33 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
-    const dropzone = await page.$('#drop');
+    const draggable = (await page.$('#drag'))!;
+    const dropzone = (await page.$('#drop'))!;
     const data = await draggable.drag({ x: 1, y: 1 });
     await dropzone.dragEnter(data);
     await dropzone.dragOver(data);
     await dropzone.drop(data);
 
-    expect(await page.evaluate(() => globalThis.didDragStart)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragEnter)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragOver)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDrop)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragStart;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragEnter;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragOver;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDrop;
+      })
+    ).toBe(true);
   });
   it('can be dragged and dropped with a single function', async () => {
     const { page, server } = getTestState();
@@ -108,14 +150,30 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
-    const dropzone = await page.$('#drop');
+    const draggable = (await page.$('#drag'))!;
+    const dropzone = (await page.$('#drop'))!;
     await draggable.dragAndDrop(dropzone);
 
-    expect(await page.evaluate(() => globalThis.didDragStart)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragEnter)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDragOver)).toBe(true);
-    expect(await page.evaluate(() => globalThis.didDrop)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragStart;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragEnter;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDragOver;
+      })
+    ).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).didDrop;
+      })
+    ).toBe(true);
   });
   it('can be disabled', async () => {
     const { page, server } = getTestState();
@@ -124,14 +182,16 @@ describeChromeOnly('Input.drag', function () {
     expect(page.isDragInterceptionEnabled()).toBe(false);
     await page.setDragInterception(true);
     expect(page.isDragInterceptionEnabled()).toBe(true);
-    const draggable = await page.$('#drag');
+    const draggable = (await page.$('#drag'))!;
     await draggable.drag({ x: 1, y: 1 });
     await page.setDragInterception(false);
 
     try {
       await draggable.drag({ x: 1, y: 1 });
     } catch (error) {
-      expect(error.message).toContain('Drag Interception is not enabled!');
+      expect((error as Error).message).toContain(
+        'Drag Interception is not enabled!'
+      );
     }
   });
 });
diff --git a/test/src/elementhandle.spec.ts b/test/src/elementhandle.spec.ts
index 8e8c3c4b81d5b..917d0d580a17b 100644
--- a/test/src/elementhandle.spec.ts
+++ b/test/src/elementhandle.spec.ts
@@ -22,7 +22,7 @@ import {
   setupTestPageAndContextHooks,
   describeFailsFirefox,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 import utils from './utils.js';
 import { ElementHandle } from '../../lib/cjs/puppeteer/common/JSHandle.js';
@@ -37,7 +37,7 @@ describe('ElementHandle specs', function () {
 
       await page.setViewport({ width: 500, height: 500 });
       await page.goto(server.PREFIX + '/grid.html');
-      const elementHandle = await page.$('.box:nth-of-type(13)');
+      const elementHandle = (await page.$('.box:nth-of-type(13)'))!;
       const box = await elementHandle.boundingBox();
       expect(box).toEqual({ x: 100, y: 50, width: 50, height: 50 });
     });
@@ -46,8 +46,8 @@ describe('ElementHandle specs', function () {
 
       await page.setViewport({ width: 500, height: 500 });
       await page.goto(server.PREFIX + '/frames/nested-frames.html');
-      const nestedFrame = page.frames()[1].childFrames()[1];
-      const elementHandle = await nestedFrame.$('div');
+      const nestedFrame = page.frames()[1]!.childFrames()[1]!;
+      const elementHandle = (await nestedFrame.$('div'))!;
       const box = await elementHandle.boundingBox();
       if (isChrome) {
         expect(box).toEqual({ x: 28, y: 182, width: 264, height: 18 });
@@ -59,7 +59,7 @@ describe('ElementHandle specs', function () {
       const { page } = getTestState();
 
       await page.setContent('<div style="display:none">hi</div>');
-      const element = await page.$('div');
+      const element = (await page.$('div'))!;
       expect(await element.boundingBox()).toBe(null);
     });
     it('should force a layout', async () => {
@@ -69,11 +69,10 @@ describe('ElementHandle specs', function () {
       await page.setContent(
         '<div style="width: 100px; height: 100px">hello</div>'
       );
-      const elementHandle = await page.$('div');
-      await page.evaluate<(element: HTMLElement) => void>(
-        (element) => (element.style.height = '200px'),
-        elementHandle
-      );
+      const elementHandle = (await page.$('div'))!;
+      await page.evaluate((element: HTMLElement) => {
+        return (element.style.height = '200px');
+      }, elementHandle);
       const box = await elementHandle.boundingBox();
       expect(box).toEqual({ x: 8, y: 8, width: 100, height: 200 });
     });
@@ -85,7 +84,7 @@ describe('ElementHandle specs', function () {
           <rect id="theRect" x="30" y="50" width="200" height="300"></rect>
         </svg>
       `);
-      const element = await page.$('#therect');
+      const element = (await page.$('#therect'))!;
       const pptrBoundingBox = await element.boundingBox();
       const webBoundingBox = await page.evaluate((e: HTMLElement) => {
         const rect = e.getBoundingClientRect();
@@ -104,14 +103,14 @@ describe('ElementHandle specs', function () {
       // Step 1: Add Frame and position it absolutely.
       await utils.attachFrame(page, 'frame1', server.PREFIX + '/resetcss.html');
       await page.evaluate(() => {
-        const frame = document.querySelector<HTMLElement>('#frame1');
+        const frame = document.querySelector<HTMLElement>('#frame1')!;
         frame.style.position = 'absolute';
         frame.style.left = '1px';
         frame.style.top = '2px';
       });
 
       // Step 2: Add div and position it absolutely inside frame.
-      const frame = page.frames()[1];
+      const frame = page.frames()[1]!;
       const divHandle = (
         await frame.evaluateHandle(() => {
           const div = document.createElement('div');
@@ -127,25 +126,25 @@ describe('ElementHandle specs', function () {
           div.style.height = '7px';
           return div;
         })
-      ).asElement();
+      ).asElement()!;
 
       // Step 3: query div's boxModel and assert box values.
-      const box = await divHandle.boxModel();
+      const box = (await divHandle.boxModel())!;
       expect(box.width).toBe(6);
       expect(box.height).toBe(7);
-      expect(box.margin[0]).toEqual({
+      expect(box.margin[0]!).toEqual({
         x: 1 + 4, // frame.left + div.left
         y: 2 + 5,
       });
-      expect(box.border[0]).toEqual({
+      expect(box.border[0]!).toEqual({
         x: 1 + 4 + 3, // frame.left + div.left + div.margin-left
         y: 2 + 5,
       });
-      expect(box.padding[0]).toEqual({
+      expect(box.padding[0]!).toEqual({
         x: 1 + 4 + 3 + 1, // frame.left + div.left + div.marginLeft + div.borderLeft
         y: 2 + 5,
       });
-      expect(box.content[0]).toEqual({
+      expect(box.content[0]!).toEqual({
         x: 1 + 4 + 3 + 1 + 2, // frame.left + div.left + div.marginLeft + div.borderLeft + dif.paddingLeft
         y: 2 + 5,
       });
@@ -155,7 +154,7 @@ describe('ElementHandle specs', function () {
       const { page } = getTestState();
 
       await page.setContent('<div style="display:none">hi</div>');
-      const element = await page.$('div');
+      const element = (await page.$('div'))!;
       expect(await element.boxModel()).toBe(null);
     });
   });
@@ -166,9 +165,9 @@ describe('ElementHandle specs', function () {
 
       await page.goto(server.EMPTY_PAGE);
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      const elementHandle = await page.$('#frame1');
+      const elementHandle = (await page.$('#frame1'))!;
       const frame = await elementHandle.contentFrame();
-      expect(frame).toBe(page.frames()[1]);
+      expect(frame).toBe(page.frames()[1]!);
     });
   });
 
@@ -178,57 +177,68 @@ describe('ElementHandle specs', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      const button = await page.$('button');
+      const button = (await page.$('button'))!;
       await button.click();
-      expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result;
+        })
+      ).toBe('Clicked');
     });
     it('should work for Shadow DOM v1', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/shadow.html');
-      const buttonHandle = await page.evaluateHandle<ElementHandle>(
+      const buttonHandle = await page.evaluateHandle<ElementHandle>(() => {
         // @ts-expect-error button is expected to be in the page's scope.
-        () => button
-      );
+        return button;
+      });
       await buttonHandle.click();
       expect(
-        await page.evaluate(
+        await page.evaluate(() => {
           // @ts-expect-error clicked is expected to be in the page's scope.
-          () => clicked
-        )
+          return clicked;
+        })
       ).toBe(true);
     });
     it('should work for TextNodes', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      const buttonTextNode = await page.evaluateHandle<ElementHandle>(
-        () => document.querySelector('button').firstChild
-      );
-      let error = null;
-      await buttonTextNode.click().catch((error_) => (error = error_));
+      const buttonTextNode = await page.evaluateHandle<ElementHandle>(() => {
+        return document.querySelector('button')!.firstChild;
+      });
+      let error!: Error;
+      await buttonTextNode.click().catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Node is not of type HTMLElement');
     });
     it('should throw for detached nodes', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      const button = await page.$('button');
-      await page.evaluate((button: HTMLElement) => button.remove(), button);
-      let error = null;
-      await button.click().catch((error_) => (error = error_));
+      const button = (await page.$('button'))!;
+      await page.evaluate((button: HTMLElement) => {
+        return button.remove();
+      }, button);
+      let error!: Error;
+      await button.click().catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Node is detached from document');
     });
     it('should throw for hidden nodes', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      const button = await page.$('button');
-      await page.evaluate(
-        (button: HTMLElement) => (button.style.display = 'none'),
-        button
-      );
-      const error = await button.click().catch((error_) => error_);
+      const button = (await page.$('button'))!;
+      await page.evaluate((button: HTMLElement) => {
+        return (button.style.display = 'none');
+      }, button);
+      const error = await button.click().catch((error_) => {
+        return error_;
+      });
       expect(error.message).toBe(
         'Node is either not clickable or not an HTMLElement'
       );
@@ -237,12 +247,13 @@ describe('ElementHandle specs', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/button.html');
-      const button = await page.$('button');
-      await page.evaluate(
-        (button: HTMLElement) => (button.parentElement.style.display = 'none'),
-        button
-      );
-      const error = await button.click().catch((error_) => error_);
+      const button = (await page.$('button'))!;
+      await page.evaluate((button: HTMLElement) => {
+        return (button.parentElement!.style.display = 'none');
+      }, button);
+      const error = await button.click().catch((error_) => {
+        return error_;
+      });
       expect(error.message).toBe(
         'Node is either not clickable or not an HTMLElement'
       );
@@ -251,8 +262,10 @@ describe('ElementHandle specs', function () {
       const { page } = getTestState();
 
       await page.setContent('hello<br>goodbye');
-      const br = await page.$('br');
-      const error = await br.click().catch((error_) => error_);
+      const br = (await page.$('br'))!;
+      const error = await br.click().catch((error_) => {
+        return error_;
+      });
       expect(error.message).toBe(
         'Node is either not clickable or not an HTMLElement'
       );
@@ -267,17 +280,19 @@ describe('ElementHandle specs', function () {
       await page.setContent(
         '<div id="not-foo"></div><div class="bar">bar2</div><div class="foo">Foo1</div>'
       );
-      let element = await waitFor;
+      let element = (await waitFor)!;
       expect(element).toBeDefined();
 
       const innerWaitFor = element.waitForSelector('.bar');
       await element.evaluate((el) => {
         el.innerHTML = '<div class="bar">bar1</div>';
       });
-      element = await innerWaitFor;
+      element = (await innerWaitFor)!;
       expect(element).toBeDefined();
       expect(
-        await element.evaluate((el: HTMLElement) => el.innerText)
+        await element.evaluate((el) => {
+          return (el as HTMLElement).innerText;
+        })
       ).toStrictEqual('bar1');
     });
   });
@@ -298,14 +313,18 @@ describe('ElementHandle specs', function () {
         </div>`
       );
 
-      const el2 = await page.waitForSelector('#el1');
+      const el2 = (await page.waitForSelector('#el1'))!;
 
       expect(
-        await (await el2.waitForXPath('//div')).evaluate((el) => el.id)
+        await (await el2.waitForXPath('//div'))!.evaluate((el) => {
+          return el.id;
+        })
       ).toStrictEqual('el2');
 
       expect(
-        await (await el2.waitForXPath('.//div')).evaluate((el) => el.id)
+        await (await el2.waitForXPath('.//div'))!.evaluate((el) => {
+          return el.id;
+        })
       ).toStrictEqual('el2');
     });
   });
@@ -315,10 +334,12 @@ describe('ElementHandle specs', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/scrollable.html');
-      const button = await page.$('#button-6');
+      const button = (await page.$('#button-6'))!;
       await button.hover();
       expect(
-        await page.evaluate(() => document.querySelector('button:hover').id)
+        await page.evaluate(() => {
+          return document.querySelector('button:hover')!.id;
+        })
       ).toBe('button-6');
     });
   });
@@ -329,7 +350,7 @@ describe('ElementHandle specs', function () {
 
       await page.goto(server.PREFIX + '/offscreenbuttons.html');
       for (let i = 0; i < 11; ++i) {
-        const button = await page.$('#btn' + i);
+        const button = (await page.$('#btn' + i))!;
         // All but last button are visible.
         const visible = i < 10;
         expect(await button.isIntersectingViewport()).toBe(visible);
@@ -341,7 +362,7 @@ describe('ElementHandle specs', function () {
       await page.goto(server.PREFIX + '/offscreenbuttons.html');
       // a button almost cannot be seen
       // sometimes we expect to return false by isIntersectingViewport1
-      const button = await page.$('#btn11');
+      const button = (await page.$('#btn11'))!;
       expect(
         await button.isIntersectingViewport({
           threshold: 0.001,
@@ -354,7 +375,7 @@ describe('ElementHandle specs', function () {
       await page.goto(server.PREFIX + '/offscreenbuttons.html');
       // a button almost cannot be seen
       // sometimes we expect to return false by isIntersectingViewport1
-      const button = await page.$('#btn0');
+      const button = (await page.$('#btn0'))!;
       expect(
         await button.isIntersectingViewport({
           threshold: 1,
@@ -374,15 +395,15 @@ describe('ElementHandle specs', function () {
 
       // Register.
       puppeteer.registerCustomQueryHandler('getById', {
-        queryOne: (element, selector) =>
-          document.querySelector(`[id="${selector}"]`),
+        queryOne: (_element, selector) => {
+          return document.querySelector(`[id="${selector}"]`);
+        },
       });
-      const element = await page.$('getById/foo');
+      const element = (await page.$('getById/foo'))!;
       expect(
-        await page.evaluate<(element: HTMLElement) => string>(
-          (element) => element.id,
-          element
-        )
+        await page.evaluate<(element: HTMLElement) => string>((element) => {
+          return element.id;
+        }, element)
       ).toBe('foo');
       const handlerNamesAfterRegistering = puppeteer.customQueryHandlerNames();
       expect(handlerNamesAfterRegistering.includes('getById')).toBeTruthy();
@@ -407,7 +428,9 @@ describe('ElementHandle specs', function () {
       try {
         const { puppeteer } = getTestState();
         puppeteer.registerCustomQueryHandler('1/2/3', {
-          queryOne: () => document.querySelector('foo'),
+          queryOne: () => {
+            return document.querySelector('foo');
+          },
         });
         throw new Error(
           'Custom query handler name was invalid - throw expected'
@@ -424,18 +447,20 @@ describe('ElementHandle specs', function () {
         '<div id="not-foo"></div><div class="foo">Foo1</div><div class="foo baz">Foo2</div>'
       );
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryAll: (element, selector) =>
-          document.querySelectorAll(`.${selector}`),
+        queryAll: (_element, selector) => {
+          return document.querySelectorAll(`.${selector}`);
+        },
       });
       const elements = await page.$$('getByClass/foo');
       const classNames = await Promise.all(
-        elements.map(
-          async (element) =>
-            await page.evaluate<(element: HTMLElement) => string>(
-              (element) => element.className,
-              element
-            )
-        )
+        elements.map(async (element) => {
+          return await page.evaluate<(element: HTMLElement) => string>(
+            (element) => {
+              return element.className;
+            },
+            element
+          );
+        })
       );
 
       expect(classNames).toStrictEqual(['foo', 'foo baz']);
@@ -446,20 +471,22 @@ describe('ElementHandle specs', function () {
         '<div id="not-foo"></div><div class="foo">Foo1</div><div class="foo baz">Foo2</div>'
       );
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryAll: (element, selector) =>
-          document.querySelectorAll(`.${selector}`),
+        queryAll: (_element, selector) => {
+          return document.querySelectorAll(`.${selector}`);
+        },
+      });
+      const elements = await page.$$eval('getByClass/foo', (divs) => {
+        return divs.length;
       });
-      const elements = await page.$$eval(
-        'getByClass/foo',
-        (divs) => divs.length
-      );
 
       expect(elements).toBe(2);
     });
     it('should wait correctly with waitForSelector', async () => {
       const { page, puppeteer } = getTestState();
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryOne: (element, selector) => element.querySelector(`.${selector}`),
+        queryOne: (element, selector) => {
+          return element.querySelector(`.${selector}`);
+        },
       });
       const waitFor = page.waitForSelector('getByClass/foo');
 
@@ -475,7 +502,9 @@ describe('ElementHandle specs', function () {
     it('should wait correctly with waitForSelector on an element', async () => {
       const { page, puppeteer } = getTestState();
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryOne: (element, selector) => element.querySelector(`.${selector}`),
+        queryOne: (element, selector) => {
+          return element.querySelector(`.${selector}`);
+        },
       });
       const waitFor = page.waitForSelector('getByClass/foo');
 
@@ -483,7 +512,7 @@ describe('ElementHandle specs', function () {
       await page.setContent(
         '<div id="not-foo"></div><div class="bar">bar2</div><div class="foo">Foo1</div>'
       );
-      let element = await waitFor;
+      let element = (await waitFor)!;
       expect(element).toBeDefined();
 
       const innerWaitFor = element.waitForSelector('getByClass/bar');
@@ -492,10 +521,12 @@ describe('ElementHandle specs', function () {
         el.innerHTML = '<div class="bar">bar1</div>';
       });
 
-      element = await innerWaitFor;
+      element = (await innerWaitFor)!;
       expect(element).toBeDefined();
       expect(
-        await element.evaluate((el: HTMLElement) => el.innerText)
+        await element.evaluate((el) => {
+          return (el as HTMLElement).innerText;
+        })
       ).toStrictEqual('bar1');
     });
 
@@ -504,7 +535,9 @@ describe('ElementHandle specs', function () {
       sinon.stub(console, 'warn').callsFake(() => {});
       const { page, puppeteer } = getTestState();
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryOne: (element, selector) => element.querySelector(`.${selector}`),
+        queryOne: (element, selector) => {
+          return element.querySelector(`.${selector}`);
+        },
       });
       const waitFor = page.waitFor('getByClass/foo');
 
@@ -522,12 +555,15 @@ describe('ElementHandle specs', function () {
         '<div id="not-foo"></div><div class="foo"><div id="nested-foo" class="foo"/></div><div class="foo baz">Foo2</div>'
       );
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryOne: (element, selector) => element.querySelector(`.${selector}`),
-        queryAll: (element, selector) =>
-          element.querySelectorAll(`.${selector}`),
+        queryOne: (element, selector) => {
+          return element.querySelector(`.${selector}`);
+        },
+        queryAll: (element, selector) => {
+          return element.querySelectorAll(`.${selector}`);
+        },
       });
 
-      const element = await page.$('getByClass/foo');
+      const element = (await page.$('getByClass/foo'))!;
       expect(element).toBeDefined();
 
       const elements = await page.$$('getByClass/foo');
@@ -539,20 +575,26 @@ describe('ElementHandle specs', function () {
         '<div id="not-foo"></div><div class="foo">text</div><div class="foo baz">content</div>'
       );
       puppeteer.registerCustomQueryHandler('getByClass', {
-        queryOne: (element, selector) => element.querySelector(`.${selector}`),
-        queryAll: (element, selector) =>
-          element.querySelectorAll(`.${selector}`),
+        queryOne: (element, selector) => {
+          return element.querySelector(`.${selector}`);
+        },
+        queryAll: (element, selector) => {
+          return element.querySelectorAll(`.${selector}`);
+        },
       });
 
-      const txtContent = await page.$eval(
-        'getByClass/foo',
-        (div) => div.textContent
-      );
+      const txtContent = await page.$eval('getByClass/foo', (div) => {
+        return div.textContent;
+      });
       expect(txtContent).toBe('text');
 
-      const txtContents = await page.$$eval('getByClass/foo', (divs) =>
-        divs.map((d) => d.textContent).join('')
-      );
+      const txtContents = await page.$$eval('getByClass/foo', (divs) => {
+        return divs
+          .map((d) => {
+            return d.textContent;
+          })
+          .join('');
+      });
       expect(txtContents).toBe('textcontent');
     });
   });
diff --git a/test/src/emulation.spec.ts b/test/src/emulation.spec.ts
index ded262a77c170..c936c49246f41 100644
--- a/test/src/emulation.spec.ts
+++ b/test/src/emulation.spec.ts
@@ -15,24 +15,25 @@
  */
 
 import expect from 'expect';
+import { Device } from '../../lib/cjs/puppeteer/common/DeviceDescriptors.js';
 import {
   getTestState,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('Emulation', () => {
   setupTestBrowserHooks();
   setupTestPageAndContextHooks();
-  let iPhone;
-  let iPhoneLandscape;
+  let iPhone!: Device;
+  let iPhoneLandscape!: Device;
 
   before(() => {
     const { puppeteer } = getTestState();
-    iPhone = puppeteer.devices['iPhone 6'];
-    iPhoneLandscape = puppeteer.devices['iPhone 6 landscape'];
+    iPhone = puppeteer.devices['iPhone 6']!;
+    iPhoneLandscape = puppeteer.devices['iPhone 6 landscape']!;
   });
 
   describe('Page.viewport', function () {
@@ -47,26 +48,52 @@ describe('Emulation', () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/mobile.html');
-      expect(await page.evaluate(() => window.innerWidth)).toBe(800);
+      expect(
+        await page.evaluate(() => {
+          return window.innerWidth;
+        })
+      ).toBe(800);
       await page.setViewport(iPhone.viewport);
-      expect(await page.evaluate(() => window.innerWidth)).toBe(375);
+      expect(
+        await page.evaluate(() => {
+          return window.innerWidth;
+        })
+      ).toBe(375);
       await page.setViewport({ width: 400, height: 300 });
-      expect(await page.evaluate(() => window.innerWidth)).toBe(400);
+      expect(
+        await page.evaluate(() => {
+          return window.innerWidth;
+        })
+      ).toBe(400);
     });
     it('should support touch emulation', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/mobile.html');
-      expect(await page.evaluate(() => 'ontouchstart' in window)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return 'ontouchstart' in window;
+        })
+      ).toBe(false);
       await page.setViewport(iPhone.viewport);
-      expect(await page.evaluate(() => 'ontouchstart' in window)).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return 'ontouchstart' in window;
+        })
+      ).toBe(true);
       expect(await page.evaluate(dispatchTouch)).toBe('Received touch');
       await page.setViewport({ width: 100, height: 100 });
-      expect(await page.evaluate(() => 'ontouchstart' in window)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return 'ontouchstart' in window;
+        })
+      ).toBe(false);
 
       function dispatchTouch() {
-        let fulfill;
-        const promise = new Promise((x) => (fulfill = x));
+        let fulfill!: (value: string) => void;
+        const promise = new Promise((x) => {
+          fulfill = x;
+        });
         window.ontouchstart = () => {
           fulfill('Received touch');
         };
@@ -81,39 +108,51 @@ describe('Emulation', () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/detect-touch.html');
-      expect(await page.evaluate(() => document.body.textContent.trim())).toBe(
-        'NO'
-      );
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent!.trim();
+        })
+      ).toBe('NO');
       await page.setViewport(iPhone.viewport);
       await page.goto(server.PREFIX + '/detect-touch.html');
-      expect(await page.evaluate(() => document.body.textContent.trim())).toBe(
-        'YES'
-      );
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent!.trim();
+        })
+      ).toBe('YES');
     });
     it('should detect touch when applying viewport with touches', async () => {
       const { page, server } = getTestState();
 
       await page.setViewport({ width: 800, height: 600, hasTouch: true });
       await page.addScriptTag({ url: server.PREFIX + '/modernizr.js' });
-      expect(await page.evaluate(() => globalThis.Modernizr.touchevents)).toBe(
-        true
-      );
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).Modernizr.touchevents;
+        })
+      ).toBe(true);
     });
     itFailsFirefox('should support landscape emulation', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/mobile.html');
-      expect(await page.evaluate(() => screen.orientation.type)).toBe(
-        'portrait-primary'
-      );
+      expect(
+        await page.evaluate(() => {
+          return screen.orientation.type;
+        })
+      ).toBe('portrait-primary');
       await page.setViewport(iPhoneLandscape.viewport);
-      expect(await page.evaluate(() => screen.orientation.type)).toBe(
-        'landscape-primary'
-      );
+      expect(
+        await page.evaluate(() => {
+          return screen.orientation.type;
+        })
+      ).toBe('landscape-primary');
       await page.setViewport({ width: 100, height: 100 });
-      expect(await page.evaluate(() => screen.orientation.type)).toBe(
-        'portrait-primary'
-      );
+      expect(
+        await page.evaluate(() => {
+          return screen.orientation.type;
+        })
+      ).toBe('portrait-primary');
     });
   });
 
@@ -123,23 +162,32 @@ describe('Emulation', () => {
 
       await page.goto(server.PREFIX + '/mobile.html');
       await page.emulate(iPhone);
-      expect(await page.evaluate(() => window.innerWidth)).toBe(375);
-      expect(await page.evaluate(() => navigator.userAgent)).toContain(
-        'iPhone'
-      );
+      expect(
+        await page.evaluate(() => {
+          return window.innerWidth;
+        })
+      ).toBe(375);
+      expect(
+        await page.evaluate(() => {
+          return navigator.userAgent;
+        })
+      ).toContain('iPhone');
     });
     itFailsFirefox('should support clicking', async () => {
       const { page, server } = getTestState();
 
       await page.emulate(iPhone);
       await page.goto(server.PREFIX + '/input/button.html');
-      const button = await page.$('button');
-      await page.evaluate(
-        (button: HTMLElement) => (button.style.marginTop = '200px'),
-        button
-      );
+      const button = (await page.$('button'))!;
+      await page.evaluate((button: HTMLElement) => {
+        return (button.style.marginTop = '200px');
+      }, button);
       await button.click();
-      expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result;
+        })
+      ).toBe('Clicked');
     });
   });
 
@@ -147,30 +195,46 @@ describe('Emulation', () => {
     itFailsFirefox('should work', async () => {
       const { page } = getTestState();
 
-      expect(await page.evaluate(() => matchMedia('screen').matches)).toBe(
-        true
-      );
-      expect(await page.evaluate(() => matchMedia('print').matches)).toBe(
-        false
-      );
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('screen').matches;
+        })
+      ).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('print').matches;
+        })
+      ).toBe(false);
       await page.emulateMediaType('print');
-      expect(await page.evaluate(() => matchMedia('screen').matches)).toBe(
-        false
-      );
-      expect(await page.evaluate(() => matchMedia('print').matches)).toBe(true);
-      await page.emulateMediaType(null);
-      expect(await page.evaluate(() => matchMedia('screen').matches)).toBe(
-        true
-      );
-      expect(await page.evaluate(() => matchMedia('print').matches)).toBe(
-        false
-      );
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('screen').matches;
+        })
+      ).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('print').matches;
+        })
+      ).toBe(true);
+      await page.emulateMediaType();
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('screen').matches;
+        })
+      ).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return matchMedia('print').matches;
+        })
+      ).toBe(false);
     });
     it('should throw in case of bad argument', async () => {
       const { page } = getTestState();
 
-      let error = null;
-      await page.emulateMediaType('bad').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.emulateMediaType('bad').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Unsupported media type: bad');
     });
   });
@@ -183,105 +247,125 @@ describe('Emulation', () => {
         { name: 'prefers-reduced-motion', value: 'reduce' },
       ]);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-reduced-motion: reduce)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-reduced-motion: reduce)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-reduced-motion: no-preference)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-reduced-motion: no-preference)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([
         { name: 'prefers-color-scheme', value: 'light' },
       ]);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: light)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: light)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: dark)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: dark)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([
         { name: 'prefers-color-scheme', value: 'dark' },
       ]);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: dark)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: dark)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: light)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: light)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([
         { name: 'prefers-reduced-motion', value: 'reduce' },
         { name: 'prefers-color-scheme', value: 'light' },
       ]);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-reduced-motion: reduce)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-reduced-motion: reduce)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-reduced-motion: no-preference)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-reduced-motion: no-preference)').matches;
+        })
       ).toBe(false);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: light)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: light)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(
-          () => matchMedia('(prefers-color-scheme: dark)').matches
-        )
+        await page.evaluate(() => {
+          return matchMedia('(prefers-color-scheme: dark)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([{ name: 'color-gamut', value: 'srgb' }]);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: p3)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: p3)').matches;
+        })
       ).toBe(false);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: srgb)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: srgb)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: rec2020)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: rec2020)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([{ name: 'color-gamut', value: 'p3' }]);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: p3)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: p3)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: srgb)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: srgb)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: rec2020)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: rec2020)').matches;
+        })
       ).toBe(false);
       await page.emulateMediaFeatures([
         { name: 'color-gamut', value: 'rec2020' },
       ]);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: p3)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: p3)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: srgb)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: srgb)').matches;
+        })
       ).toBe(true);
       expect(
-        await page.evaluate(() => matchMedia('(color-gamut: rec2020)').matches)
+        await page.evaluate(() => {
+          return matchMedia('(color-gamut: rec2020)').matches;
+        })
       ).toBe(true);
     });
     it('should throw in case of bad argument', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .emulateMediaFeatures([{ name: 'bad', value: '' }])
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toBe('Unsupported media feature: bad');
     });
   });
@@ -291,25 +375,37 @@ describe('Emulation', () => {
       const { page } = getTestState();
 
       await page.evaluate(() => {
-        globalThis.date = new Date(1479579154987);
+        (globalThis as any).date = new Date(1479579154987);
       });
       await page.emulateTimezone('America/Jamaica');
-      expect(await page.evaluate(() => globalThis.date.toString())).toBe(
-        'Sat Nov 19 2016 13:12:34 GMT-0500 (Eastern Standard Time)'
-      );
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).date.toString();
+        })
+      ).toBe('Sat Nov 19 2016 13:12:34 GMT-0500 (Eastern Standard Time)');
 
       await page.emulateTimezone('Pacific/Honolulu');
-      expect(await page.evaluate(() => globalThis.date.toString())).toBe(
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).date.toString();
+        })
+      ).toBe(
         'Sat Nov 19 2016 08:12:34 GMT-1000 (Hawaii-Aleutian Standard Time)'
       );
 
       await page.emulateTimezone('America/Buenos_Aires');
-      expect(await page.evaluate(() => globalThis.date.toString())).toBe(
-        'Sat Nov 19 2016 15:12:34 GMT-0300 (Argentina Standard Time)'
-      );
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).date.toString();
+        })
+      ).toBe('Sat Nov 19 2016 15:12:34 GMT-0300 (Argentina Standard Time)');
 
       await page.emulateTimezone('Europe/Berlin');
-      expect(await page.evaluate(() => globalThis.date.toString())).toBe(
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).date.toString();
+        })
+      ).toBe(
         'Sat Nov 19 2016 19:12:34 GMT+0100 (Central European Standard Time)'
       );
     });
@@ -317,10 +413,14 @@ describe('Emulation', () => {
     it('should throw for invalid timezone IDs', async () => {
       const { page } = getTestState();
 
-      let error = null;
-      await page.emulateTimezone('Foo/Bar').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.emulateTimezone('Foo/Bar').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Invalid timezone ID: Foo/Bar');
-      await page.emulateTimezone('Baz/Qux').catch((error_) => (error = error_));
+      await page.emulateTimezone('Baz/Qux').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Invalid timezone ID: Baz/Qux');
     });
   });
@@ -378,11 +478,13 @@ describe('Emulation', () => {
     it('should throw for invalid vision deficiencies', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         // @ts-expect-error deliberately passign invalid deficiency
         .emulateVisionDeficiency('invalid')
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toBe('Unsupported vision deficiency: invalid');
     });
   });
@@ -391,8 +493,8 @@ describe('Emulation', () => {
     it('should change navigator.connection.effectiveType', async () => {
       const { page, puppeteer } = getTestState();
 
-      const slow3G = puppeteer.networkConditions['Slow 3G'];
-      const fast3G = puppeteer.networkConditions['Fast 3G'];
+      const slow3G = puppeteer.networkConditions['Slow 3G']!;
+      const fast3G = puppeteer.networkConditions['Fast 3G']!;
 
       expect(
         await page.evaluate('window.navigator.connection.effectiveType')
diff --git a/test/src/evaluation.spec.ts b/test/src/evaluation.spec.ts
index 0935e7c46b274..bb6e3c96e76b0 100644
--- a/test/src/evaluation.spec.ts
+++ b/test/src/evaluation.spec.ts
@@ -22,7 +22,7 @@ import {
   setupTestPageAndContextHooks,
   itFailsFirefox,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 const bigint = typeof BigInt !== 'undefined';
 
@@ -34,55 +34,79 @@ describe('Evaluation specs', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => 7 * 3);
+      const result = await page.evaluate(() => {
+        return 7 * 3;
+      });
       expect(result).toBe(21);
     });
     (bigint ? it : xit)('should transfer BigInt', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a: bigint) => a, BigInt(42));
+      const result = await page.evaluate((a: bigint) => {
+        return a;
+      }, BigInt(42));
       expect(result).toBe(BigInt(42));
     });
     it('should transfer NaN', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a, NaN);
+      const result = await page.evaluate((a) => {
+        return a;
+      }, NaN);
       expect(Object.is(result, NaN)).toBe(true);
     });
     it('should transfer -0', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a, -0);
+      const result = await page.evaluate((a) => {
+        return a;
+      }, -0);
       expect(Object.is(result, -0)).toBe(true);
     });
     it('should transfer Infinity', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a, Infinity);
+      const result = await page.evaluate((a) => {
+        return a;
+      }, Infinity);
       expect(Object.is(result, Infinity)).toBe(true);
     });
     it('should transfer -Infinity', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a, -Infinity);
+      const result = await page.evaluate((a) => {
+        return a;
+      }, -Infinity);
       expect(Object.is(result, -Infinity)).toBe(true);
     });
     it('should transfer arrays', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a, [1, 2, 3]);
+      const result = await page.evaluate(
+        (a) => {
+          return a;
+        },
+        [1, 2, 3]
+      );
       expect(result).toEqual([1, 2, 3]);
     });
     it('should transfer arrays as arrays, not objects', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => Array.isArray(a), [1, 2, 3]);
+      const result = await page.evaluate(
+        (a) => {
+          return Array.isArray(a);
+        },
+        [1, 2, 3]
+      );
       expect(result).toBe(true);
     });
     it('should modify global environment', async () => {
       const { page } = getTestState();
 
-      await page.evaluate(() => (globalThis.globalVar = 123));
+      await page.evaluate(() => {
+        return ((globalThis as any).globalVar = 123);
+      });
       expect(await page.evaluate('globalVar')).toBe(123);
     });
     it('should evaluate in the page context', async () => {
@@ -96,18 +120,22 @@ describe('Evaluation specs', function () {
       async () => {
         const { page } = getTestState();
 
-        expect(await page.evaluate(() => [Symbol('foo4')])).toBe(undefined);
+        expect(
+          await page.evaluate(() => {
+            return [Symbol('foo4')];
+          })
+        ).toBe(undefined);
       }
     );
     it('should work with function shorthands', async () => {
       const { page } = getTestState();
 
       const a = {
-        sum(a, b) {
+        sum(a: number, b: number) {
           return a + b;
         },
 
-        async mult(a, b) {
+        async mult(a: number, b: number) {
           return a * b;
         },
       };
@@ -117,27 +145,36 @@ describe('Evaluation specs', function () {
     it('should work with unicode chars', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate((a) => a['中文字符'], {
-        中文字符: 42,
-      });
+      const result = await page.evaluate(
+        (a) => {
+          return a['中文字符'];
+        },
+        {
+          中文字符: 42,
+        }
+      );
       expect(result).toBe(42);
     });
     itFailsFirefox('should throw when evaluation triggers reload', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .evaluate(() => {
           location.reload();
           return new Promise(() => {});
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain('Protocol error');
     });
     it('should await promise', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => Promise.resolve(8 * 7));
+      const result = await page.evaluate(() => {
+        return Promise.resolve(8 * 7);
+      });
       expect(result).toBe(56);
     });
     it('should work right after framenavigated', async () => {
@@ -145,7 +182,9 @@ describe('Evaluation specs', function () {
 
       let frameEvaluation = null;
       page.on('framenavigated', async (frame) => {
-        frameEvaluation = frame.evaluate(() => 6 * 7);
+        frameEvaluation = frame.evaluate(() => {
+          return 6 * 7;
+        });
       });
       await page.goto(server.EMPTY_PAGE);
       expect(await frameEvaluation).toBe(42);
@@ -154,50 +193,63 @@ describe('Evaluation specs', function () {
       const { page } = getTestState();
 
       // Setup inpage callback, which calls Page.evaluate
-      await page.exposeFunction('callController', async function (a, b) {
-        return await page.evaluate<(a: number, b: number) => number>(
-          (a, b) => a * b,
-          a,
-          b
-        );
-      });
+      await page.exposeFunction(
+        'callController',
+        async function (a: number, b: number) {
+          return await page.evaluate(
+            (a: number, b: number): number => {
+              return a * b;
+            },
+            a,
+            b
+          );
+        }
+      );
       const result = await page.evaluate(async function () {
-        return await globalThis.callController(9, 3);
+        return await (globalThis as any).callController(9, 3);
       });
       expect(result).toBe(27);
     });
     it('should reject promise with exception', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
-        // @ts-expect-error we know the object doesn't exist
-        .evaluate(() => notExistingObject.property)
-        .catch((error_) => (error = error_));
+        .evaluate(() => {
+          // @ts-expect-error we know the object doesn't exist
+          return notExistingObject.property;
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
       expect(error.message).toContain('notExistingObject');
     });
     it('should support thrown strings as error messages', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .evaluate(() => {
           throw 'qwerty';
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
       expect(error.message).toContain('qwerty');
     });
     it('should support thrown numbers as error messages', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .evaluate(() => {
           throw 100500;
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
       expect(error.message).toContain('100500');
     });
@@ -205,46 +257,60 @@ describe('Evaluation specs', function () {
       const { page } = getTestState();
 
       const object = { foo: 'bar!' };
-      const result = await page.evaluate((a) => a, object);
+      const result = await page.evaluate((a) => {
+        return a;
+      }, object);
       expect(result).not.toBe(object);
       expect(result).toEqual(object);
     });
     (bigint ? it : xit)('should return BigInt', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => BigInt(42));
+      const result = await page.evaluate(() => {
+        return BigInt(42);
+      });
       expect(result).toBe(BigInt(42));
     });
     it('should return NaN', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => NaN);
+      const result = await page.evaluate(() => {
+        return NaN;
+      });
       expect(Object.is(result, NaN)).toBe(true);
     });
     it('should return -0', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => -0);
+      const result = await page.evaluate(() => {
+        return -0;
+      });
       expect(Object.is(result, -0)).toBe(true);
     });
     it('should return Infinity', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => Infinity);
+      const result = await page.evaluate(() => {
+        return Infinity;
+      });
       expect(Object.is(result, Infinity)).toBe(true);
     });
     it('should return -Infinity', async () => {
       const { page } = getTestState();
 
-      const result = await page.evaluate(() => -Infinity);
+      const result = await page.evaluate(() => {
+        return -Infinity;
+      });
       expect(Object.is(result, -Infinity)).toBe(true);
     });
-    it('should accept "undefined" as one of multiple parameters', async () => {
+    it('should accept "null" as one of multiple parameters', async () => {
       const { page } = getTestState();
 
       const result = await page.evaluate(
-        (a, b) => Object.is(a, undefined) && Object.is(b, 'foo'),
-        undefined,
+        (a, b) => {
+          return Object.is(a, null) && Object.is(b, 'foo');
+        },
+        null,
         'foo'
       );
       expect(result).toBe(true);
@@ -252,14 +318,22 @@ describe('Evaluation specs', function () {
     it('should properly serialize null fields', async () => {
       const { page } = getTestState();
 
-      expect(await page.evaluate(() => ({ a: undefined }))).toEqual({});
+      expect(
+        await page.evaluate(() => {
+          return { a: undefined };
+        })
+      ).toEqual({});
     });
     itFailsFirefox(
       'should return undefined for non-serializable objects',
       async () => {
         const { page } = getTestState();
 
-        expect(await page.evaluate(() => window)).toBe(undefined);
+        expect(
+          await page.evaluate(() => {
+            return window;
+          })
+        ).toBe(undefined);
       }
     );
     itFailsFirefox('should fail for circular object', async () => {
@@ -268,7 +342,7 @@ describe('Evaluation specs', function () {
       const result = await page.evaluate(() => {
         const a: { [x: string]: any } = {};
         const b = { a };
-        a.b = b;
+        a['b'] = b;
         return a;
       });
       expect(result).toBe(undefined);
@@ -276,15 +350,21 @@ describe('Evaluation specs', function () {
     itFailsFirefox('should be able to throw a tricky error', async () => {
       const { page } = getTestState();
 
-      const windowHandle = await page.evaluateHandle(() => window);
+      const windowHandle = await page.evaluateHandle(() => {
+        return window;
+      });
       const errorText = await windowHandle
         .jsonValue<string>()
-        .catch((error_) => error_.message);
+        .catch((error_) => {
+          return error_.message;
+        });
       const error = await page
         .evaluate<(errorText: string) => Error>((errorText) => {
           throw new Error(errorText);
         }, errorText)
-        .catch((error_) => error_);
+        .catch((error_) => {
+          return error_;
+        });
       expect(error.message).toContain(errorText);
     });
     it('should accept a string', async () => {
@@ -309,24 +389,27 @@ describe('Evaluation specs', function () {
       const { page } = getTestState();
 
       await page.setContent('<section>42</section>');
-      const element = await page.$('section');
-      const text = await page.evaluate<(e: HTMLElement) => string>(
-        (e) => e.textContent,
-        element
-      );
+      const element = (await page.$('section'))!;
+      const text = await page.evaluate((e) => {
+        return e.textContent;
+      }, element);
       expect(text).toBe('42');
     });
     it('should throw if underlying element was disposed', async () => {
       const { page } = getTestState();
 
       await page.setContent('<section>39</section>');
-      const element = await page.$('section');
+      const element = (await page.$('section'))!;
       expect(element).toBeTruthy();
       await element.dispose();
-      let error = null;
+      let error!: Error;
       await page
-        .evaluate((e: HTMLElement) => e.textContent, element)
-        .catch((error_) => (error = error_));
+        .evaluate((e: HTMLElement) => {
+          return e.textContent;
+        }, element)
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain('JSHandle is disposed');
     });
     itFailsFirefox(
@@ -335,11 +418,15 @@ describe('Evaluation specs', function () {
         const { page, server } = getTestState();
 
         await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-        const bodyHandle = await page.frames()[1].$('body');
-        let error = null;
+        const bodyHandle = await page.frames()[1]!.$('body');
+        let error!: Error;
         await page
-          .evaluate((body: HTMLElement) => body.innerHTML, bodyHandle)
-          .catch((error_) => (error = error_));
+          .evaluate((body: HTMLElement) => {
+            return body.innerHTML;
+          }, bodyHandle)
+          .catch((error_) => {
+            return (error = error_);
+          });
         expect(error).toBeTruthy();
         expect(error.message).toContain(
           'JSHandles can be evaluated only in the context they were created'
@@ -363,11 +450,17 @@ describe('Evaluation specs', function () {
 
       await Promise.all([
         page.waitForNavigation(),
-        executionContext.evaluate(() => window.location.reload()),
+        executionContext.evaluate(() => {
+          return window.location.reload();
+        }),
       ]);
       const error = await executionContext
-        .evaluate(() => null)
-        .catch((error_) => error_);
+        .evaluate(() => {
+          return null;
+        })
+        .catch((error_) => {
+          return error_;
+        });
       expect((error as Error).message).toContain('navigation');
     });
     itFailsFirefox(
@@ -386,23 +479,24 @@ describe('Evaluation specs', function () {
     it('should transfer 100Mb of data from page to node.js', async function () {
       const { page } = getTestState();
 
-      const a = await page.evaluate<() => string>(() =>
-        Array(100 * 1024 * 1024 + 1).join('a')
-      );
+      const a = await page.evaluate<() => string>(() => {
+        return Array(100 * 1024 * 1024 + 1).join('a');
+      });
       expect(a.length).toBe(100 * 1024 * 1024);
     });
     it('should throw error with detailed information on exception inside promise ', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
-        .evaluate(
-          () =>
-            new Promise(() => {
-              throw new Error('Error in promise');
-            })
-        )
-        .catch((error_) => (error = error_));
+        .evaluate(() => {
+          return new Promise(() => {
+            throw new Error('Error in promise');
+          });
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain('Error in promise');
     });
   });
@@ -412,26 +506,38 @@ describe('Evaluation specs', function () {
       const { page, server } = getTestState();
 
       await page.evaluateOnNewDocument(function () {
-        globalThis.injected = 123;
+        (globalThis as any).injected = 123;
       });
       await page.goto(server.PREFIX + '/tamperable.html');
-      expect(await page.evaluate(() => globalThis.result)).toBe(123);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result;
+        })
+      ).toBe(123);
     });
     it('should work with CSP', async () => {
       const { page, server } = getTestState();
 
       server.setCSP('/empty.html', 'script-src ' + server.PREFIX);
       await page.evaluateOnNewDocument(function () {
-        globalThis.injected = 123;
+        (globalThis as any).injected = 123;
       });
       await page.goto(server.PREFIX + '/empty.html');
-      expect(await page.evaluate(() => globalThis.injected)).toBe(123);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).injected;
+        })
+      ).toBe(123);
 
       // Make sure CSP works.
-      await page
-        .addScriptTag({ content: 'window.e = 10;' })
-        .catch((error) => void error);
-      expect(await page.evaluate(() => (window as any).e)).toBe(undefined);
+      await page.addScriptTag({ content: 'window.e = 10;' }).catch((error) => {
+        return void error;
+      });
+      expect(
+        await page.evaluate(() => {
+          return (window as any).e;
+        })
+      ).toBe(undefined);
     });
   });
 
@@ -442,10 +548,22 @@ describe('Evaluation specs', function () {
       await page.goto(server.EMPTY_PAGE);
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       expect(page.frames().length).toBe(2);
-      await page.frames()[0].evaluate(() => (globalThis.FOO = 'foo'));
-      await page.frames()[1].evaluate(() => (globalThis.FOO = 'bar'));
-      expect(await page.frames()[0].evaluate(() => globalThis.FOO)).toBe('foo');
-      expect(await page.frames()[1].evaluate(() => globalThis.FOO)).toBe('bar');
+      await page.frames()[0]!.evaluate(() => {
+        return ((globalThis as any).FOO = 'foo');
+      });
+      await page.frames()[1]!.evaluate(() => {
+        return ((globalThis as any).FOO = 'bar');
+      });
+      expect(
+        await page.frames()[0]!.evaluate(() => {
+          return (globalThis as any).FOO;
+        })
+      ).toBe('foo');
+      expect(
+        await page.frames()[1]!.evaluate(() => {
+          return (globalThis as any).FOO;
+        })
+      ).toBe('bar');
     });
     it('should have correct execution contexts', async () => {
       const { page, server } = getTestState();
@@ -453,10 +571,14 @@ describe('Evaluation specs', function () {
       await page.goto(server.PREFIX + '/frames/one-frame.html');
       expect(page.frames().length).toBe(2);
       expect(
-        await page.frames()[0].evaluate(() => document.body.textContent.trim())
+        await page.frames()[0]!.evaluate(() => {
+          return document.body.textContent!.trim();
+        })
       ).toBe('');
       expect(
-        await page.frames()[1].evaluate(() => document.body.textContent.trim())
+        await page.frames()[1]!.evaluate(() => {
+          return document.body.textContent!.trim();
+        })
       ).toBe(`Hi, I'm frame`);
     });
     it('should execute after cross-site navigation', async () => {
@@ -464,13 +586,17 @@ describe('Evaluation specs', function () {
 
       await page.goto(server.EMPTY_PAGE);
       const mainFrame = page.mainFrame();
-      expect(await mainFrame.evaluate(() => window.location.href)).toContain(
-        'localhost'
-      );
+      expect(
+        await mainFrame.evaluate(() => {
+          return window.location.href;
+        })
+      ).toContain('localhost');
       await page.goto(server.CROSS_PROCESS_PREFIX + '/empty.html');
-      expect(await mainFrame.evaluate(() => window.location.href)).toContain(
-        '127'
-      );
+      expect(
+        await mainFrame.evaluate(() => {
+          return window.location.href;
+        })
+      ).toContain('127');
     });
   });
 });
diff --git a/test/src/fixtures.spec.ts b/test/src/fixtures.spec.ts
index ec68dbe151eba..3dd75de2b215f 100644
--- a/test/src/fixtures.spec.ts
+++ b/test/src/fixtures.spec.ts
@@ -17,7 +17,7 @@
 /* eslint-disable @typescript-eslint/no-var-requires */
 
 import expect from 'expect';
-import { getTestState, itHeadlessOnly } from './mocha-utils'; // eslint-disable-line import/extensions
+import { getTestState, itHeadlessOnly } from './mocha-utils.js';
 
 import path from 'path';
 
@@ -36,8 +36,12 @@ describe('Fixtures', function () {
       puppeteerPath,
       JSON.stringify(options),
     ]);
-    res.stderr.on('data', (data) => (dumpioData += data.toString('utf8')));
-    await new Promise((resolve) => res.on('close', resolve));
+    res.stderr.on('data', (data) => {
+      return (dumpioData += data.toString('utf8'));
+    });
+    await new Promise((resolve) => {
+      return res.on('close', resolve);
+    });
     expect(dumpioData).toContain('message from dumpio');
   });
   it('should dump browser process stderr', async () => {
@@ -51,8 +55,12 @@ describe('Fixtures', function () {
       puppeteerPath,
       JSON.stringify(options),
     ]);
-    res.stderr.on('data', (data) => (dumpioData += data.toString('utf8')));
-    await new Promise((resolve) => res.on('close', resolve));
+    res.stderr.on('data', (data) => {
+      return (dumpioData += data.toString('utf8'));
+    });
+    await new Promise((resolve) => {
+      return res.on('close', resolve);
+    });
     expect(dumpioData).toContain('DevTools listening on ws://');
   });
   it('should close the browser when the node process closes', async () => {
@@ -68,10 +76,10 @@ describe('Fixtures', function () {
       puppeteerPath,
       JSON.stringify(options),
     ]);
-    let wsEndPointCallback;
-    const wsEndPointPromise = new Promise<string>(
-      (x) => (wsEndPointCallback = x)
-    );
+    let wsEndPointCallback: (value: string) => void;
+    const wsEndPointPromise = new Promise<string>((x) => {
+      return (wsEndPointCallback = x);
+    });
     let output = '';
     res.stdout.on('data', (data) => {
       output += data;
@@ -83,13 +91,17 @@ describe('Fixtures', function () {
       browserWSEndpoint: await wsEndPointPromise,
     });
     const promises = [
-      new Promise((resolve) => browser.once('disconnected', resolve)),
-      new Promise((resolve) => res.on('close', resolve)),
+      new Promise((resolve) => {
+        return browser.once('disconnected', resolve);
+      }),
+      new Promise((resolve) => {
+        return res.on('close', resolve);
+      }),
     ];
     if (process.platform === 'win32') {
       execSync(`taskkill /pid ${res.pid} /T /F`);
     } else {
-      process.kill(res.pid);
+      process.kill(res.pid!);
     }
     await Promise.all(promises);
   });
diff --git a/test/src/frame.spec.ts b/test/src/frame.spec.ts
index 18e6c448a320a..b5f82ce6a26b5 100644
--- a/test/src/frame.spec.ts
+++ b/test/src/frame.spec.ts
@@ -14,15 +14,16 @@
  * limitations under the License.
  */
 
-import utils from './utils.js';
 import expect from 'expect';
+import { CDPSession } from '../../lib/cjs/puppeteer/common/Connection.js';
+import { Frame } from '../../lib/cjs/puppeteer/common/FrameManager.js';
 import {
   getTestState,
+  itFailsFirefox,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-  itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import { CDPSession } from '../../lib/cjs/puppeteer/common/Connection.js';
+} from './mocha-utils.js';
+import utils, { dumpFrames } from './utils.js';
 
 describe('Frame specs', function () {
   setupTestBrowserHooks();
@@ -36,8 +37,8 @@ describe('Frame specs', function () {
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       expect(page.frames().length).toBe(2);
       const [frame1, frame2] = page.frames();
-      const context1 = await frame1.executionContext();
-      const context2 = await frame2.executionContext();
+      const context1 = await frame1!.executionContext();
+      const context2 = await frame2!.executionContext();
       expect(context1).toBeTruthy();
       expect(context2).toBeTruthy();
       expect(context1 !== context2).toBeTruthy();
@@ -45,12 +46,20 @@ describe('Frame specs', function () {
       expect(context2.frame()).toBe(frame2);
 
       await Promise.all([
-        context1.evaluate(() => (globalThis.a = 1)),
-        context2.evaluate(() => (globalThis.a = 2)),
+        context1.evaluate(() => {
+          return ((globalThis as any).a = 1);
+        }),
+        context2.evaluate(() => {
+          return ((globalThis as any).a = 2);
+        }),
       ]);
       const [a1, a2] = await Promise.all([
-        context1.evaluate(() => globalThis.a),
-        context2.evaluate(() => globalThis.a),
+        context1.evaluate(() => {
+          return (globalThis as any).a;
+        }),
+        context2.evaluate(() => {
+          return (globalThis as any).a;
+        }),
       ]);
       expect(a1).toBe(1);
       expect(a2).toBe(2);
@@ -63,7 +72,9 @@ describe('Frame specs', function () {
 
       await page.goto(server.EMPTY_PAGE);
       const mainFrame = page.mainFrame();
-      const windowHandle = await mainFrame.evaluateHandle(() => window);
+      const windowHandle = await mainFrame.evaluateHandle(() => {
+        return window;
+      });
       expect(windowHandle).toBeTruthy();
     });
   });
@@ -72,10 +83,20 @@ describe('Frame specs', function () {
     itFailsFirefox('should throw for detached frames', async () => {
       const { page, server } = getTestState();
 
-      const frame1 = await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
+      const frame1 = (await utils.attachFrame(
+        page,
+        'frame1',
+        server.EMPTY_PAGE
+      ))!;
       await utils.detachFrame(page, 'frame1');
-      let error = null;
-      await frame1.evaluate(() => 7 * 8).catch((error_) => (error = error_));
+      let error!: Error;
+      await frame1
+        .evaluate(() => {
+          return 7 * 8;
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain(
         'Execution context is not available in detached frame'
       );
@@ -89,7 +110,9 @@ describe('Frame specs', function () {
       // This test checks if Frame.evaluate allows a readonly array to be an argument.
       // See https://github.com/puppeteer/puppeteer/issues/6953.
       const readonlyArray: readonly string[] = ['a', 'b', 'c'];
-      await mainFrame.evaluate((arr) => arr, readonlyArray);
+      await mainFrame.evaluate((arr) => {
+        return arr;
+      }, readonlyArray);
     });
   });
 
@@ -98,7 +121,7 @@ describe('Frame specs', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/frames/nested-frames.html');
-      expect(utils.dumpFrames(page.mainFrame())).toEqual([
+      expect(dumpFrames(page.mainFrame())).toEqual([
         'http://localhost:<PORT>/frames/nested-frames.html',
         '    http://localhost:<PORT>/frames/two-frames.html (2frames)',
         '        http://localhost:<PORT>/frames/frame.html (uno)',
@@ -113,25 +136,31 @@ describe('Frame specs', function () {
 
         await page.goto(server.EMPTY_PAGE);
         // validate frameattached events
-        const attachedFrames = [];
-        page.on('frameattached', (frame) => attachedFrames.push(frame));
+        const attachedFrames: Frame[] = [];
+        page.on('frameattached', (frame) => {
+          return attachedFrames.push(frame);
+        });
         await utils.attachFrame(page, 'frame1', './assets/frame.html');
         expect(attachedFrames.length).toBe(1);
-        expect(attachedFrames[0].url()).toContain('/assets/frame.html');
+        expect(attachedFrames[0]!.url()).toContain('/assets/frame.html');
 
         // validate framenavigated events
-        const navigatedFrames = [];
-        page.on('framenavigated', (frame) => navigatedFrames.push(frame));
+        const navigatedFrames: Frame[] = [];
+        page.on('framenavigated', (frame) => {
+          return navigatedFrames.push(frame);
+        });
         await utils.navigateFrame(page, 'frame1', './empty.html');
         expect(navigatedFrames.length).toBe(1);
-        expect(navigatedFrames[0].url()).toBe(server.EMPTY_PAGE);
+        expect(navigatedFrames[0]!.url()).toBe(server.EMPTY_PAGE);
 
         // validate framedetached events
-        const detachedFrames = [];
-        page.on('framedetached', (frame) => detachedFrames.push(frame));
+        const detachedFrames: Frame[] = [];
+        page.on('framedetached', (frame) => {
+          return detachedFrames.push(frame);
+        });
         await utils.detachFrame(page, 'frame1');
         expect(detachedFrames.length).toBe(1);
-        expect(detachedFrames[0].isDetached()).toBe(true);
+        expect(detachedFrames[0]!.isDetached()).toBe(true);
       }
     );
     it('should send "framenavigated" when navigating on anchor URLs', async () => {
@@ -156,8 +185,12 @@ describe('Frame specs', function () {
       const { page, server } = getTestState();
 
       let hasEvents = false;
-      page.on('frameattached', () => (hasEvents = true));
-      page.on('framedetached', () => (hasEvents = true));
+      page.on('frameattached', () => {
+        return (hasEvents = true);
+      });
+      page.on('framedetached', () => {
+        return (hasEvents = true);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(hasEvents).toBe(false);
     });
@@ -167,9 +200,15 @@ describe('Frame specs', function () {
       let attachedFrames = [];
       let detachedFrames = [];
       let navigatedFrames = [];
-      page.on('frameattached', (frame) => attachedFrames.push(frame));
-      page.on('framedetached', (frame) => detachedFrames.push(frame));
-      page.on('framenavigated', (frame) => navigatedFrames.push(frame));
+      page.on('frameattached', (frame) => {
+        return attachedFrames.push(frame);
+      });
+      page.on('framedetached', (frame) => {
+        return detachedFrames.push(frame);
+      });
+      page.on('framenavigated', (frame) => {
+        return navigatedFrames.push(frame);
+      });
       await page.goto(server.PREFIX + '/frames/nested-frames.html');
       expect(attachedFrames.length).toBe(4);
       expect(detachedFrames.length).toBe(0);
@@ -189,9 +228,15 @@ describe('Frame specs', function () {
       let attachedFrames = [];
       let detachedFrames = [];
       let navigatedFrames = [];
-      page.on('frameattached', (frame) => attachedFrames.push(frame));
-      page.on('framedetached', (frame) => detachedFrames.push(frame));
-      page.on('framenavigated', (frame) => navigatedFrames.push(frame));
+      page.on('frameattached', (frame) => {
+        return attachedFrames.push(frame);
+      });
+      page.on('framedetached', (frame) => {
+        return detachedFrames.push(frame);
+      });
+      page.on('framenavigated', (frame) => {
+        return navigatedFrames.push(frame);
+      });
       await page.goto(server.PREFIX + '/frames/frameset.html');
       expect(attachedFrames.length).toBe(4);
       expect(detachedFrames.length).toBe(0);
@@ -212,11 +257,13 @@ describe('Frame specs', function () {
       await page.evaluate(async (url: string) => {
         const frame = document.createElement('iframe');
         frame.src = url;
-        document.body.shadowRoot.appendChild(frame);
-        await new Promise((x) => (frame.onload = x));
+        document.body.shadowRoot!.appendChild(frame);
+        await new Promise((x) => {
+          return (frame.onload = x);
+        });
       }, server.EMPTY_PAGE);
       expect(page.frames().length).toBe(2);
-      expect(page.frames()[1].url()).toBe(server.EMPTY_PAGE);
+      expect(page.frames()[1]!.url()).toBe(server.EMPTY_PAGE);
     });
     itFailsFirefox('should report frame.name()', async () => {
       const { page, server } = getTestState();
@@ -227,20 +274,22 @@ describe('Frame specs', function () {
         frame.name = 'theFrameName';
         frame.src = url;
         document.body.appendChild(frame);
-        return new Promise((x) => (frame.onload = x));
+        return new Promise((x) => {
+          return (frame.onload = x);
+        });
       }, server.EMPTY_PAGE);
-      expect(page.frames()[0].name()).toBe('');
-      expect(page.frames()[1].name()).toBe('theFrameId');
-      expect(page.frames()[2].name()).toBe('theFrameName');
+      expect(page.frames()[0]!.name()).toBe('');
+      expect(page.frames()[1]!.name()).toBe('theFrameId');
+      expect(page.frames()[2]!.name()).toBe('theFrameName');
     });
     itFailsFirefox('should report frame.parent()', async () => {
       const { page, server } = getTestState();
 
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       await utils.attachFrame(page, 'frame2', server.EMPTY_PAGE);
-      expect(page.frames()[0].parentFrame()).toBe(null);
-      expect(page.frames()[1].parentFrame()).toBe(page.mainFrame());
-      expect(page.frames()[2].parentFrame()).toBe(page.mainFrame());
+      expect(page.frames()[0]!.parentFrame()).toBe(null);
+      expect(page.frames()[1]!.parentFrame()).toBe(page.mainFrame());
+      expect(page.frames()[2]!.parentFrame()).toBe(page.mainFrame());
     });
     itFailsFirefox(
       'should report different frame instance when frame re-attaches',
@@ -253,13 +302,15 @@ describe('Frame specs', function () {
           server.EMPTY_PAGE
         );
         await page.evaluate(() => {
-          globalThis.frame = document.querySelector('#frame1');
-          globalThis.frame.remove();
+          (globalThis as any).frame = document.querySelector('#frame1');
+          (globalThis as any).frame.remove();
         });
-        expect(frame1.isDetached()).toBe(true);
+        expect(frame1!.isDetached()).toBe(true);
         const [frame2] = await Promise.all([
           utils.waitEvent(page, 'frameattached'),
-          page.evaluate(() => document.body.appendChild(globalThis.frame)),
+          page.evaluate(() => {
+            return document.body.appendChild((globalThis as any).frame);
+          }),
         ]);
         expect(frame2.isDetached()).toBe(false);
         expect(frame1).not.toBe(frame2);
@@ -271,7 +322,7 @@ describe('Frame specs', function () {
       await page.goto(server.PREFIX + '/frames/one-frame-url-fragment.html');
 
       expect(page.frames().length).toBe(2);
-      expect(page.frames()[1].url()).toBe(
+      expect(page.frames()[1]!.url()).toBe(
         server.PREFIX + '/frames/frame.html?param=value#fragment'
       );
     });
@@ -281,11 +332,11 @@ describe('Frame specs', function () {
       await page.setViewport({ width: 1000, height: 1000 });
       await page.goto(server.PREFIX + '/frames/lazy-frame.html');
 
-      expect(page.frames().map((frame) => frame._hasStartedLoading)).toEqual([
-        true,
-        true,
-        false,
-      ]);
+      expect(
+        page.frames().map((frame) => {
+          return frame._hasStartedLoading;
+        })
+      ).toEqual([true, true, false]);
     });
   });
 
diff --git a/test/src/golden-utils.js b/test/src/golden-utils.ts
similarity index 50%
rename from test/src/golden-utils.js
rename to test/src/golden-utils.ts
index e6e7384217044..1c150dc42607d 100644
--- a/test/src/golden-utils.js
+++ b/test/src/golden-utils.ts
@@ -1,4 +1,3 @@
-// @ts-nocheck
 /**
  * Copyright 2017 Google Inc. All rights reserved.
  *
@@ -14,45 +13,61 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-const path = require('path');
-const fs = require('fs');
-const Diff = require('text-diff');
-const mime = require('mime');
-const PNG = require('pngjs').PNG;
-const jpeg = require('jpeg-js');
-const pixelmatch = require('pixelmatch');
+import assert from 'assert';
+import diff from 'diff';
+import fs from 'fs';
+import jpeg from 'jpeg-js';
+import mime from 'mime';
+import path from 'path';
+import pixelmatch from 'pixelmatch';
+import { PNG } from 'pngjs';
 
-module.exports = { compare };
+interface DiffFile {
+  diff: string | Buffer;
+  ext?: string;
+}
+
+const GoldenComparators = new Map<
+  string,
+  (
+    actualBuffer: string | Buffer,
+    expectedBuffer: string | Buffer,
+    mimeType: string
+  ) => DiffFile | undefined
+>();
 
-const GoldenComparators = {
-  'image/png': compareImages,
-  'image/jpeg': compareImages,
-  'text/plain': compareText,
+const addSuffix = (
+  filePath: string,
+  suffix: string,
+  customExtension?: string
+): string => {
+  const dirname = path.dirname(filePath);
+  const ext = path.extname(filePath);
+  const name = path.basename(filePath, ext);
+  return path.join(dirname, name + suffix + (customExtension || ext));
 };
 
-/**
- * @param {?Object} actualBuffer
- * @param {!Buffer} expectedBuffer
- * @param {!string} mimeType
- * @returns {?{diff: (!Object:undefined), errorMessage: (string|undefined)}}
- */
-function compareImages(actualBuffer, expectedBuffer, mimeType) {
-  if (!actualBuffer || !(actualBuffer instanceof Buffer)) {
-    return { errorMessage: 'Actual result should be Buffer.' };
-  }
+const compareImages = (
+  actualBuffer: string | Buffer,
+  expectedBuffer: string | Buffer,
+  mimeType: string
+): DiffFile | undefined => {
+  assert(typeof actualBuffer !== 'string');
+  assert(typeof expectedBuffer !== 'string');
 
   const actual =
     mimeType === 'image/png'
       ? PNG.sync.read(actualBuffer)
       : jpeg.decode(actualBuffer);
+
   const expected =
     mimeType === 'image/png'
       ? PNG.sync.read(expectedBuffer)
       : jpeg.decode(expectedBuffer);
   if (expected.width !== actual.width || expected.height !== actual.height) {
-    return {
-      errorMessage: `Sizes differ: expected image ${expected.width}px X ${expected.height}px, but got ${actual.width}px X ${actual.height}px. `,
-    };
+    throw new Error(
+      `Sizes differ: expected image ${expected.width}px X ${expected.height}px, but got ${actual.width}px X ${actual.height}px.`
+    );
   }
   const diff = new PNG({ width: expected.width, height: expected.height });
   const count = pixelmatch(
@@ -63,64 +78,68 @@ function compareImages(actualBuffer, expectedBuffer, mimeType) {
     expected.height,
     { threshold: 0.1 }
   );
-  return count > 0 ? { diff: PNG.sync.write(diff) } : null;
-}
+  return count > 0 ? { diff: PNG.sync.write(diff) } : undefined;
+};
 
-/**
- * @param {?Object} actual
- * @param {!Buffer} expectedBuffer
- * @returns {?{diff: (!Object:undefined), errorMessage: (string|undefined)}}
- */
-function compareText(actual, expectedBuffer) {
-  if (typeof actual !== 'string') {
-    return { errorMessage: 'Actual result should be string' };
-  }
+const compareText = (
+  actual: string | Buffer,
+  expectedBuffer: string | Buffer
+): DiffFile | undefined => {
+  assert(typeof actual === 'string');
   const expected = expectedBuffer.toString('utf-8');
   if (expected === actual) {
-    return null;
+    return;
   }
-  const diff = new Diff();
-  const result = diff.main(expected, actual);
-  diff.cleanupSemantic(result);
-  let html = diff.prettyHtml(result);
-  const diffStylePath = path.join(__dirname, 'diffstyle.css');
-  html = `<link rel="stylesheet" href="file://${diffStylePath}">` + html;
+  const result = diff.diffLines(expected, actual);
+  const html = result.reduce((text, change) => {
+    text += change.added
+      ? `<span class='ins'>${change.value}</span>`
+      : change.removed
+      ? `<span class='del'>${change.value}</span>`
+      : change.value;
+    return text;
+  }, `<link rel="stylesheet" href="file://${path.join(__dirname, 'diffstyle.css')}">`);
   return {
     diff: html,
-    diffExtension: '.html',
+    ext: '.html',
   };
-}
+};
 
-/**
- * @param {?Object} actual
- * @param {string} goldenName
- * @returns {!{pass: boolean, message: (undefined|string)}}
- */
-function compare(goldenPath, outputPath, actual, goldenName) {
+GoldenComparators.set('image/png', compareImages);
+GoldenComparators.set('image/jpeg', compareImages);
+GoldenComparators.set('text/plain', compareText);
+
+export const compare = (
+  goldenPath: string,
+  outputPath: string,
+  actual: string | Buffer,
+  goldenName: string
+): { pass: true } | { pass: false; message: string } => {
   goldenPath = path.normalize(goldenPath);
   outputPath = path.normalize(outputPath);
   const expectedPath = path.join(goldenPath, goldenName);
   const actualPath = path.join(outputPath, goldenName);
 
-  const messageSuffix =
-    'Output is saved in "' + path.basename(outputPath + '" directory');
+  const messageSuffix = `Output is saved in "${path.basename(
+    outputPath + '" directory'
+  )}`;
 
   if (!fs.existsSync(expectedPath)) {
     ensureOutputDir();
     fs.writeFileSync(actualPath, actual);
     return {
       pass: false,
-      message: goldenName + ' is missing in golden results. ' + messageSuffix,
+      message: `${goldenName} is missing in golden results. ${messageSuffix}`,
     };
   }
   const expected = fs.readFileSync(expectedPath);
   const mimeType = mime.getType(goldenName);
-  const comparator = GoldenComparators[mimeType];
+  assert(mimeType);
+  const comparator = GoldenComparators.get(mimeType);
   if (!comparator) {
     return {
       pass: false,
-      message:
-        'Failed to find comparator with type ' + mimeType + ': ' + goldenName,
+      message: `Failed to find comparator with type ${mimeType}: ${goldenName}`,
     };
   }
   const result = comparator(actual, expected, mimeType);
@@ -135,18 +154,14 @@ function compare(goldenPath, outputPath, actual, goldenName) {
     // Copy expected to the output/ folder for convenience.
     fs.writeFileSync(addSuffix(actualPath, '-expected'), expected);
   }
-  if (result.diff) {
-    const diffPath = addSuffix(actualPath, '-diff', result.diffExtension);
+  if (result) {
+    const diffPath = addSuffix(actualPath, '-diff', result.ext);
     fs.writeFileSync(diffPath, result.diff);
   }
 
-  let message = goldenName + ' mismatch!';
-  if (result.errorMessage) {
-    message += ' ' + result.errorMessage;
-  }
   return {
     pass: false,
-    message: message + ' ' + messageSuffix,
+    message: `${goldenName} mismatch! ${messageSuffix}`,
   };
 
   function ensureOutputDir() {
@@ -154,17 +169,4 @@ function compare(goldenPath, outputPath, actual, goldenName) {
       fs.mkdirSync(outputPath);
     }
   }
-}
-
-/**
- * @param {string} filePath
- * @param {string} suffix
- * @param {string=} customExtension
- * @returns {string}
- */
-function addSuffix(filePath, suffix, customExtension) {
-  const dirname = path.dirname(filePath);
-  const ext = path.extname(filePath);
-  const name = path.basename(filePath, ext);
-  return path.join(dirname, name + suffix + (customExtension || ext));
-}
+};
diff --git a/test/src/headful.spec.ts b/test/src/headful.spec.ts
index 1eeb67bf0c2ed..371ba425cd1d2 100644
--- a/test/src/headful.spec.ts
+++ b/test/src/headful.spec.ts
@@ -14,17 +14,21 @@
  * limitations under the License.
  */
 
-import path from 'path';
-import os from 'os';
+import expect from 'expect';
 import fs from 'fs';
+import os from 'os';
+import path from 'path';
+import rimraf from 'rimraf';
 import { promisify } from 'util';
-import expect from 'expect';
 import {
-  getTestState,
+  PuppeteerLaunchOptions,
+  PuppeteerNode,
+} from '../../lib/cjs/puppeteer/node/Puppeteer.js';
+import {
   describeChromeOnly,
+  getTestState,
   itFailsWindows,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import rimraf from 'rimraf';
+} from './mocha-utils.js';
 
 const rmAsync = promisify(rimraf);
 const mkdtempAsync = promisify(fs.mkdtemp);
@@ -39,12 +43,22 @@ describeChromeOnly('headful tests', function () {
    */
   this.timeout(20 * 1000);
 
-  let headfulOptions;
-  let headlessOptions;
-  let extensionOptions;
-  let forcedOopifOptions;
-  let devtoolsOptions;
-  const browsers = [];
+  let headfulOptions: PuppeteerLaunchOptions | undefined;
+  let headlessOptions: PuppeteerLaunchOptions & { headless: boolean };
+  let extensionOptions: PuppeteerLaunchOptions & {
+    headless: boolean;
+    args: string[];
+  };
+  let forcedOopifOptions: PuppeteerLaunchOptions & {
+    headless: boolean;
+    devtools: boolean;
+    args: string[];
+  };
+  let devtoolsOptions: PuppeteerLaunchOptions & {
+    headless: boolean;
+    devtools: boolean;
+  };
+  const browsers: any[] = [];
 
   beforeEach(() => {
     const { server, defaultBrowserOptions } = getTestState();
@@ -81,7 +95,7 @@ describeChromeOnly('headful tests', function () {
     });
   });
 
-  async function launchBrowser(puppeteer, options) {
+  async function launchBrowser(puppeteer: PuppeteerNode, options: any) {
     const browser = await puppeteer.launch(options);
     browsers.push(browser);
     return browser;
@@ -106,7 +120,9 @@ describeChromeOnly('headful tests', function () {
       );
       const page = await browserWithExtension.newPage();
       const backgroundPageTarget = await browserWithExtension.waitForTarget(
-        (target) => target.type() === 'background_page'
+        (target: { type: () => string }) => {
+          return target.type() === 'background_page';
+        }
       );
       await page.close();
       await browserWithExtension.close();
@@ -119,11 +135,21 @@ describeChromeOnly('headful tests', function () {
         extensionOptions
       );
       const backgroundPageTarget = await browserWithExtension.waitForTarget(
-        (target) => target.type() === 'background_page'
+        (target: { type: () => string }) => {
+          return target.type() === 'background_page';
+        }
       );
-      const page = await backgroundPageTarget.page();
-      expect(await page.evaluate(() => 2 * 3)).toBe(6);
-      expect(await page.evaluate(() => globalThis.MAGIC)).toBe(42);
+      const page = (await backgroundPageTarget.page())!;
+      expect(
+        await page.evaluate(() => {
+          return 2 * 3;
+        })
+      ).toBe(6);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).MAGIC;
+        })
+      ).toBe(42);
       await browserWithExtension.close();
     });
     it('target.page() should return a DevTools page if custom isPageTarget is provided', async function () {
@@ -140,18 +166,24 @@ describeChromeOnly('headful tests', function () {
           );
         },
       });
-      const devtoolsPageTarget = await browser.waitForTarget(
-        (target) => target.type() === 'other'
-      );
-      const page = await devtoolsPageTarget.page();
-      expect(await page.evaluate(() => 2 * 3)).toBe(6);
+      const devtoolsPageTarget = await browser.waitForTarget((target) => {
+        return target.type() === 'other';
+      });
+      const page = (await devtoolsPageTarget.page())!;
+      expect(
+        await page.evaluate(() => {
+          return 2 * 3;
+        })
+      ).toBe(6);
       expect(await browser.pages()).toContainEqual(page);
       await browser.close();
     });
     it('should have default url when launching browser', async function () {
       const { puppeteer } = getTestState();
       const browser = await launchBrowser(puppeteer, extensionOptions);
-      const pages = (await browser.pages()).map((page) => page.url());
+      const pages = (await browser.pages()).map((page: { url: () => any }) => {
+        return page.url();
+      });
       expect(pages).toEqual(['about:blank']);
       await browser.close();
     });
@@ -169,11 +201,10 @@ describeChromeOnly('headful tests', function () {
         );
         const headfulPage = await headfulBrowser.newPage();
         await headfulPage.goto(server.EMPTY_PAGE);
-        await headfulPage.evaluate(
-          () =>
-            (document.cookie =
-              'foo=true; expires=Fri, 31 Dec 9999 23:59:59 GMT')
-        );
+        await headfulPage.evaluate(() => {
+          return (document.cookie =
+            'foo=true; expires=Fri, 31 Dec 9999 23:59:59 GMT');
+        });
         await headfulBrowser.close();
         // Read the cookie from headless chrome
         const headlessBrowser = await launchBrowser(
@@ -182,7 +213,9 @@ describeChromeOnly('headful tests', function () {
         );
         const headlessPage = await headlessBrowser.newPage();
         await headlessPage.goto(server.EMPTY_PAGE);
-        const cookie = await headlessPage.evaluate(() => document.cookie);
+        const cookie = await headlessPage.evaluate(() => {
+          return document.cookie;
+        });
         await headlessBrowser.close();
         // This might throw. See https://github.com/puppeteer/puppeteer/issues/2778
         await rmAsync(userDataDir).catch(() => {});
@@ -198,17 +231,23 @@ describeChromeOnly('headful tests', function () {
       const page = await browser.newPage();
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(true);
-      page.on('request', (r) => r.respond({ body: 'YO, GOOGLE.COM' }));
+      page.on('request', (r: { respond: (arg0: { body: string }) => any }) => {
+        return r.respond({ body: 'YO, GOOGLE.COM' });
+      });
       await page.evaluate(() => {
         const frame = document.createElement('iframe');
         frame.setAttribute('src', 'https://google.com/');
         document.body.appendChild(frame);
-        return new Promise((x) => (frame.onload = x));
+        return new Promise((x) => {
+          return (frame.onload = x);
+        });
       });
       await page.waitForSelector('iframe[src="https://google.com/"]');
       const urls = page
         .frames()
-        .map((frame) => frame.url())
+        .map((frame: { url: () => any }) => {
+          return frame.url();
+        })
         .sort();
       expect(urls).toEqual([server.EMPTY_PAGE, 'https://google.com/']);
       await browser.close();
@@ -221,26 +260,32 @@ describeChromeOnly('headful tests', function () {
 
       // Setup our session listeners to observe OOPIF activity.
       const session = await page.target().createCDPSession();
-      const networkEvents = [];
-      const otherSessions = [];
+      const networkEvents: any[] = [];
+      const otherSessions: any[] = [];
       await session.send('Target.setAutoAttach', {
         autoAttach: true,
         flatten: true,
         waitForDebuggerOnStart: true,
       });
-      session.on('sessionattached', async (session) => {
-        otherSessions.push(session);
-
-        session.on('Network.requestWillBeSent', (params) =>
-          networkEvents.push(params)
-        );
-        await session.send('Network.enable');
-        await session.send('Runtime.runIfWaitingForDebugger');
-      });
+      session.on(
+        'sessionattached',
+        async (session: {
+          on: (arg0: string, arg1: (params: any) => number) => void;
+          send: (arg0: string) => any;
+        }) => {
+          otherSessions.push(session);
+
+          session.on('Network.requestWillBeSent', (params: any) => {
+            return networkEvents.push(params);
+          });
+          await session.send('Network.enable');
+          await session.send('Runtime.runIfWaitingForDebugger');
+        }
+      );
 
       // Navigate to the empty page and add an OOPIF iframe with at least one request.
       await page.goto(server.EMPTY_PAGE);
-      await page.evaluate((frameUrl) => {
+      await page.evaluate((frameUrl: string) => {
         const frame = document.createElement('iframe');
         frame.setAttribute('src', frameUrl);
         document.body.appendChild(frame);
@@ -255,14 +300,16 @@ describeChromeOnly('headful tests', function () {
       expect(otherSessions).toHaveLength(1);
 
       // Resume the iframe and trigger another request.
-      const iframeSession = otherSessions[0];
+      const iframeSession = otherSessions[0]!;
       await iframeSession.send('Runtime.evaluate', {
         expression: `fetch('/fetch')`,
         awaitPromise: true,
       });
       await browser.close();
 
-      const requests = networkEvents.map((event) => event.request.url);
+      const requests = networkEvents.map((event) => {
+        return event.request.url;
+      });
       expect(requests).toContain(`http://oopifdomain:${server.PORT}/fetch`);
     });
     it('should close browser with beforeunload page', async () => {
@@ -286,7 +333,9 @@ describeChromeOnly('headful tests', function () {
       const context = await browser.createIncognitoBrowserContext();
       await Promise.all([
         context.newPage(),
-        browser.waitForTarget((target) => target.url().includes('devtools://')),
+        browser.waitForTarget((target: { url: () => string | string[] }) => {
+          return target.url().includes('devtools://');
+        }),
       ]);
       await browser.close();
     });
@@ -300,20 +349,28 @@ describeChromeOnly('headful tests', function () {
       const page2 = await browser.newPage();
 
       await page1.bringToFront();
-      expect(await page1.evaluate(() => document.visibilityState)).toBe(
-        'visible'
-      );
-      expect(await page2.evaluate(() => document.visibilityState)).toBe(
-        'hidden'
-      );
+      expect(
+        await page1.evaluate(() => {
+          return document.visibilityState;
+        })
+      ).toBe('visible');
+      expect(
+        await page2.evaluate(() => {
+          return document.visibilityState;
+        })
+      ).toBe('hidden');
 
       await page2.bringToFront();
-      expect(await page1.evaluate(() => document.visibilityState)).toBe(
-        'hidden'
-      );
-      expect(await page2.evaluate(() => document.visibilityState)).toBe(
-        'visible'
-      );
+      expect(
+        await page1.evaluate(() => {
+          return document.visibilityState;
+        })
+      ).toBe('hidden');
+      expect(
+        await page2.evaluate(() => {
+          return document.visibilityState;
+        })
+      ).toBe('visible');
 
       await page1.close();
       await page2.close();
@@ -340,7 +397,7 @@ describeChromeOnly('headful tests', function () {
       const promises = [];
       for (let i = 0; i < N; ++i) {
         promises.push(
-          pages[i].screenshot({
+          pages[i]!.screenshot({
             clip: { x: 50 * i, y: 0, width: 50, height: 50 },
           })
         );
@@ -349,7 +406,11 @@ describeChromeOnly('headful tests', function () {
       for (let i = 0; i < N; ++i) {
         expect(screenshots[i]).toBeGolden(`grid-cell-${i}.png`);
       }
-      await Promise.all(pages.map((page) => page.close()));
+      await Promise.all(
+        pages.map((page) => {
+          return page.close();
+        })
+      );
 
       await browser.close();
     });
diff --git a/test/src/idle_override.spec.ts b/test/src/idle_override.spec.ts
index 18e7da3986c70..1025c1f66ddda 100644
--- a/test/src/idle_override.spec.ts
+++ b/test/src/idle_override.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describeFailsFirefox('Emulate idle state', () => {
   setupTestBrowserHooks();
@@ -29,7 +29,7 @@ describeFailsFirefox('Emulate idle state', () => {
   async function getIdleState() {
     const { page } = getTestState();
 
-    const stateElement = await page.$('#state');
+    const stateElement = (await page.$('#state'))!;
     return await page.evaluate((element: HTMLElement) => {
       return element.innerText;
     }, stateElement);
diff --git a/test/src/ignorehttpserrors.spec.ts b/test/src/ignorehttpserrors.spec.ts
index 688cee8843e15..7d1c53ee86725 100644
--- a/test/src/ignorehttpserrors.spec.ts
+++ b/test/src/ignorehttpserrors.spec.ts
@@ -16,20 +16,26 @@
 
 import expect from 'expect';
 import { TLSSocket } from 'tls';
+import {
+  Browser,
+  BrowserContext,
+} from '../../lib/cjs/puppeteer/common/Browser.js';
+import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
+import { HTTPResponse } from '../../lib/cjs/puppeteer/common/HTTPResponse.js';
 import {
   getTestState,
   describeFailsFirefox,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('ignoreHTTPSErrors', function () {
   /* Note that this test creates its own browser rather than use
    * the one provided by the test set-up as we need one
    * with ignoreHTTPSErrors set to true
    */
-  let browser;
-  let context;
-  let page;
+  let browser!: Browser;
+  let context: BrowserContext;
+  let page!: Page;
 
   before(async () => {
     const { defaultBrowserOptions, puppeteer } = getTestState();
@@ -42,7 +48,6 @@ describe('ignoreHTTPSErrors', function () {
 
   after(async () => {
     await browser.close();
-    browser = null;
   });
 
   beforeEach(async () => {
@@ -52,8 +57,6 @@ describe('ignoreHTTPSErrors', function () {
 
   afterEach(async () => {
     await context.close();
-    context = null;
-    page = null;
   });
 
   describeFailsFirefox('Response.securityDetails', function () {
@@ -64,10 +67,10 @@ describe('ignoreHTTPSErrors', function () {
         httpsServer.waitForRequest('/empty.html'),
         page.goto(httpsServer.EMPTY_PAGE),
       ]);
-      const securityDetails = response.securityDetails();
+      const securityDetails = response!.securityDetails()!;
       expect(securityDetails.issuer()).toBe('puppeteer-tests');
       const protocol = (serverRequest.socket as TLSSocket)
-        .getProtocol()
+        .getProtocol()!
         .replace('v', ' ');
       expect(securityDetails.protocol()).toBe(protocol);
       expect(securityDetails.subjectName()).toBe('puppeteer-tests');
@@ -81,24 +84,26 @@ describe('ignoreHTTPSErrors', function () {
     it('should be |null| for non-secure requests', async () => {
       const { server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.securityDetails()).toBe(null);
     });
     it('Network redirects should report SecurityDetails', async () => {
       const { httpsServer } = getTestState();
 
       httpsServer.setRedirect('/plzredirect', '/empty.html');
-      const responses = [];
-      page.on('response', (response) => responses.push(response));
+      const responses: HTTPResponse[] = [];
+      page.on('response', (response) => {
+        return responses.push(response);
+      });
       const [serverRequest] = await Promise.all([
         httpsServer.waitForRequest('/plzredirect'),
         page.goto(httpsServer.PREFIX + '/plzredirect'),
       ]);
       expect(responses.length).toBe(2);
-      expect(responses[0].status()).toBe(302);
-      const securityDetails = responses[0].securityDetails();
+      expect(responses[0]!.status()).toBe(302);
+      const securityDetails = responses[0]!.securityDetails()!;
       const protocol = (serverRequest.socket as TLSSocket)
-        .getProtocol()
+        .getProtocol()!
         .replace('v', ' ');
       expect(securityDetails.protocol()).toBe(protocol);
     });
@@ -107,25 +112,27 @@ describe('ignoreHTTPSErrors', function () {
   it('should work', async () => {
     const { httpsServer } = getTestState();
 
-    let error = null;
-    const response = await page
-      .goto(httpsServer.EMPTY_PAGE)
-      .catch((error_) => (error = error_));
-    expect(error).toBe(null);
+    let error!: Error;
+    const response = await page.goto(httpsServer.EMPTY_PAGE).catch((error_) => {
+      return (error = error_);
+    });
+    expect(error).toBeUndefined();
     expect(response.ok()).toBe(true);
   });
   itFailsFirefox('should work with request interception', async () => {
     const { httpsServer } = getTestState();
 
     await page.setRequestInterception(true);
-    page.on('request', (request) => request.continue());
-    const response = await page.goto(httpsServer.EMPTY_PAGE);
+    page.on('request', (request) => {
+      return request.continue();
+    });
+    const response = (await page.goto(httpsServer.EMPTY_PAGE))!;
     expect(response.status()).toBe(200);
   });
   itFailsFirefox('should work with mixed content', async () => {
     const { server, httpsServer } = getTestState();
 
-    httpsServer.setRoute('/mixedcontent.html', (req, res) => {
+    httpsServer.setRoute('/mixedcontent.html', (_req, res) => {
       res.end(`<iframe src=${server.EMPTY_PAGE}></iframe>`);
     });
     await page.goto(httpsServer.PREFIX + '/mixedcontent.html', {
@@ -134,7 +141,7 @@ describe('ignoreHTTPSErrors', function () {
     expect(page.frames().length).toBe(2);
     // Make sure blocked iframe has functional execution context
     // @see https://github.com/puppeteer/puppeteer/issues/2709
-    expect(await page.frames()[0].evaluate('1 + 2')).toBe(3);
-    expect(await page.frames()[1].evaluate('2 + 3')).toBe(5);
+    expect(await page.frames()[0]!.evaluate('1 + 2')).toBe(3);
+    expect(await page.frames()[1]!.evaluate('2 + 3')).toBe(5);
   });
 });
diff --git a/test/src/input.spec.ts b/test/src/input.spec.ts
index ad96a47c7fc94..f7eca8b308d90 100644
--- a/test/src/input.spec.ts
+++ b/test/src/input.spec.ts
@@ -21,7 +21,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 const FILE_TO_UPLOAD = path.join(__dirname, '/../assets/file-to-upload.txt');
 
@@ -35,33 +35,42 @@ describe('input tests', function () {
 
       await page.goto(server.PREFIX + '/input/fileupload.html');
       const filePath = path.relative(process.cwd(), FILE_TO_UPLOAD);
-      const input = await page.$('input');
+      const input = (await page.$('input'))!;
       await page.evaluate((e: HTMLElement) => {
-        globalThis._inputEvents = [];
-        e.addEventListener('change', (ev) =>
-          globalThis._inputEvents.push(ev.type)
-        );
-        e.addEventListener('input', (ev) =>
-          globalThis._inputEvents.push(ev.type)
-        );
+        (globalThis as any)._inputEvents = [];
+        e.addEventListener('change', (ev) => {
+          return (globalThis as any)._inputEvents.push(ev.type);
+        });
+        e.addEventListener('input', (ev) => {
+          return (globalThis as any)._inputEvents.push(ev.type);
+        });
       }, input);
       await input.uploadFile(filePath);
       expect(
-        await page.evaluate((e: HTMLInputElement) => e.files[0].name, input)
+        await page.evaluate((e: HTMLInputElement) => {
+          return e.files![0]!.name;
+        }, input)
       ).toBe('file-to-upload.txt');
       expect(
-        await page.evaluate((e: HTMLInputElement) => e.files[0].type, input)
+        await page.evaluate((e: HTMLInputElement) => {
+          return e.files![0]!.type;
+        }, input)
       ).toBe('text/plain');
-      expect(await page.evaluate(() => globalThis._inputEvents)).toEqual([
-        'input',
-        'change',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any)._inputEvents;
+        })
+      ).toEqual(['input', 'change']);
       expect(
         await page.evaluate((e: HTMLInputElement) => {
           const reader = new FileReader();
-          const promise = new Promise((fulfill) => (reader.onload = fulfill));
-          reader.readAsText(e.files[0]);
-          return promise.then(() => reader.result);
+          const promise = new Promise((fulfill) => {
+            return (reader.onload = fulfill);
+          });
+          reader.readAsText(e.files![0]!);
+          return promise.then(() => {
+            return reader.result;
+          });
         }, input)
       ).toBe('contents of the file');
     });
@@ -94,28 +103,30 @@ describe('input tests', function () {
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
-      await page
-        .waitForFileChooser({ timeout: 1 })
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitForFileChooser({ timeout: 1 }).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should respect default timeout when there is no custom timeout', async () => {
       const { page, puppeteer } = getTestState();
 
       page.setDefaultTimeout(1);
-      let error = null;
-      await page.waitForFileChooser().catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitForFileChooser().catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should prioritize exact timeout over default timeout', async () => {
       const { page, puppeteer } = getTestState();
 
       page.setDefaultTimeout(0);
-      let error = null;
-      await page
-        .waitForFileChooser({ timeout: 1 })
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitForFileChooser({ timeout: 1 }).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should work with no timeout', async () => {
@@ -123,13 +134,13 @@ describe('input tests', function () {
 
       const [chooser] = await Promise.all([
         page.waitForFileChooser({ timeout: 0 }),
-        page.evaluate(() =>
-          setTimeout(() => {
+        page.evaluate(() => {
+          return setTimeout(() => {
             const el = document.createElement('input');
             el.type = 'file';
             el.click();
-          }, 50)
-        ),
+          }, 50);
+        }),
       ]);
       expect(chooser).toBeTruthy();
     });
@@ -140,7 +151,9 @@ describe('input tests', function () {
       const [fileChooser1, fileChooser2] = await Promise.all([
         page.waitForFileChooser(),
         page.waitForFileChooser(),
-        page.$eval('input', (input: HTMLInputElement) => input.click()),
+        page.$eval('input', (input) => {
+          return (input as HTMLInputElement).click();
+        }),
       ]);
       expect(fileChooser1 === fileChooser2).toBe(true);
     });
@@ -159,36 +172,43 @@ describe('input tests', function () {
       ]);
       await Promise.all([
         chooser.accept([FILE_TO_UPLOAD]),
-        new Promise((x) => page.once('metrics', x)),
+        new Promise((x) => {
+          return page.once('metrics', x);
+        }),
       ]);
       expect(
-        await page.$eval(
-          'input',
-          (input: HTMLInputElement) => input.files.length
-        )
+        await page.$eval('input', (input) => {
+          return (input as HTMLInputElement).files!.length;
+        })
       ).toBe(1);
       expect(
-        await page.$eval(
-          'input',
-          (input: HTMLInputElement) => input.files[0].name
-        )
+        await page.$eval('input', (input) => {
+          return (input as HTMLInputElement).files![0]!.name;
+        })
       ).toBe('file-to-upload.txt');
     });
     it('should be able to read selected file', async () => {
       const { page } = getTestState();
 
       await page.setContent(`<input type=file>`);
-      page
-        .waitForFileChooser()
-        .then((chooser) => chooser.accept([FILE_TO_UPLOAD]));
+      page.waitForFileChooser().then((chooser) => {
+        return chooser.accept([FILE_TO_UPLOAD]);
+      });
       expect(
-        await page.$eval('input', async (picker: HTMLInputElement) => {
-          picker.click();
-          await new Promise((x) => (picker.oninput = x));
+        await page.$eval('input', async (picker) => {
+          const pick = picker as HTMLInputElement;
+          pick.click();
+          await new Promise((x) => {
+            return (pick.oninput = x);
+          });
           const reader = new FileReader();
-          const promise = new Promise((fulfill) => (reader.onload = fulfill));
-          reader.readAsText(picker.files[0]);
-          return promise.then(() => reader.result);
+          const promise = new Promise((fulfill) => {
+            return (reader.onload = fulfill);
+          });
+          reader.readAsText(pick.files![0]!);
+          return promise.then(() => {
+            return reader.result;
+          });
         })
       ).toBe('contents of the file');
     });
@@ -196,22 +216,30 @@ describe('input tests', function () {
       const { page } = getTestState();
 
       await page.setContent(`<input type=file>`);
-      page
-        .waitForFileChooser()
-        .then((chooser) => chooser.accept([FILE_TO_UPLOAD]));
+      page.waitForFileChooser().then((chooser) => {
+        return chooser.accept([FILE_TO_UPLOAD]);
+      });
       expect(
-        await page.$eval('input', async (picker: HTMLInputElement) => {
-          picker.click();
-          await new Promise((x) => (picker.oninput = x));
-          return picker.files.length;
+        await page.$eval('input', async (picker) => {
+          const pick = picker as HTMLInputElement;
+          pick.click();
+          await new Promise((x) => {
+            return (pick.oninput = x);
+          });
+          return pick.files!.length;
         })
       ).toBe(1);
-      page.waitForFileChooser().then((chooser) => chooser.accept([]));
+      page.waitForFileChooser().then((chooser) => {
+        return chooser.accept([]);
+      });
       expect(
-        await page.$eval('input', async (picker: HTMLInputElement) => {
-          picker.click();
-          await new Promise((x) => (picker.oninput = x));
-          return picker.files.length;
+        await page.$eval('input', async (picker) => {
+          const pick = picker as HTMLInputElement;
+          pick.click();
+          await new Promise((x) => {
+            return (pick.oninput = x);
+          });
+          return pick.files!.length;
         })
       ).toBe(0);
     });
@@ -223,7 +251,7 @@ describe('input tests', function () {
         page.waitForFileChooser(),
         page.click('input'),
       ]);
-      let error = null;
+      let error!: Error;
       await chooser
         .accept([
           path.relative(
@@ -232,7 +260,9 @@ describe('input tests', function () {
           ),
           path.relative(process.cwd(), __dirname + '/../assets/pptr.png'),
         ])
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).not.toBe(null);
     });
     it('should succeed even for non-existent files', async () => {
@@ -243,27 +273,34 @@ describe('input tests', function () {
         page.waitForFileChooser(),
         page.click('input'),
       ]);
-      let error: Error | undefined;
-      await chooser
-        .accept(['file-does-not-exist.txt'])
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await chooser.accept(['file-does-not-exist.txt']).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeUndefined();
     });
     it('should error on read of non-existent files', async () => {
       const { page } = getTestState();
 
       await page.setContent(`<input type=file>`);
-      page
-        .waitForFileChooser()
-        .then((chooser) => chooser.accept(['file-does-not-exist.txt']));
+      page.waitForFileChooser().then((chooser) => {
+        return chooser.accept(['file-does-not-exist.txt']);
+      });
       expect(
-        await page.$eval('input', async (picker: HTMLInputElement) => {
-          picker.click();
-          await new Promise((x) => (picker.oninput = x));
+        await page.$eval('input', async (picker) => {
+          const pick = picker as HTMLInputElement;
+          pick.click();
+          await new Promise((x) => {
+            return (pick.oninput = x);
+          });
           const reader = new FileReader();
-          const promise = new Promise((fulfill) => (reader.onerror = fulfill));
-          reader.readAsText(picker.files[0]);
-          return promise.then(() => false);
+          const promise = new Promise((fulfill) => {
+            return (reader.onerror = fulfill);
+          });
+          reader.readAsText(pick.files![0]!);
+          return promise.then(() => {
+            return false;
+          });
         })
       ).toBeFalsy();
     });
@@ -273,11 +310,15 @@ describe('input tests', function () {
       await page.setContent(`<input type=file>`);
       const [fileChooser] = await Promise.all([
         page.waitForFileChooser(),
-        page.$eval('input', (input: HTMLInputElement) => input.click()),
+        page.$eval('input', (input) => {
+          return (input as HTMLInputElement).click();
+        }),
       ]);
       await fileChooser.accept([]);
-      let error = null;
-      await fileChooser.accept([]).catch((error_) => (error = error_));
+      let error!: Error;
+      await fileChooser.accept([]).catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe(
         'Cannot accept FileChooser which is already handled!'
       );
@@ -294,13 +335,17 @@ describe('input tests', function () {
       await page.setContent(`<input type=file>`);
       const [fileChooser1] = await Promise.all([
         page.waitForFileChooser(),
-        page.$eval('input', (input: HTMLInputElement) => input.click()),
+        page.$eval('input', (input) => {
+          return (input as HTMLInputElement).click();
+        }),
       ]);
       await fileChooser1.cancel();
       // If this resolves, than we successfully canceled file chooser.
       await Promise.all([
         page.waitForFileChooser(),
-        page.$eval('input', (input: HTMLInputElement) => input.click()),
+        page.$eval('input', (input) => {
+          return (input as HTMLInputElement).click();
+        }),
       ]);
     });
     it('should fail when canceling file chooser twice', async () => {
@@ -309,15 +354,17 @@ describe('input tests', function () {
       await page.setContent(`<input type=file>`);
       const [fileChooser] = await Promise.all([
         page.waitForFileChooser(),
-        page.$eval('input', (input: HTMLInputElement) => input.click()),
+        page.$eval('input', (input) => {
+          return (input as HTMLElement).click();
+        }),
       ]);
       await fileChooser.cancel();
-      let error = null;
+      let error!: Error;
 
       try {
         fileChooser.cancel();
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
 
       expect(error.message).toBe(
diff --git a/test/src/jshandle.spec.ts b/test/src/jshandle.spec.ts
index 9a37ac4849c5b..4acff01aca3df 100644
--- a/test/src/jshandle.spec.ts
+++ b/test/src/jshandle.spec.ts
@@ -22,7 +22,7 @@ import {
   setupTestPageAndContextHooks,
   itFailsFirefox,
   shortWaitForArrayToHaveAtLeastNElements,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('JSHandle', function () {
   setupTestBrowserHooks();
@@ -32,24 +32,31 @@ describe('JSHandle', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const windowHandle = await page.evaluateHandle(() => window);
+      const windowHandle = await page.evaluateHandle(() => {
+        return window;
+      });
       expect(windowHandle).toBeTruthy();
     });
     it('should accept object handle as an argument', async () => {
       const { page } = getTestState();
 
-      const navigatorHandle = await page.evaluateHandle(() => navigator);
-      const text = await page.evaluate(
-        (e: Navigator) => e.userAgent,
-        navigatorHandle
-      );
+      const navigatorHandle = await page.evaluateHandle(() => {
+        return navigator;
+      });
+      const text = await page.evaluate((e: Navigator) => {
+        return e.userAgent;
+      }, navigatorHandle);
       expect(text).toContain('Mozilla');
     });
     it('should accept object handle to primitive types', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => 5);
-      const isFive = await page.evaluate((e) => Object.is(e, 5), aHandle);
+      const aHandle = await page.evaluateHandle(() => {
+        return 5;
+      });
+      const isFive = await page.evaluate((e) => {
+        return Object.is(e, 5);
+      }, aHandle);
       expect(isFive).toBeTruthy();
     });
     it('should warn about recursive objects', async () => {
@@ -57,31 +64,44 @@ describe('JSHandle', function () {
 
       const test: { obj?: unknown } = {};
       test.obj = test;
-      let error = null;
+      let error!: Error;
       await page
-        // @ts-expect-error we are deliberately passing a bad type here (nested object)
-        .evaluateHandle((opts) => opts.elem, { test })
-        .catch((error_) => (error = error_));
+        .evaluateHandle(
+          (opts) => {
+            return opts.elem;
+          },
+          // @ts-expect-error we are deliberately passing a bad type here (nested object)
+          { test }
+        )
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain('Recursive objects are not allowed.');
     });
     it('should accept object handle to unserializable value', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => Infinity);
-      expect(await page.evaluate((e) => Object.is(e, Infinity), aHandle)).toBe(
-        true
-      );
+      const aHandle = await page.evaluateHandle(() => {
+        return Infinity;
+      });
+      expect(
+        await page.evaluate((e) => {
+          return Object.is(e, Infinity);
+        }, aHandle)
+      ).toBe(true);
     });
     it('should use the same JS wrappers', async () => {
       const { page } = getTestState();
 
       const aHandle = await page.evaluateHandle(() => {
-        globalThis.FOO = 123;
+        (globalThis as any).FOO = 123;
         return window;
       });
-      expect(await page.evaluate((e: { FOO: number }) => e.FOO, aHandle)).toBe(
-        123
-      );
+      expect(
+        await page.evaluate((e: { FOO: number }) => {
+          return e.FOO;
+        }, aHandle)
+      ).toBe(123);
     });
   });
 
@@ -89,11 +109,13 @@ describe('JSHandle', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => ({
-        one: 1,
-        two: 2,
-        three: 3,
-      }));
+      const aHandle = await page.evaluateHandle(() => {
+        return {
+          one: 1,
+          two: 2,
+          three: 3,
+        };
+      });
       const twoHandle = await aHandle.getProperty('two');
       expect(await twoHandle.jsonValue()).toEqual(2);
     });
@@ -101,11 +123,13 @@ describe('JSHandle', function () {
     it('should return a JSHandle even if the property does not exist', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => ({
-        one: 1,
-        two: 2,
-        three: 3,
-      }));
+      const aHandle = await page.evaluateHandle(() => {
+        return {
+          one: 1,
+          two: 2,
+          three: 3,
+        };
+      });
       const undefinedHandle = await aHandle.getProperty('doesnotexist');
       expect(undefinedHandle).toBeInstanceOf(JSHandle);
       expect(await undefinedHandle.jsonValue()).toBe(undefined);
@@ -116,7 +140,9 @@ describe('JSHandle', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => ({ foo: 'bar' }));
+      const aHandle = await page.evaluateHandle(() => {
+        return { foo: 'bar' };
+      });
       const json = await aHandle.jsonValue<Record<string, string>>();
       expect(json).toEqual({ foo: 'bar' });
     });
@@ -124,7 +150,9 @@ describe('JSHandle', function () {
     it('works with jsonValues that are not objects', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => ['a', 'b']);
+      const aHandle = await page.evaluateHandle(() => {
+        return ['a', 'b'];
+      });
       const json = await aHandle.jsonValue<string[]>();
       expect(json).toEqual(['a', 'b']);
     });
@@ -132,7 +160,9 @@ describe('JSHandle', function () {
     it('works with jsonValues that are primitives', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => 'foo');
+      const aHandle = await page.evaluateHandle(() => {
+        return 'foo';
+      });
       const json = await aHandle.jsonValue<string>();
       expect(json).toEqual('foo');
     });
@@ -140,9 +170,9 @@ describe('JSHandle', function () {
     itFailsFirefox('should not work with dates', async () => {
       const { page } = getTestState();
 
-      const dateHandle = await page.evaluateHandle(
-        () => new Date('2017-09-26T00:00:00.000Z')
-      );
+      const dateHandle = await page.evaluateHandle(() => {
+        return new Date('2017-09-26T00:00:00.000Z');
+      });
       const json = await dateHandle.jsonValue();
       expect(json).toEqual({});
     });
@@ -150,8 +180,10 @@ describe('JSHandle', function () {
       const { page, isChrome } = getTestState();
 
       const windowHandle = await page.evaluateHandle('window');
-      let error = null;
-      await windowHandle.jsonValue().catch((error_) => (error = error_));
+      let error!: Error;
+      await windowHandle.jsonValue().catch((error_) => {
+        return (error = error_);
+      });
       if (isChrome) {
         expect(error.message).toContain('Object reference chain is too long');
       } else {
@@ -164,11 +196,13 @@ describe('JSHandle', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => ({
-        foo: 'bar',
-      }));
+      const aHandle = await page.evaluateHandle(() => {
+        return {
+          foo: 'bar',
+        };
+      });
       const properties = await aHandle.getProperties();
-      const foo = properties.get('foo');
+      const foo = properties.get('foo')!;
       expect(foo).toBeTruthy();
       expect(await foo.jsonValue()).toBe('bar');
     });
@@ -192,8 +226,8 @@ describe('JSHandle', function () {
         return new B();
       });
       const properties = await aHandle.getProperties();
-      expect(await properties.get('a').jsonValue()).toBe('1');
-      expect(await properties.get('b').jsonValue()).toBe('2');
+      expect(await properties.get('a')!.jsonValue()).toBe('1');
+      expect(await properties.get('b')!.jsonValue()).toBe('2');
     });
   });
 
@@ -201,14 +235,18 @@ describe('JSHandle', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => document.body);
+      const aHandle = await page.evaluateHandle(() => {
+        return document.body;
+      });
       const element = aHandle.asElement();
       expect(element).toBeTruthy();
     });
     it('should return null for non-elements', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => 2);
+      const aHandle = await page.evaluateHandle(() => {
+        return 2;
+      });
       const element = aHandle.asElement();
       expect(element).toBeFalsy();
     });
@@ -216,16 +254,15 @@ describe('JSHandle', function () {
       const { page } = getTestState();
 
       await page.setContent('<div>ee!</div>');
-      const aHandle = await page.evaluateHandle(
-        () => document.querySelector('div').firstChild
-      );
+      const aHandle = await page.evaluateHandle(() => {
+        return document.querySelector('div')!.firstChild;
+      });
       const element = aHandle.asElement();
       expect(element).toBeTruthy();
       expect(
-        await page.evaluate(
-          (e: HTMLElement) => e.nodeType === Node.TEXT_NODE,
-          element
-        )
+        await page.evaluate((e: HTMLElement) => {
+          return e.nodeType === Node.TEXT_NODE;
+        }, element)
       );
     });
   });
@@ -234,15 +271,21 @@ describe('JSHandle', function () {
     it('should work for primitives', async () => {
       const { page } = getTestState();
 
-      const numberHandle = await page.evaluateHandle(() => 2);
+      const numberHandle = await page.evaluateHandle(() => {
+        return 2;
+      });
       expect(numberHandle.toString()).toBe('JSHandle:2');
-      const stringHandle = await page.evaluateHandle(() => 'a');
+      const stringHandle = await page.evaluateHandle(() => {
+        return 'a';
+      });
       expect(stringHandle.toString()).toBe('JSHandle:a');
     });
     it('should work for complicated objects', async () => {
       const { page } = getTestState();
 
-      const aHandle = await page.evaluateHandle(() => window);
+      const aHandle = await page.evaluateHandle(() => {
+        return window;
+      });
       expect(aHandle.toString()).toBe('JSHandle@object');
     });
     it('should work with different subtypes', async () => {
@@ -315,9 +358,11 @@ describe('JSHandle', function () {
         `;
       });
       await page.evaluate(async () => {
-        return new Promise((resolve) => window.requestAnimationFrame(resolve));
+        return new Promise((resolve) => {
+          return window.requestAnimationFrame(resolve);
+        });
       });
-      const divHandle = await page.$('div');
+      const divHandle = (await page.$('div'))!;
       expect(await divHandle.clickablePoint()).toEqual({
         x: 45 + 60, // margin + middle point offset
         y: 45 + 30, // margin + middle point offset
@@ -343,10 +388,12 @@ describe('JSHandle', function () {
         `;
       });
       await page.evaluate(async () => {
-        return new Promise((resolve) => window.requestAnimationFrame(resolve));
+        return new Promise((resolve) => {
+          return window.requestAnimationFrame(resolve);
+        });
       });
-      const frame = page.frames()[1];
-      const divHandle = await frame.$('div');
+      const frame = page.frames()[1]!;
+      const divHandle = (await frame.$('div'))!;
       expect(await divHandle.clickablePoint()).toEqual({
         x: 20 + 45 + 60, // iframe pos + margin + middle point offset
         y: 20 + 45 + 30, // iframe pos + margin + middle point offset
@@ -367,7 +414,7 @@ describe('JSHandle', function () {
     itFailsFirefox('should work', async () => {
       const { page } = getTestState();
 
-      const clicks = [];
+      const clicks: [x: number, y: number][] = [];
 
       await page.exposeFunction('reportClick', (x: number, y: number): void => {
         clicks.push([x, y]);
@@ -384,7 +431,7 @@ describe('JSHandle', function () {
         });
       });
 
-      const divHandle = await page.$('div');
+      const divHandle = (await page.$('div'))!;
       await divHandle.click();
       await divHandle.click({
         offset: {
diff --git a/test/src/keyboard.spec.ts b/test/src/keyboard.spec.ts
index bde9649c3224c..2288869e16f72 100644
--- a/test/src/keyboard.spec.ts
+++ b/test/src/keyboard.spec.ts
@@ -22,7 +22,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import { KeyInput } from '../../lib/cjs/puppeteer/common/USKeyboardLayout.js';
 
 describe('Keyboard', function () {
@@ -40,16 +40,20 @@ describe('Keyboard', function () {
     const text = 'Hello world. I am the text that was typed!';
     await page.keyboard.type(text);
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe(text);
   });
   itFailsFirefox('should press the metaKey', async () => {
     const { page, isFirefox } = getTestState();
 
     await page.evaluate(() => {
-      (window as any).keyPromise = new Promise((resolve) =>
-        document.addEventListener('keydown', (event) => resolve(event.key))
-      );
+      (window as any).keyPromise = new Promise((resolve) => {
+        return document.addEventListener('keydown', (event) => {
+          return resolve(event.key);
+        });
+      });
     });
     await page.keyboard.press('Meta');
     expect(await page.evaluate('keyPromise')).toBe(
@@ -62,14 +66,18 @@ describe('Keyboard', function () {
     await page.goto(server.PREFIX + '/input/textarea.html');
     await page.type('textarea', 'Hello World!');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('Hello World!');
     for (let i = 0; i < 'World!'.length; i++) {
       page.keyboard.press('ArrowLeft');
     }
     await page.keyboard.type('inserted ');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('Hello inserted World!');
     page.keyboard.down('Shift');
     for (let i = 0; i < 'inserted '.length; i++) {
@@ -78,26 +86,38 @@ describe('Keyboard', function () {
     page.keyboard.up('Shift');
     await page.keyboard.press('Backspace');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('Hello World!');
   });
   it('should send a character with ElementHandle.press', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/textarea.html');
-    const textarea = await page.$('textarea');
+    const textarea = (await page.$('textarea'))!;
     await textarea.press('a');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('a');
 
-    await page.evaluate(() =>
-      window.addEventListener('keydown', (e) => e.preventDefault(), true)
-    );
+    await page.evaluate(() => {
+      return window.addEventListener(
+        'keydown',
+        (e) => {
+          return e.preventDefault();
+        },
+        true
+      );
+    });
 
     await textarea.press('b');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('a');
   });
   itFailsFirefox(
@@ -106,10 +126,12 @@ describe('Keyboard', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/textarea.html');
-      const textarea = await page.$('textarea');
+      const textarea = (await page.$('textarea'))!;
       await textarea.press('a', { text: 'ё' });
       expect(
-        await page.evaluate(() => document.querySelector('textarea').value)
+        await page.evaluate(() => {
+          return document.querySelector('textarea')!.value;
+        })
       ).toBe('ё');
     }
   );
@@ -120,14 +142,24 @@ describe('Keyboard', function () {
     await page.focus('textarea');
     await page.keyboard.sendCharacter('嗨');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('嗨');
-    await page.evaluate(() =>
-      window.addEventListener('keydown', (e) => e.preventDefault(), true)
-    );
+    await page.evaluate(() => {
+      return window.addEventListener(
+        'keydown',
+        (e) => {
+          return e.preventDefault();
+        },
+        true
+      );
+    });
     await page.keyboard.sendCharacter('a');
     expect(
-      await page.evaluate(() => document.querySelector('textarea').value)
+      await page.evaluate(() => {
+        return document.querySelector('textarea')!.value;
+      })
     ).toBe('嗨a');
   });
   itFailsFirefox('should report shiftKey', async () => {
@@ -142,7 +174,11 @@ describe('Keyboard', function () {
     ]);
     for (const [modifierKey, modifierCode] of codeForKey) {
       await keyboard.down(modifierKey);
-      expect(await page.evaluate(() => globalThis.getResult())).toBe(
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).getResult();
+        })
+      ).toBe(
         'Keydown: ' +
           modifierKey +
           ' ' +
@@ -156,7 +192,11 @@ describe('Keyboard', function () {
       await keyboard.down('!');
       // Shift+! will generate a keypress
       if (modifierKey === 'Shift') {
-        expect(await page.evaluate(() => globalThis.getResult())).toBe(
+        expect(
+          await page.evaluate(() => {
+            return (globalThis as any).getResult();
+          })
+        ).toBe(
           'Keydown: ! Digit1 49 [' +
             modifierKey +
             ']\nKeypress: ! Digit1 33 33 [' +
@@ -164,17 +204,25 @@ describe('Keyboard', function () {
             ']'
         );
       } else {
-        expect(await page.evaluate(() => globalThis.getResult())).toBe(
-          'Keydown: ! Digit1 49 [' + modifierKey + ']'
-        );
+        expect(
+          await page.evaluate(() => {
+            return (globalThis as any).getResult();
+          })
+        ).toBe('Keydown: ! Digit1 49 [' + modifierKey + ']');
       }
 
       await keyboard.up('!');
-      expect(await page.evaluate(() => globalThis.getResult())).toBe(
-        'Keyup: ! Digit1 49 [' + modifierKey + ']'
-      );
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).getResult();
+        })
+      ).toBe('Keyup: ! Digit1 49 [' + modifierKey + ']');
       await keyboard.up(modifierKey);
-      expect(await page.evaluate(() => globalThis.getResult())).toBe(
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).getResult();
+        })
+      ).toBe(
         'Keyup: ' +
           modifierKey +
           ' ' +
@@ -191,36 +239,52 @@ describe('Keyboard', function () {
     await page.goto(server.PREFIX + '/input/keyboard.html');
     const keyboard = page.keyboard;
     await keyboard.down('Control');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keydown: Control ControlLeft 17 [Control]'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keydown: Control ControlLeft 17 [Control]');
     await keyboard.down('Alt');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keydown: Alt AltLeft 18 [Alt Control]'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keydown: Alt AltLeft 18 [Alt Control]');
     await keyboard.down(';');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keydown: ; Semicolon 186 [Alt Control]'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keydown: ; Semicolon 186 [Alt Control]');
     await keyboard.up(';');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keyup: ; Semicolon 186 [Alt Control]'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keyup: ; Semicolon 186 [Alt Control]');
     await keyboard.up('Control');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keyup: Control ControlLeft 17 [Alt]'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keyup: Control ControlLeft 17 [Alt]');
     await keyboard.up('Alt');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
-      'Keyup: Alt AltLeft 18 []'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe('Keyup: Alt AltLeft 18 []');
   });
   it('should send proper codes while typing', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/keyboard.html');
     await page.keyboard.type('!');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe(
       [
         'Keydown: ! Digit1 49 []',
         'Keypress: ! Digit1 33 33 []',
@@ -228,7 +292,11 @@ describe('Keyboard', function () {
       ].join('\n')
     );
     await page.keyboard.type('^');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe(
       [
         'Keydown: ^ Digit6 54 []',
         'Keypress: ^ Digit6 94 94 []',
@@ -243,7 +311,11 @@ describe('Keyboard', function () {
     const keyboard = page.keyboard;
     await keyboard.down('Shift');
     await page.keyboard.type('~');
-    expect(await page.evaluate(() => globalThis.getResult())).toBe(
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toBe(
       [
         'Keydown: Shift ShiftLeft 16 [Shift]',
         'Keydown: ~ Backquote 192 [Shift]', // 192 is ` keyCode
@@ -275,33 +347,59 @@ describe('Keyboard', function () {
       );
     });
     await page.keyboard.type('Hello World!');
-    expect(await page.evaluate(() => globalThis.textarea.value)).toBe(
-      'He Wrd!'
-    );
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).textarea.value;
+      })
+    ).toBe('He Wrd!');
   });
   itFailsFirefox('should specify repeat property', async () => {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/textarea.html');
     await page.focus('textarea');
-    await page.evaluate(() =>
-      document
-        .querySelector('textarea')
-        .addEventListener('keydown', (e) => (globalThis.lastEvent = e), true)
-    );
+    await page.evaluate(() => {
+      return document.querySelector('textarea')!.addEventListener(
+        'keydown',
+        (e) => {
+          return ((globalThis as any).lastEvent = e);
+        },
+        true
+      );
+    });
     await page.keyboard.down('a');
-    expect(await page.evaluate(() => globalThis.lastEvent.repeat)).toBe(false);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).lastEvent.repeat;
+      })
+    ).toBe(false);
     await page.keyboard.press('a');
-    expect(await page.evaluate(() => globalThis.lastEvent.repeat)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).lastEvent.repeat;
+      })
+    ).toBe(true);
 
     await page.keyboard.down('b');
-    expect(await page.evaluate(() => globalThis.lastEvent.repeat)).toBe(false);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).lastEvent.repeat;
+      })
+    ).toBe(false);
     await page.keyboard.down('b');
-    expect(await page.evaluate(() => globalThis.lastEvent.repeat)).toBe(true);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).lastEvent.repeat;
+      })
+    ).toBe(true);
 
     await page.keyboard.up('a');
     await page.keyboard.down('a');
-    expect(await page.evaluate(() => globalThis.lastEvent.repeat)).toBe(false);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).lastEvent.repeat;
+      })
+    ).toBe(false);
   });
   itFailsFirefox('should type all kinds of characters', async () => {
     const { page, server } = getTestState();
@@ -319,11 +417,13 @@ describe('Keyboard', function () {
     await page.evaluate(() => {
       window.addEventListener(
         'keydown',
-        (event) => (globalThis.keyLocation = event.location),
+        (event) => {
+          return ((globalThis as any).keyLocation = event.location);
+        },
         true
       );
     });
-    const textarea = await page.$('textarea');
+    const textarea = (await page.$('textarea'))!;
 
     await textarea.press('Digit5');
     expect(await page.evaluate('keyLocation')).toBe(0);
@@ -343,15 +443,21 @@ describe('Keyboard', function () {
     let error = await page.keyboard
       // @ts-expect-error bad input
       .press('NotARealKey')
-      .catch((error_) => error_);
+      .catch((error_) => {
+        return error_;
+      });
     expect(error.message).toBe('Unknown key: "NotARealKey"');
 
     // @ts-expect-error bad input
-    error = await page.keyboard.press('ё').catch((error_) => error_);
+    error = await page.keyboard.press('ё').catch((error_) => {
+      return error_;
+    });
     expect(error && error.message).toBe('Unknown key: "ё"');
 
     // @ts-expect-error bad input
-    error = await page.keyboard.press('😊').catch((error_) => error_);
+    error = await page.keyboard.press('😊').catch((error_) => {
+      return error_;
+    });
     expect(error && error.message).toBe('Unknown key: "😊"');
   });
   itFailsFirefox('should type emoji', async () => {
@@ -360,10 +466,9 @@ describe('Keyboard', function () {
     await page.goto(server.PREFIX + '/input/textarea.html');
     await page.type('textarea', '👹 Tokyo street Japan 🇯🇵');
     expect(
-      await page.$eval(
-        'textarea',
-        (textarea: HTMLInputElement) => textarea.value
-      )
+      await page.$eval('textarea', (textarea) => {
+        return (textarea as HTMLInputElement).value;
+      })
     ).toBe('👹 Tokyo street Japan 🇯🇵');
   });
   itFailsFirefox('should type emoji into an iframe', async () => {
@@ -375,23 +480,22 @@ describe('Keyboard', function () {
       'emoji-test',
       server.PREFIX + '/input/textarea.html'
     );
-    const frame = page.frames()[1];
-    const textarea = await frame.$('textarea');
+    const frame = page.frames()[1]!;
+    const textarea = (await frame.$('textarea'))!;
     await textarea.type('👹 Tokyo street Japan 🇯🇵');
     expect(
-      await frame.$eval(
-        'textarea',
-        (textarea: HTMLInputElement) => textarea.value
-      )
+      await frame.$eval('textarea', (textarea) => {
+        return (textarea as HTMLInputElement).value;
+      })
     ).toBe('👹 Tokyo street Japan 🇯🇵');
   });
   itFailsFirefox('should press the meta key', async () => {
     const { page, isFirefox } = getTestState();
 
     await page.evaluate(() => {
-      globalThis.result = null;
+      (globalThis as any).result = null;
       document.addEventListener('keydown', (event) => {
-        globalThis.result = [event.key, event.code, event.metaKey];
+        (globalThis as any).result = [event.key, event.code, event.metaKey];
       });
     });
     await page.keyboard.press('Meta');
diff --git a/test/src/launcher.spec.ts b/test/src/launcher.spec.ts
index e01a5d7015bca..13620065c8e21 100644
--- a/test/src/launcher.spec.ts
+++ b/test/src/launcher.spec.ts
@@ -13,24 +13,25 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import Protocol from 'devtools-protocol';
+import expect from 'expect';
 import fs from 'fs';
 import os from 'os';
 import path from 'path';
+import rimraf from 'rimraf';
 import sinon from 'sinon';
+import { TLSSocket } from 'tls';
 import { promisify } from 'util';
-import Protocol from 'devtools-protocol';
+import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
+import { Product } from '../../lib/cjs/puppeteer/common/Product.js';
 import {
   getTestState,
   itChromeOnly,
   itFailsFirefox,
   itFirefoxOnly,
   itOnlyRegularInstall,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import utils from './utils.js';
-import expect from 'expect';
-import rimraf from 'rimraf';
-import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
-import { TLSSocket } from 'tls';
 
 const mkdtempAsync = promisify(fs.mkdtemp);
 const readFileAsync = promisify(fs.readFile);
@@ -73,7 +74,7 @@ describe('Launcher specs', function () {
         expect(await browserFetcher.canDownload('100000')).toBe(false);
         expect(await browserFetcher.canDownload(expectedRevision)).toBe(true);
 
-        revisionInfo = await browserFetcher.download(expectedRevision);
+        revisionInfo = (await browserFetcher.download(expectedRevision))!;
         expect(revisionInfo.local).toBe(true);
         expect(await readFileAsync(revisionInfo.executablePath, 'utf8')).toBe(
           'LINUX BINARY\n'
@@ -118,7 +119,7 @@ describe('Launcher specs', function () {
         expect(await browserFetcher.canDownload('100000')).toBe(false);
         expect(await browserFetcher.canDownload(expectedVersion)).toBe(true);
 
-        revisionInfo = await browserFetcher.download(expectedVersion);
+        revisionInfo = (await browserFetcher.download(expectedVersion))!;
         expect(revisionInfo.local).toBe(true);
         expect(await readFileAsync(revisionInfo.executablePath, 'utf8')).toBe(
           'FIREFOX LINUX BINARY\n'
@@ -147,7 +148,9 @@ describe('Launcher specs', function () {
         const page = await remote.newPage();
         const navigationPromise = page
           .goto(server.PREFIX + '/one-style.html', { timeout: 60000 })
-          .catch((error_) => error_);
+          .catch((error_) => {
+            return error_;
+          });
         await server.waitForRequest('/one-style.css');
         remote.disconnect();
         const error = await navigationPromise;
@@ -170,7 +173,9 @@ describe('Launcher specs', function () {
         const page = await remote.newPage();
         const watchdog = page
           .waitForSelector('div', { timeout: 60000 })
-          .catch((error_) => error_);
+          .catch((error_) => {
+            return error_;
+          });
         remote.disconnect();
         const error = await watchdog;
         expect(error.message).toContain('Protocol error');
@@ -187,8 +192,12 @@ describe('Launcher specs', function () {
         });
         const newPage = await remote.newPage();
         const results = await Promise.all([
-          newPage.waitForRequest(server.EMPTY_PAGE).catch((error) => error),
-          newPage.waitForResponse(server.EMPTY_PAGE).catch((error) => error),
+          newPage.waitForRequest(server.EMPTY_PAGE).catch((error) => {
+            return error;
+          }),
+          newPage.waitForResponse(server.EMPTY_PAGE).catch((error) => {
+            return error;
+          }),
           browser.close(),
         ]);
         for (let i = 0; i < 2; i++) {
@@ -204,10 +213,14 @@ describe('Launcher specs', function () {
         const { defaultBrowserOptions, puppeteer } = getTestState();
         const browser = await puppeteer.launch(defaultBrowserOptions);
         const page = await browser.newPage();
-        let error = null;
+        let error!: Error;
         const neverResolves = page
-          .evaluate(() => new Promise(() => {}))
-          .catch((error_) => (error = error_));
+          .evaluate(() => {
+            return new Promise(() => {});
+          })
+          .catch((error_) => {
+            return (error = error_);
+          });
         await browser.close();
         await neverResolves;
         expect(error.message).toContain('Protocol error');
@@ -215,11 +228,13 @@ describe('Launcher specs', function () {
       it('should reject if executable path is invalid', async () => {
         const { defaultBrowserOptions, puppeteer } = getTestState();
 
-        let waitError = null;
+        let waitError!: Error;
         const options = Object.assign({}, defaultBrowserOptions, {
           executablePath: 'random-invalid-path',
         });
-        await puppeteer.launch(options).catch((error) => (waitError = error));
+        await puppeteer.launch(options).catch((error) => {
+          return (waitError = error);
+        });
         expect(waitError.message).toContain('Failed to launch');
       });
       it('userDataDir option', async () => {
@@ -315,13 +330,19 @@ describe('Launcher specs', function () {
         const browser = await puppeteer.launch(options);
         const page = await browser.newPage();
         await page.goto(server.EMPTY_PAGE);
-        await page.evaluate(() => (localStorage.hey = 'hello'));
+        await page.evaluate(() => {
+          return (localStorage['hey'] = 'hello');
+        });
         await browser.close();
 
         const browser2 = await puppeteer.launch(options);
         const page2 = await browser2.newPage();
         await page2.goto(server.EMPTY_PAGE);
-        expect(await page2.evaluate(() => localStorage.hey)).toBe('hello');
+        expect(
+          await page2.evaluate(() => {
+            return localStorage['hey'];
+          })
+        ).toBe('hello');
         await browser2.close();
         // This might throw. See https://github.com/puppeteer/puppeteer/issues/2778
         await rmAsync(userDataDir).catch(() => {});
@@ -336,19 +357,20 @@ describe('Launcher specs', function () {
         const browser = await puppeteer.launch(options);
         const page = await browser.newPage();
         await page.goto(server.EMPTY_PAGE);
-        await page.evaluate(
-          () =>
-            (document.cookie =
-              'doSomethingOnlyOnce=true; expires=Fri, 31 Dec 9999 23:59:59 GMT')
-        );
+        await page.evaluate(() => {
+          return (document.cookie =
+            'doSomethingOnlyOnce=true; expires=Fri, 31 Dec 9999 23:59:59 GMT');
+        });
         await browser.close();
 
         const browser2 = await puppeteer.launch(options);
         const page2 = await browser2.newPage();
         await page2.goto(server.EMPTY_PAGE);
-        expect(await page2.evaluate(() => document.cookie)).toBe(
-          'doSomethingOnlyOnce=true'
-        );
+        expect(
+          await page2.evaluate(() => {
+            return document.cookie;
+          })
+        ).toBe('doSomethingOnlyOnce=true');
         await browser2.close();
         // This might throw. See https://github.com/puppeteer/puppeteer/issues/2778
         await rmAsync(userDataDir).catch(() => {});
@@ -422,16 +444,16 @@ describe('Launcher specs', function () {
           const browser = await puppeteer.launch(
             Object.assign({}, defaultBrowserOptions, {
               // Ignore first and third default argument.
-              ignoreDefaultArgs: [defaultArgs[0], defaultArgs[2]],
+              ignoreDefaultArgs: [defaultArgs[0]!, defaultArgs[2]],
             })
           );
-          const spawnargs = browser.process().spawnargs;
+          const spawnargs = browser.process()!.spawnargs;
           if (!spawnargs) {
             throw new Error('spawnargs not present');
           }
-          expect(spawnargs.indexOf(defaultArgs[0])).toBe(-1);
-          expect(spawnargs.indexOf(defaultArgs[1])).not.toBe(-1);
-          expect(spawnargs.indexOf(defaultArgs[2])).toBe(-1);
+          expect(spawnargs.indexOf(defaultArgs[0]!)).toBe(-1);
+          expect(spawnargs.indexOf(defaultArgs[1]!)).not.toBe(-1);
+          expect(spawnargs.indexOf(defaultArgs[2]!)).toBe(-1);
           await browser.close();
         }
       );
@@ -444,22 +466,24 @@ describe('Launcher specs', function () {
           const browser = await puppeteer.launch(
             Object.assign({}, defaultBrowserOptions, {
               // Only the first argument is fixed, others are optional.
-              ignoreDefaultArgs: [defaultArgs[0]],
+              ignoreDefaultArgs: [defaultArgs[0]!],
             })
           );
-          const spawnargs = browser.process().spawnargs;
+          const spawnargs = browser.process()!.spawnargs;
           if (!spawnargs) {
             throw new Error('spawnargs not present');
           }
-          expect(spawnargs.indexOf(defaultArgs[0])).toBe(-1);
-          expect(spawnargs.indexOf(defaultArgs[1])).not.toBe(-1);
+          expect(spawnargs.indexOf(defaultArgs[0]!)).toBe(-1);
+          expect(spawnargs.indexOf(defaultArgs[1]!)).not.toBe(-1);
           await browser.close();
         }
       );
       it('should have default URL when launching browser', async function () {
         const { defaultBrowserOptions, puppeteer } = getTestState();
         const browser = await puppeteer.launch(defaultBrowserOptions);
-        const pages = (await browser.pages()).map((page) => page.url());
+        const pages = (await browser.pages()).map((page) => {
+          return page.url();
+        });
         expect(pages).toEqual(['about:blank']);
         await browser.close();
       });
@@ -473,7 +497,7 @@ describe('Launcher specs', function () {
           const browser = await puppeteer.launch(options);
           const pages = await browser.pages();
           expect(pages.length).toBe(1);
-          const page = pages[0];
+          const page = pages[0]!;
           if (page.url() !== server.EMPTY_PAGE) {
             await page.waitForNavigation();
           }
@@ -486,8 +510,10 @@ describe('Launcher specs', function () {
         const options = Object.assign({}, defaultBrowserOptions, {
           timeout: 1,
         });
-        let error = null;
-        await puppeteer.launch(options).catch((error_) => (error = error_));
+        let error!: Error;
+        await puppeteer.launch(options).catch((error_) => {
+          return (error = error_);
+        });
         expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
       });
       it('should set the default viewport', async () => {
@@ -550,8 +576,10 @@ describe('Launcher specs', function () {
           pipe: true,
         });
 
-        let error = null;
-        await puppeteer.launch(options).catch((error_) => (error = error_));
+        let error!: Error;
+        await puppeteer.launch(options).catch((error_) => {
+          return (error = error_);
+        });
         expect(error.message).toContain('either pipe or debugging port');
       });
       itChromeOnly(
@@ -575,11 +603,11 @@ describe('Launcher specs', function () {
     });
 
     describe('Puppeteer.launch', function () {
-      let productName;
+      let productName!: Product;
 
       before(async () => {
         const { puppeteer } = getTestState();
-        productName = puppeteer._productName;
+        productName = puppeteer._productName!;
       });
 
       after(async () => {
@@ -639,11 +667,19 @@ describe('Launcher specs', function () {
           browserWSEndpoint: originalBrowser.wsEndpoint(),
         });
         const page = await otherBrowser.newPage();
-        expect(await page.evaluate(() => 7 * 8)).toBe(56);
+        expect(
+          await page.evaluate(() => {
+            return 7 * 8;
+          })
+        ).toBe(56);
         otherBrowser.disconnect();
 
         const secondPage = await originalBrowser.newPage();
-        expect(await secondPage.evaluate(() => 7 * 6)).toBe(42);
+        expect(
+          await secondPage.evaluate(() => {
+            return 7 * 6;
+          })
+        ).toBe(42);
         await originalBrowser.close();
       });
       it('should be able to close remote browser', async () => {
@@ -670,16 +706,18 @@ describe('Launcher specs', function () {
           ignoreHTTPSErrors: true,
         });
         const page = await browser.newPage();
-        let error = null;
+        let error!: Error;
         const [serverRequest, response] = await Promise.all([
           httpsServer.waitForRequest('/empty.html'),
-          page.goto(httpsServer.EMPTY_PAGE).catch((error_) => (error = error_)),
+          page.goto(httpsServer.EMPTY_PAGE).catch((error_) => {
+            return (error = error_);
+          }),
         ]);
-        expect(error).toBe(null);
+        expect(error).toBeUndefined();
         expect(response.ok()).toBe(true);
         expect(response.securityDetails()).toBeTruthy();
         const protocol = (serverRequest.socket as TLSSocket)
-          .getProtocol()
+          .getProtocol()!
           .replace('v', ' ');
         expect(response.securityDetails().protocol()).toBe(protocol);
         await page.close();
@@ -700,8 +738,9 @@ describe('Launcher specs', function () {
 
         const browser = await puppeteer.connect({
           browserWSEndpoint,
-          targetFilter: (targetInfo: Protocol.Target.TargetInfo) =>
-            !targetInfo.url?.includes('should-be-ignored'),
+          targetFilter: (targetInfo: Protocol.Target.TargetInfo) => {
+            return !targetInfo.url?.includes('should-be-ignored');
+          },
         });
 
         const pages = await browser.pages();
@@ -711,10 +750,13 @@ describe('Launcher specs', function () {
         await browser.disconnect();
         await originalBrowser.close();
 
-        expect(pages.map((p: Page) => p.url()).sort()).toEqual([
-          'about:blank',
-          server.EMPTY_PAGE,
-        ]);
+        expect(
+          pages
+            .map((p: Page) => {
+              return p.url();
+            })
+            .sort()
+        ).toEqual(['about:blank', server.EMPTY_PAGE]);
       });
       itFailsFirefox(
         'should be able to reconnect to a disconnected browser',
@@ -729,10 +771,9 @@ describe('Launcher specs', function () {
 
           const browser = await puppeteer.connect({ browserWSEndpoint });
           const pages = await browser.pages();
-          const restoredPage = pages.find(
-            (page) =>
-              page.url() === server.PREFIX + '/frames/nested-frames.html'
-          );
+          const restoredPage = pages.find((page) => {
+            return page.url() === server.PREFIX + '/frames/nested-frames.html';
+          })!;
           expect(utils.dumpFrames(restoredPage.mainFrame())).toEqual([
             'http://localhost:<PORT>/frames/nested-frames.html',
             '    http://localhost:<PORT>/frames/two-frames.html (2frames)',
@@ -740,7 +781,11 @@ describe('Launcher specs', function () {
             '        http://localhost:<PORT>/frames/frame.html (dos)',
             '    http://localhost:<PORT>/frames/frame.html (aframe)',
           ]);
-          expect(await restoredPage.evaluate(() => 7 * 8)).toBe(56);
+          expect(
+            await restoredPage.evaluate(() => {
+              return 7 * 8;
+            })
+          ).toBe(56);
           await browser.close();
         }
       );
@@ -755,13 +800,23 @@ describe('Launcher specs', function () {
             browserWSEndpoint: browserOne.wsEndpoint(),
           });
           const [page1, page2] = await Promise.all([
-            new Promise<Page>((x) =>
-              browserOne.once('targetcreated', (target) => x(target.page()))
-            ),
+            new Promise<Page>((x) => {
+              return browserOne.once('targetcreated', (target) => {
+                return x(target.page());
+              });
+            }),
             browserTwo.newPage(),
           ]);
-          expect(await page1.evaluate(() => 7 * 8)).toBe(56);
-          expect(await page2.evaluate(() => 7 * 6)).toBe(42);
+          expect(
+            await page1.evaluate(() => {
+              return 7 * 8;
+            })
+          ).toBe(56);
+          expect(
+            await page2.evaluate(() => {
+              return 7 * 6;
+            })
+          ).toBe(42);
           await browserOne.close();
         }
       );
@@ -775,12 +830,14 @@ describe('Launcher specs', function () {
 
         const browserTwo = await puppeteer.connect({ browserWSEndpoint });
         const pages = await browserTwo.pages();
-        const pageTwo = pages.find((page) => page.url() === server.EMPTY_PAGE);
+        const pageTwo = pages.find((page) => {
+          return page.url() === server.EMPTY_PAGE;
+        })!;
         await pageTwo.reload();
         const bodyHandle = await pageTwo.waitForSelector('body', {
           timeout: 10000,
         });
-        await bodyHandle.dispose();
+        await bodyHandle!.dispose();
         await browserTwo.close();
       });
     });
@@ -802,13 +859,15 @@ describe('Launcher specs', function () {
         const sandbox = sinon.createSandbox();
 
         beforeEach(() => {
-          process.env.PUPPETEER_EXECUTABLE_PATH = '';
+          process.env['PUPPETEER_EXECUTABLE_PATH'] = '';
           sandbox
             .stub(process.env, 'PUPPETEER_EXECUTABLE_PATH')
             .value('SOME_CUSTOM_EXECUTABLE');
         });
 
-        afterEach(() => sandbox.restore());
+        afterEach(() => {
+          return sandbox.restore();
+        });
 
         it('its value is returned', async () => {
           const { puppeteer } = getTestState();
@@ -840,13 +899,15 @@ describe('Launcher specs', function () {
             const sandbox = sinon.createSandbox();
 
             beforeEach(() => {
-              process.env.PUPPETEER_EXECUTABLE_PATH = '';
+              process.env['PUPPETEER_EXECUTABLE_PATH'] = '';
               sandbox
                 .stub(process.env, 'PUPPETEER_EXECUTABLE_PATH')
                 .value('SOME_CUSTOM_EXECUTABLE');
             });
 
-            afterEach(() => sandbox.restore());
+            afterEach(() => {
+              return sandbox.restore();
+            });
 
             it('its value is returned', async () => {
               const { puppeteer } = getTestState();
@@ -888,10 +949,16 @@ describe('Launcher specs', function () {
       const { server, puppeteer, defaultBrowserOptions } = getTestState();
 
       const browser = await puppeteer.launch(defaultBrowserOptions);
-      const events = [];
-      browser.on('targetcreated', () => events.push('CREATED'));
-      browser.on('targetchanged', () => events.push('CHANGED'));
-      browser.on('targetdestroyed', () => events.push('DESTROYED'));
+      const events: string[] = [];
+      browser.on('targetcreated', () => {
+        return events.push('CREATED');
+      });
+      browser.on('targetchanged', () => {
+        return events.push('CHANGED');
+      });
+      browser.on('targetdestroyed', () => {
+        return events.push('DESTROYED');
+      });
       const page = await browser.newPage();
       await page.goto(server.EMPTY_PAGE);
       await page.close();
@@ -911,9 +978,15 @@ describe('Launcher specs', function () {
       let disconnectedOriginal = 0;
       let disconnectedRemote1 = 0;
       let disconnectedRemote2 = 0;
-      originalBrowser.on('disconnected', () => ++disconnectedOriginal);
-      remoteBrowser1.on('disconnected', () => ++disconnectedRemote1);
-      remoteBrowser2.on('disconnected', () => ++disconnectedRemote2);
+      originalBrowser.on('disconnected', () => {
+        return ++disconnectedOriginal;
+      });
+      remoteBrowser1.on('disconnected', () => {
+        return ++disconnectedRemote1;
+      });
+      remoteBrowser2.on('disconnected', () => {
+        return ++disconnectedRemote2;
+      });
 
       await Promise.all([
         utils.waitEvent(remoteBrowser2, 'disconnected'),
diff --git a/test/src/mocha-utils.ts b/test/src/mocha-utils.ts
index 8e92f2e038590..e23837e6b1eba 100644
--- a/test/src/mocha-utils.ts
+++ b/test/src/mocha-utils.ts
@@ -25,12 +25,15 @@ import {
   Browser,
   BrowserContext,
 } from '../../lib/cjs/puppeteer/common/Browser.js';
-import { isErrorLike } from '../../lib/cjs/puppeteer/common/util.js';
 import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
-import { PuppeteerNode } from '../../lib/cjs/puppeteer/node/Puppeteer.js';
+import { isErrorLike } from '../../lib/cjs/puppeteer/common/util.js';
+import {
+  PuppeteerLaunchOptions,
+  PuppeteerNode,
+} from '../../lib/cjs/puppeteer/node/Puppeteer.js';
 import puppeteer from '../../lib/cjs/puppeteer/puppeteer.js';
-import utils from './utils.js';
 import { TestServer } from '../../utils/testserver/lib/index.js';
+import { extendExpectWithToBeGolden } from './utils.js';
 
 const setupServer = async () => {
   const assetsPath = path.join(__dirname, '../assets');
@@ -55,8 +58,9 @@ const setupServer = async () => {
   return { server, httpsServer };
 };
 
-export const getTestState = (): PuppeteerTestState =>
-  state as PuppeteerTestState;
+export const getTestState = (): PuppeteerTestState => {
+  return state as PuppeteerTestState;
+};
 
 const product =
   process.env['PRODUCT'] || process.env['PUPPETEER_PRODUCT'] || 'Chromium';
@@ -127,7 +131,7 @@ const setupGoldenAssertions = (): void => {
   if (fs.existsSync(OUTPUT_DIR)) {
     rimraf.sync(OUTPUT_DIR);
   }
-  utils.extendExpectWithToBeGolden(GOLDEN_DIR, OUTPUT_DIR);
+  extendExpectWithToBeGolden(GOLDEN_DIR, OUTPUT_DIR);
 };
 
 setupGoldenAssertions();
@@ -137,9 +141,7 @@ interface PuppeteerTestState {
   context: BrowserContext;
   page: Page;
   puppeteer: PuppeteerNode;
-  defaultBrowserOptions: {
-    [x: string]: any;
-  };
+  defaultBrowserOptions: PuppeteerLaunchOptions;
   server: TestServer;
   httpsServer: TestServer;
   isFirefox: boolean;
@@ -304,16 +306,16 @@ export const mochaHooks = {
   ],
 
   beforeEach: async (): Promise<void> => {
-    state.server.reset();
-    state.httpsServer.reset();
+    state.server!.reset();
+    state.httpsServer!.reset();
   },
 
   afterAll: [
     async (): Promise<void> => {
-      await state.server.stop();
-      state.server = null;
-      await state.httpsServer.stop();
-      state.httpsServer = null;
+      await state.server!.stop();
+      state.server = undefined;
+      await state.httpsServer!.stop();
+      state.httpsServer = undefined;
     },
   ],
 
@@ -357,6 +359,8 @@ export const shortWaitForArrayToHaveAtLeastNElements = async (
     if (data.length >= minLength) {
       break;
     }
-    await new Promise((resolve) => setTimeout(resolve, timeout));
+    await new Promise((resolve) => {
+      return setTimeout(resolve, timeout);
+    });
   }
 };
diff --git a/test/src/mouse.spec.ts b/test/src/mouse.spec.ts
index 289954ba021cf..1c0073aadd0a4 100644
--- a/test/src/mouse.spec.ts
+++ b/test/src/mouse.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import { KeyInput } from '../../lib/cjs/puppeteer/common/USKeyboardLayout.js';
 
 interface Dimensions {
@@ -31,7 +31,7 @@ interface Dimensions {
 }
 
 function dimensions(): Dimensions {
-  const rect = document.querySelector('textarea').getBoundingClientRect();
+  const rect = document.querySelector('textarea')!.getBoundingClientRect();
   return {
     x: rect.left,
     y: rect.top,
@@ -47,7 +47,7 @@ describe('Mouse', function () {
     const { page } = getTestState();
 
     await page.evaluate(() => {
-      globalThis.clickPromise = new Promise((resolve) => {
+      (globalThis as any).clickPromise = new Promise((resolve) => {
         document.addEventListener('click', (event) => {
           resolve({
             type: event.type,
@@ -61,9 +61,9 @@ describe('Mouse', function () {
       });
     });
     await page.mouse.click(50, 60);
-    const event = await page.evaluate<() => MouseEvent>(
-      () => globalThis.clickPromise
-    );
+    const event = await page.evaluate<() => MouseEvent>(() => {
+      return (globalThis as any).clickPromise;
+    });
     expect(event.type).toBe('click');
     expect(event.detail).toBe(1);
     expect(event.clientX).toBe(50);
@@ -96,10 +96,12 @@ describe('Mouse', function () {
       "This is the text that we are going to try to select. Let's see how it goes.";
     await page.keyboard.type(text);
     // Firefox needs an extra frame here after typing or it will fail to set the scrollTop
-    await page.evaluate(() => new Promise(requestAnimationFrame));
-    await page.evaluate(
-      () => (document.querySelector('textarea').scrollTop = 0)
-    );
+    await page.evaluate(() => {
+      return new Promise(requestAnimationFrame);
+    });
+    await page.evaluate(() => {
+      return (document.querySelector('textarea')!.scrollTop = 0);
+    });
     const { x, y } = await page.evaluate(dimensions);
     await page.mouse.move(x + 2, y + 2);
     await page.mouse.down();
@@ -107,7 +109,7 @@ describe('Mouse', function () {
     await page.mouse.up();
     expect(
       await page.evaluate(() => {
-        const textarea = document.querySelector('textarea');
+        const textarea = document.querySelector('textarea')!;
         return textarea.value.substring(
           textarea.selectionStart,
           textarea.selectionEnd
@@ -121,15 +123,21 @@ describe('Mouse', function () {
     await page.goto(server.PREFIX + '/input/scrollable.html');
     await page.hover('#button-6');
     expect(
-      await page.evaluate(() => document.querySelector('button:hover').id)
+      await page.evaluate(() => {
+        return document.querySelector('button:hover')!.id;
+      })
     ).toBe('button-6');
     await page.hover('#button-2');
     expect(
-      await page.evaluate(() => document.querySelector('button:hover').id)
+      await page.evaluate(() => {
+        return document.querySelector('button:hover')!.id;
+      })
     ).toBe('button-2');
     await page.hover('#button-91');
     expect(
-      await page.evaluate(() => document.querySelector('button:hover').id)
+      await page.evaluate(() => {
+        return document.querySelector('button:hover')!.id;
+      })
     ).toBe('button-91');
   });
   itFailsFirefox(
@@ -138,10 +146,15 @@ describe('Mouse', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/scrollable.html');
-      await page.evaluate(() => delete window.Node);
+      await page.evaluate(() => {
+        // @ts-expect-error Expected.
+        return delete window.Node;
+      });
       await page.hover('#button-6');
       expect(
-        await page.evaluate(() => document.querySelector('button:hover').id)
+        await page.evaluate(() => {
+          return document.querySelector('button:hover')!.id;
+        })
       ).toBe('button-6');
     }
   );
@@ -149,11 +162,15 @@ describe('Mouse', function () {
     const { page, server, isFirefox } = getTestState();
 
     await page.goto(server.PREFIX + '/input/scrollable.html');
-    await page.evaluate(() =>
-      document
-        .querySelector('#button-3')
-        .addEventListener('mousedown', (e) => (globalThis.lastEvent = e), true)
-    );
+    await page.evaluate(() => {
+      return document.querySelector('#button-3')!.addEventListener(
+        'mousedown',
+        (e) => {
+          return ((globalThis as any).lastEvent = e);
+        },
+        true
+      );
+    });
     const modifiers = new Map<KeyInput, string>([
       ['Shift', 'shiftKey'],
       ['Control', 'ctrlKey'],
@@ -162,13 +179,15 @@ describe('Mouse', function () {
     ]);
     // In Firefox, the Meta modifier only exists on Mac
     if (isFirefox && os.platform() !== 'darwin') {
-      delete modifiers['Meta'];
+      modifiers.delete('Meta');
     }
     for (const [modifier, key] of modifiers) {
       await page.keyboard.down(modifier);
       await page.click('#button-3');
       if (
-        !(await page.evaluate((mod: string) => globalThis.lastEvent[mod], key))
+        !(await page.evaluate((mod: string) => {
+          return (globalThis as any).lastEvent[mod];
+        }, key))
       ) {
         throw new Error(key + ' should be true');
       }
@@ -177,9 +196,11 @@ describe('Mouse', function () {
     await page.click('#button-3');
     for (const [modifier, key] of modifiers) {
       if (
-        await page.evaluate((mod: string) => globalThis.lastEvent[mod], key)
+        await page.evaluate((mod: string) => {
+          return (globalThis as any).lastEvent[mod];
+        }, key)
       ) {
-        throw new Error(modifiers[modifier] + ' should be false');
+        throw new Error(modifiers.get(modifier) + ' should be false');
       }
     }
   });
@@ -187,8 +208,8 @@ describe('Mouse', function () {
     const { page, server } = getTestState();
 
     await page.goto(server.PREFIX + '/input/wheel.html');
-    const elem = await page.$('div');
-    const boundingBoxBefore = await elem.boundingBox();
+    const elem = (await page.$('div'))!;
+    const boundingBoxBefore = (await elem.boundingBox())!;
     expect(boundingBoxBefore).toMatchObject({
       width: 115,
       height: 115,
@@ -211,9 +232,9 @@ describe('Mouse', function () {
 
     await page.mouse.move(100, 100);
     await page.evaluate(() => {
-      globalThis.result = [];
+      (globalThis as any).result = [];
       document.addEventListener('mousemove', (event) => {
-        globalThis.result.push([event.clientX, event.clientY]);
+        (globalThis as any).result.push([event.clientX, event.clientY]);
       });
     });
     await page.mouse.move(200, 300, { steps: 5 });
@@ -234,7 +255,7 @@ describe('Mouse', function () {
     await page.goto(server.CROSS_PROCESS_PREFIX + '/mobile.html');
     await page.evaluate(() => {
       document.addEventListener('click', (event) => {
-        globalThis.result = { x: event.clientX, y: event.clientY };
+        (globalThis as any).result = { x: event.clientX, y: event.clientY };
       });
     });
 
diff --git a/test/src/navigation.spec.ts b/test/src/navigation.spec.ts
index 50a482c3a9e95..c31626babcf06 100644
--- a/test/src/navigation.spec.ts
+++ b/test/src/navigation.spec.ts
@@ -22,8 +22,10 @@ import {
   setupTestPageAndContextHooks,
   itFailsFirefox,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import os from 'os';
+import { ServerResponse } from 'http';
+import { HTTPRequest } from '../../lib/cjs/puppeteer/common/HTTPRequest.js';
 
 describe('navigation', function () {
   setupTestBrowserHooks();
@@ -62,31 +64,35 @@ describe('navigation', function () {
     it('should return response when page changes its URL after load', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/historyapi.html');
+      const response = (await page.goto(server.PREFIX + '/historyapi.html'))!;
       expect(response.status()).toBe(200);
     });
     itFailsFirefox('should work with subframes return 204', async () => {
       const { page, server } = getTestState();
 
-      server.setRoute('/frames/frame.html', (req, res) => {
+      server.setRoute('/frames/frame.html', (_req, res) => {
         res.statusCode = 204;
         res.end();
       });
-      let error = null;
+      let error!: Error;
       await page
         .goto(server.PREFIX + '/frames/one-frame.html')
-        .catch((error_) => (error = error_));
-      expect(error).toBe(null);
+        .catch((error_) => {
+          return (error = error_);
+        });
+      expect(error).toBeUndefined();
     });
     itFailsFirefox('should fail when server returns 204', async () => {
       const { page, server, isChrome } = getTestState();
 
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.statusCode = 204;
         res.end();
       });
-      let error = null;
-      await page.goto(server.EMPTY_PAGE).catch((error_) => (error = error_));
+      let error!: Error;
+      await page.goto(server.EMPTY_PAGE).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).not.toBe(null);
       if (isChrome) {
         expect(error.message).toContain('net::ERR_ABORTED');
@@ -100,7 +106,7 @@ describe('navigation', function () {
       const response = await page.goto(server.EMPTY_PAGE, {
         waitUntil: 'domcontentloaded',
       });
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
     });
     it('should work when page calls history API in beforeunload', async () => {
       const { page, server } = getTestState();
@@ -109,12 +115,14 @@ describe('navigation', function () {
       await page.evaluate(() => {
         window.addEventListener(
           'beforeunload',
-          () => history.replaceState(null, 'initial', window.location.href),
+          () => {
+            return history.replaceState(null, 'initial', window.location.href);
+          },
           false
         );
       });
       const response = await page.goto(server.PREFIX + '/grid.html');
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
     });
     itFailsFirefox(
       'should navigate to empty page with networkidle0',
@@ -124,7 +132,7 @@ describe('navigation', function () {
         const response = await page.goto(server.EMPTY_PAGE, {
           waitUntil: 'networkidle0',
         });
-        expect(response.status()).toBe(200);
+        expect(response!.status()).toBe(200);
       }
     );
     itFailsFirefox(
@@ -135,14 +143,16 @@ describe('navigation', function () {
         const response = await page.goto(server.EMPTY_PAGE, {
           waitUntil: 'networkidle2',
         });
-        expect(response.status()).toBe(200);
+        expect(response!.status()).toBe(200);
       }
     );
     itFailsFirefox('should fail when navigating to bad url', async () => {
       const { page, isChrome } = getTestState();
 
-      let error = null;
-      await page.goto('asdfasdf').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.goto('asdfasdf').catch((error_) => {
+        return (error = error_);
+      });
       if (isChrome) {
         expect(error.message).toContain('Cannot navigate to invalid URL');
       } else {
@@ -165,15 +175,21 @@ describe('navigation', function () {
 
       // Make sure that network events do not emit 'undefined'.
       // @see https://crbug.com/750469
-      const requests = [];
-      page.on('request', () => requests.push('request'));
-      page.on('requestfinished', () => requests.push('requestfinished'));
-      page.on('requestfailed', () => requests.push('requestfailed'));
+      const requests: string[] = [];
+      page.on('request', () => {
+        return requests.push('request');
+      });
+      page.on('requestfinished', () => {
+        return requests.push('requestfinished');
+      });
+      page.on('requestfailed', () => {
+        return requests.push('requestfailed');
+      });
 
-      let error = null;
-      await page
-        .goto(httpsServer.EMPTY_PAGE)
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await page.goto(httpsServer.EMPTY_PAGE).catch((error_) => {
+        return (error = error_);
+      });
       if (isChrome) {
         expect(error.message).toContain(EXPECTED_SSL_CERT_MESSAGE);
       } else {
@@ -181,18 +197,20 @@ describe('navigation', function () {
       }
 
       expect(requests.length).toBe(2);
-      expect(requests[0]).toBe('request');
-      expect(requests[1]).toBe('requestfailed');
+      expect(requests[0]!).toBe('request');
+      expect(requests[1]!).toBe('requestfailed');
     });
     it('should fail when navigating to bad SSL after redirects', async () => {
       const { page, server, httpsServer, isChrome } = getTestState();
 
       server.setRedirect('/redirect/1.html', '/redirect/2.html');
       server.setRedirect('/redirect/2.html', '/empty.html');
-      let error = null;
+      let error!: Error;
       await page
         .goto(httpsServer.PREFIX + '/redirect/1.html')
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       if (isChrome) {
         expect(error.message).toContain(EXPECTED_SSL_CERT_MESSAGE);
       } else {
@@ -202,11 +220,13 @@ describe('navigation', function () {
     it('should throw if networkidle is passed as an option', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         // @ts-expect-error purposefully passing an old option
         .goto(server.EMPTY_PAGE, { waitUntil: 'networkidle' })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain(
         '"networkidle" option is no longer supported'
       );
@@ -214,10 +234,12 @@ describe('navigation', function () {
     it('should fail when main resources failed to load', async () => {
       const { page, isChrome } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
         .goto('http://localhost:44123/non-existing-url')
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       if (isChrome) {
         expect(error.message).toContain('net::ERR_CONNECTION_REFUSED');
       } else {
@@ -229,10 +251,12 @@ describe('navigation', function () {
 
       // Hang for request to the empty.html
       server.setRoute('/empty.html', () => {});
-      let error = null;
+      let error!: Error;
       await page
         .goto(server.PREFIX + '/empty.html', { timeout: 1 })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain('Navigation timeout of 1 ms exceeded');
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
@@ -241,11 +265,11 @@ describe('navigation', function () {
 
       // Hang for request to the empty.html
       server.setRoute('/empty.html', () => {});
-      let error = null;
+      let error!: Error;
       page.setDefaultNavigationTimeout(1);
-      await page
-        .goto(server.PREFIX + '/empty.html')
-        .catch((error_) => (error = error_));
+      await page.goto(server.PREFIX + '/empty.html').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain('Navigation timeout of 1 ms exceeded');
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
@@ -254,11 +278,11 @@ describe('navigation', function () {
 
       // Hang for request to the empty.html
       server.setRoute('/empty.html', () => {});
-      let error = null;
+      let error!: Error;
       page.setDefaultTimeout(1);
-      await page
-        .goto(server.PREFIX + '/empty.html')
-        .catch((error_) => (error = error_));
+      await page.goto(server.PREFIX + '/empty.html').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain('Navigation timeout of 1 ms exceeded');
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
@@ -267,43 +291,47 @@ describe('navigation', function () {
 
       // Hang for request to the empty.html
       server.setRoute('/empty.html', () => {});
-      let error = null;
+      let error!: Error;
       page.setDefaultTimeout(0);
       page.setDefaultNavigationTimeout(1);
-      await page
-        .goto(server.PREFIX + '/empty.html')
-        .catch((error_) => (error = error_));
+      await page.goto(server.PREFIX + '/empty.html').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain('Navigation timeout of 1 ms exceeded');
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should disable timeout when its set to 0', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       let loaded = false;
-      page.once('load', () => (loaded = true));
+      page.once('load', () => {
+        return (loaded = true);
+      });
       await page
         .goto(server.PREFIX + '/grid.html', { timeout: 0, waitUntil: ['load'] })
-        .catch((error_) => (error = error_));
-      expect(error).toBe(null);
+        .catch((error_) => {
+          return (error = error_);
+        });
+      expect(error).toBeUndefined();
       expect(loaded).toBe(true);
     });
     it('should work when navigating to valid url', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.ok()).toBe(true);
     });
     itFailsFirefox('should work when navigating to data url', async () => {
       const { page } = getTestState();
 
-      const response = await page.goto('data:text/html,hello');
+      const response = (await page.goto('data:text/html,hello'))!;
       expect(response.ok()).toBe(true);
     });
     it('should work when navigating to 404', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/not-found');
+      const response = (await page.goto(server.PREFIX + '/not-found'))!;
       expect(response.ok()).toBe(false);
       expect(response.status()).toBe(404);
     });
@@ -313,7 +341,7 @@ describe('navigation', function () {
       server.setRedirect('/redirect/1.html', '/redirect/2.html');
       server.setRedirect('/redirect/2.html', '/redirect/3.html');
       server.setRedirect('/redirect/3.html', server.EMPTY_PAGE);
-      const response = await page.goto(server.PREFIX + '/redirect/1.html');
+      const response = (await page.goto(server.PREFIX + '/redirect/1.html'))!;
       expect(response.ok()).toBe(true);
       expect(response.url()).toBe(server.EMPTY_PAGE);
     });
@@ -322,20 +350,20 @@ describe('navigation', function () {
       async () => {
         const { page, server } = getTestState();
 
-        let responses = [];
+        let responses: ServerResponse[] = [];
         // Hold on to a bunch of requests without answering.
-        server.setRoute('/fetch-request-a.js', (req, res) =>
-          responses.push(res)
-        );
-        server.setRoute('/fetch-request-b.js', (req, res) =>
-          responses.push(res)
-        );
-        server.setRoute('/fetch-request-c.js', (req, res) =>
-          responses.push(res)
-        );
-        server.setRoute('/fetch-request-d.js', (req, res) =>
-          responses.push(res)
-        );
+        server.setRoute('/fetch-request-a.js', (_req, res) => {
+          return responses.push(res);
+        });
+        server.setRoute('/fetch-request-b.js', (_req, res) => {
+          return responses.push(res);
+        });
+        server.setRoute('/fetch-request-c.js', (_req, res) => {
+          return responses.push(res);
+        });
+        server.setRoute('/fetch-request-d.js', (_req, res) => {
+          return responses.push(res);
+        });
         const initialFetchResourcesRequested = Promise.all([
           server.waitForRequest('/fetch-request-a.js'),
           server.waitForRequest('/fetch-request-b.js'),
@@ -355,10 +383,14 @@ describe('navigation', function () {
         );
         // Track when the navigation gets completed.
         let navigationFinished = false;
-        navigationPromise.then(() => (navigationFinished = true));
+        navigationPromise.then(() => {
+          return (navigationFinished = true);
+        });
 
         // Wait for the page's 'load' event.
-        await new Promise((fulfill) => page.once('load', fulfill));
+        await new Promise((fulfill) => {
+          return page.once('load', fulfill);
+        });
         expect(navigationFinished).toBe(false);
 
         // Wait for the initial three resources to be requested.
@@ -387,7 +419,7 @@ describe('navigation', function () {
           response.end(`File not found`);
         }
 
-        const response = await navigationPromise;
+        const response = (await navigationPromise)!;
         // Expect navigation to succeed.
         expect(response.ok()).toBe(true);
       }
@@ -396,7 +428,9 @@ describe('navigation', function () {
       const { page, server } = getTestState();
 
       let warning = null;
-      const warningHandler = (w) => (warning = w);
+      const warningHandler: NodeJS.WarningListener = (w) => {
+        return (warning = w);
+      };
       process.on('warning', warningHandler);
       for (let i = 0; i < 20; ++i) {
         await page.goto(server.EMPTY_PAGE);
@@ -408,7 +442,9 @@ describe('navigation', function () {
       const { page } = getTestState();
 
       let warning = null;
-      const warningHandler = (w) => (warning = w);
+      const warningHandler: NodeJS.WarningListener = (w) => {
+        return (warning = w);
+      };
       process.on('warning', warningHandler);
       for (let i = 0; i < 20; ++i) {
         await page.goto('asdf').catch(() => {
@@ -422,7 +458,9 @@ describe('navigation', function () {
       const { context, server } = getTestState();
 
       let warning = null;
-      const warningHandler = (w) => (warning = w);
+      const warningHandler: NodeJS.WarningListener = (w) => {
+        return (warning = w);
+      };
       process.on('warning', warningHandler);
       await Promise.all(
         [...Array(20)].map(async () => {
@@ -439,16 +477,15 @@ describe('navigation', function () {
       async () => {
         const { page } = getTestState();
 
-        const requests = [];
-        page.on(
-          'request',
-          (request) => !utils.isFavicon(request) && requests.push(request)
-        );
+        const requests: HTTPRequest[] = [];
+        page.on('request', (request) => {
+          return !utils.isFavicon(request) && requests.push(request);
+        });
         const dataURL = 'data:text/html,<div>yo</div>';
-        const response = await page.goto(dataURL);
+        const response = (await page.goto(dataURL))!;
         expect(response.status()).toBe(200);
         expect(requests.length).toBe(1);
-        expect(requests[0].url()).toBe(dataURL);
+        expect(requests[0]!.url()).toBe(dataURL);
       }
     );
     itFailsFirefox(
@@ -456,22 +493,21 @@ describe('navigation', function () {
       async () => {
         const { page, server } = getTestState();
 
-        const requests = [];
-        page.on(
-          'request',
-          (request) => !utils.isFavicon(request) && requests.push(request)
-        );
-        const response = await page.goto(server.EMPTY_PAGE + '#hash');
+        const requests: HTTPRequest[] = [];
+        page.on('request', (request) => {
+          return !utils.isFavicon(request) && requests.push(request);
+        });
+        const response = (await page.goto(server.EMPTY_PAGE + '#hash'))!;
         expect(response.status()).toBe(200);
         expect(response.url()).toBe(server.EMPTY_PAGE);
         expect(requests.length).toBe(1);
-        expect(requests[0].url()).toBe(server.EMPTY_PAGE);
+        expect(requests[0]!.url()).toBe(server.EMPTY_PAGE);
       }
     );
     it('should work with self requesting page', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/self-request.html');
+      const response = (await page.goto(server.PREFIX + '/self-request.html'))!;
       expect(response.status()).toBe(200);
       expect(response.url()).toContain('self-request.html');
     });
@@ -479,11 +515,11 @@ describe('navigation', function () {
       const { page, httpsServer } = getTestState();
 
       const url = httpsServer.PREFIX + '/redirect/1.html';
-      let error = null;
+      let error!: Error;
       try {
         await page.goto(url);
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toContain(url);
     });
@@ -510,19 +546,20 @@ describe('navigation', function () {
       await page.goto(server.EMPTY_PAGE);
       const [response] = await Promise.all([
         page.waitForNavigation(),
-        page.evaluate(
-          (url: string) => (window.location.href = url),
-          server.PREFIX + '/grid.html'
-        ),
+        page.evaluate((url: string) => {
+          return (window.location.href = url);
+        }, server.PREFIX + '/grid.html'),
       ]);
-      expect(response.ok()).toBe(true);
-      expect(response.url()).toContain('grid.html');
+      expect(response!.ok()).toBe(true);
+      expect(response!.url()).toContain('grid.html');
     });
     it('should work with both domcontentloaded and load', async () => {
       const { page, server } = getTestState();
 
-      let response = null;
-      server.setRoute('/one-style.css', (req, res) => (response = res));
+      let response!: ServerResponse;
+      server.setRoute('/one-style.css', (_req, res) => {
+        return (response = res);
+      });
       const navigationPromise = page.goto(server.PREFIX + '/one-style.html');
       const domContentLoadedPromise = page.waitForNavigation({
         waitUntil: 'domcontentloaded',
@@ -533,7 +570,9 @@ describe('navigation', function () {
         .waitForNavigation({
           waitUntil: ['load', 'domcontentloaded'],
         })
-        .then(() => (bothFired = true));
+        .then(() => {
+          return (bothFired = true);
+        });
 
       await server.waitForRequest('/one-style.css');
       await domContentLoadedPromise;
@@ -637,7 +676,9 @@ describe('navigation', function () {
           });
         });
         await Promise.all([
-          frame.evaluate(() => window.stop()),
+          frame.evaluate(() => {
+            return window.stop();
+          }),
           navigationPromise,
         ]);
       }
@@ -651,15 +692,15 @@ describe('navigation', function () {
       await page.goto(server.EMPTY_PAGE);
       await page.goto(server.PREFIX + '/grid.html');
 
-      let response = await page.goBack();
+      let response = (await page.goBack())!;
       expect(response.ok()).toBe(true);
       expect(response.url()).toContain(server.EMPTY_PAGE);
 
-      response = await page.goForward();
+      response = (await page.goForward())!;
       expect(response.ok()).toBe(true);
       expect(response.url()).toContain('/grid.html');
 
-      response = await page.goForward();
+      response = (await page.goForward())!;
       expect(response).toBe(null);
     });
     itFailsFirefox('should work with HistoryAPI', async () => {
@@ -686,12 +727,12 @@ describe('navigation', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/frames/one-frame.html');
-      expect(page.frames()[0].url()).toContain('/frames/one-frame.html');
-      expect(page.frames()[1].url()).toContain('/frames/frame.html');
+      expect(page.frames()[0]!.url()).toContain('/frames/one-frame.html');
+      expect(page.frames()[1]!.url()).toContain('/frames/frame.html');
 
-      const response = await page.frames()[1].goto(server.EMPTY_PAGE);
+      const response = (await page.frames()[1]!.goto(server.EMPTY_PAGE))!;
       expect(response.ok()).toBe(true);
-      expect(response.frame()).toBe(page.frames()[1]);
+      expect(response.frame()).toBe(page.frames()[1]!);
     });
     it('should reject when frame detaches', async () => {
       const { page, server } = getTestState();
@@ -700,12 +741,16 @@ describe('navigation', function () {
 
       server.setRoute('/empty.html', () => {});
       const navigationPromise = page
-        .frames()[1]
+        .frames()[1]!
         .goto(server.EMPTY_PAGE)
-        .catch((error_) => error_);
+        .catch((error_) => {
+          return error_;
+        });
       await server.waitForRequest('/empty.html');
 
-      await page.$eval('iframe', (frame) => frame.remove());
+      await page.$eval('iframe', (frame) => {
+        return frame.remove();
+      });
       const error = await navigationPromise;
       expect(error.message).toBe('Navigating frame was detached');
     });
@@ -722,20 +767,20 @@ describe('navigation', function () {
         utils.attachFrame(page, 'frame3', server.EMPTY_PAGE),
       ]);
       // Navigate all frames to the same URL.
-      const serverResponses = [];
-      server.setRoute('/one-style.html', (req, res) =>
-        serverResponses.push(res)
-      );
+      const serverResponses: ServerResponse[] = [];
+      server.setRoute('/one-style.html', (_req, res) => {
+        return serverResponses.push(res);
+      });
       const navigations = [];
       for (let i = 0; i < 3; ++i) {
-        navigations.push(frames[i].goto(server.PREFIX + '/one-style.html'));
+        navigations.push(frames[i]!.goto(server.PREFIX + '/one-style.html'));
         await server.waitForRequest('/one-style.html');
       }
       // Respond from server out-of-order.
       const serverResponseTexts = ['AAA', 'BBB', 'CCC'];
       for (const i of [1, 2, 0]) {
-        serverResponses[i].end(serverResponseTexts[i]);
-        const response = await navigations[i];
+        serverResponses[i]!.end(serverResponseTexts[i]);
+        const response = (await navigations[i])!;
         expect(response.frame()).toBe(frames[i]);
         expect(await response.text()).toBe(serverResponseTexts[i]);
       }
@@ -747,35 +792,38 @@ describe('navigation', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/frames/one-frame.html');
-      const frame = page.frames()[1];
+      const frame = page.frames()[1]!;
       const [response] = await Promise.all([
         frame.waitForNavigation(),
-        frame.evaluate(
-          (url: string) => (window.location.href = url),
-          server.PREFIX + '/grid.html'
-        ),
+        frame.evaluate((url: string) => {
+          return (window.location.href = url);
+        }, server.PREFIX + '/grid.html'),
       ]);
-      expect(response.ok()).toBe(true);
-      expect(response.url()).toContain('grid.html');
-      expect(response.frame()).toBe(frame);
+      expect(response!.ok()).toBe(true);
+      expect(response!.url()).toContain('grid.html');
+      expect(response!.frame()).toBe(frame);
       expect(page.url()).toContain('/frames/one-frame.html');
     });
     it('should fail when frame detaches', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/frames/one-frame.html');
-      const frame = page.frames()[1];
+      const frame = page.frames()[1]!;
 
       server.setRoute('/empty.html', () => {});
-      let error = null;
-      const navigationPromise = frame
-        .waitForNavigation()
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      const navigationPromise = frame.waitForNavigation().catch((error_) => {
+        return (error = error_);
+      });
       await Promise.all([
         server.waitForRequest('/empty.html'),
-        frame.evaluate(() => ((window as any).location = '/empty.html')),
+        frame.evaluate(() => {
+          return ((window as any).location = '/empty.html');
+        }),
       ]);
-      await page.$eval('iframe', (frame) => frame.remove());
+      await page.$eval('iframe', (frame) => {
+        return frame.remove();
+      });
       await navigationPromise;
       expect(error.message).toBe('Navigating frame was detached');
     });
@@ -786,9 +834,15 @@ describe('navigation', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
-      await page.evaluate(() => (globalThis._foo = 10));
+      await page.evaluate(() => {
+        return ((globalThis as any)._foo = 10);
+      });
       await page.reload();
-      expect(await page.evaluate(() => globalThis._foo)).toBe(undefined);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any)._foo;
+        })
+      ).toBe(undefined);
     });
   });
 });
diff --git a/test/src/network.spec.ts b/test/src/network.spec.ts
index b1106293e73f7..5d3310e6441f3 100644
--- a/test/src/network.spec.ts
+++ b/test/src/network.spec.ts
@@ -26,8 +26,10 @@ import {
   describeFailsFirefox,
   itChromeOnly,
   itFirefoxOnly,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import { HTTPResponse } from '../../lib/cjs/puppeteer/api-docs-entry.js';
+} from './mocha-utils.js';
+import { HTTPRequest } from '../../lib/cjs/puppeteer/common/HTTPRequest.js';
+import { HTTPResponse } from '../../lib/cjs/puppeteer/common/HTTPResponse.js';
+import { ServerResponse } from 'http';
 
 describe('network', function () {
   setupTestBrowserHooks();
@@ -37,36 +39,35 @@ describe('network', function () {
     it('should fire for navigation requests', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
-      await page.goto(server.EMPTY_PAGE);
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(requests.length).toBe(1);
     });
     it('should fire for iframes', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
-      await page.goto(server.EMPTY_PAGE);
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       expect(requests.length).toBe(2);
     });
     it('should fire for fetches', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
-      await page.goto(server.EMPTY_PAGE);
-      await page.evaluate(() => fetch('/empty.html'));
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
+      await page.evaluate(() => {
+        return fetch('/empty.html');
+      });
       expect(requests.length).toBe(2);
     });
   });
@@ -74,57 +75,56 @@ describe('network', function () {
     it('should work for main frame navigation request', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
-      await page.goto(server.EMPTY_PAGE);
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(requests.length).toBe(1);
-      expect(requests[0].frame()).toBe(page.mainFrame());
+      expect(requests[0]!.frame()).toBe(page.mainFrame());
     });
     itFailsFirefox('should work for subframe navigation request', async () => {
       const { page, server } = getTestState();
 
-      await page.goto(server.EMPTY_PAGE);
-      const requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
+      (await page.goto(server.EMPTY_PAGE))!;
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
       expect(requests.length).toBe(1);
-      expect(requests[0].frame()).toBe(page.frames()[1]);
+      expect(requests[0]!.frame()).toBe(page.frames()[1]!);
     });
     it('should work for fetch requests', async () => {
       const { page, server } = getTestState();
 
-      await page.goto(server.EMPTY_PAGE);
-      let requests = [];
-      page.on(
-        'request',
-        (request) => !utils.isFavicon(request) && requests.push(request)
-      );
-      await page.evaluate(() => fetch('/digits/1.png'));
-      requests = requests.filter(
-        (request) => !request.url().includes('favicon')
-      );
+      (await page.goto(server.EMPTY_PAGE))!;
+      let requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return !utils.isFavicon(request) && requests.push(request);
+      });
+      await page.evaluate(() => {
+        return fetch('/digits/1.png');
+      });
+      requests = requests.filter((request) => {
+        return !request.url().includes('favicon');
+      });
       expect(requests.length).toBe(1);
-      expect(requests[0].frame()).toBe(page.mainFrame());
+      expect(requests[0]!.frame()).toBe(page.mainFrame());
     });
   });
 
   describe('Request.headers', function () {
     itChromeOnly('should define Chrome as user agent header', async () => {
       const { page, server } = getTestState();
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.request().headers()['user-agent']).toContain('Chrome');
     });
 
     itFirefoxOnly('should define Firefox as user agent header', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.request().headers()['user-agent']).toContain('Firefox');
     });
   });
@@ -133,11 +133,11 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.setHeader('foo', 'bar');
         res.end();
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.headers()['foo']).toBe('bar');
     });
   });
@@ -147,9 +147,12 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       const initiators = new Map();
-      page.on('request', (request) =>
-        initiators.set(request.url().split('/').pop(), request.initiator())
-      );
+      page.on('request', (request) => {
+        return initiators.set(
+          request.url().split('/').pop(),
+          request.initiator()
+        );
+      });
       await page.goto(server.PREFIX + '/initiator.html');
 
       expect(initiators.get('initiator.html').type).toBe('other');
@@ -171,11 +174,11 @@ describe('network', function () {
       );
       expect(initiators.get('initiator.js').type).toBe('parser');
       expect(initiators.get('injectedfile.js').type).toBe('script');
-      expect(initiators.get('injectedfile.js').stack.callFrames[0].url).toBe(
+      expect(initiators.get('injectedfile.js').stack.callFrames[0]!.url).toBe(
         server.PREFIX + '/initiator.js'
       );
       expect(initiators.get('injectedstyle.css').type).toBe('script');
-      expect(initiators.get('injectedstyle.css').stack.callFrames[0].url).toBe(
+      expect(initiators.get('injectedstyle.css').stack.callFrames[0]!.url).toBe(
         server.PREFIX + '/initiator.js'
       );
       expect(initiators.get('initiator.js').url).toBe(
@@ -188,7 +191,7 @@ describe('network', function () {
     it('should return |false| for non-cached content', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.fromCache()).toBe(false);
     });
 
@@ -196,12 +199,12 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       const responses = new Map();
-      page.on(
-        'response',
-        (r) =>
+      page.on('response', (r) => {
+        return (
           !utils.isFavicon(r.request()) &&
           responses.set(r.url().split('/').pop(), r)
-      );
+        );
+      });
 
       // Load and re-load to make sure it's cached.
       await page.goto(server.PREFIX + '/cached/one-style.html');
@@ -219,7 +222,7 @@ describe('network', function () {
     it('should return |false| for non-service-worker content', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.fromServiceWorker()).toBe(false);
     });
 
@@ -227,16 +230,19 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       const responses = new Map();
-      page.on(
-        'response',
-        (r) => !utils.isFavicon(r) && responses.set(r.url().split('/').pop(), r)
-      );
+      page.on('response', (r) => {
+        return (
+          !utils.isFavicon(r) && responses.set(r.url().split('/').pop(), r)
+        );
+      });
 
       // Load and re-load to make sure serviceworker is installed and running.
       await page.goto(server.PREFIX + '/serviceworkers/fetch/sw.html', {
         waitUntil: 'networkidle2',
       });
-      await page.evaluate(async () => await globalThis.activationPromise);
+      await page.evaluate(async () => {
+        return await (globalThis as any).activationPromise;
+      });
       await page.reload();
 
       expect(responses.size).toBe(2);
@@ -251,27 +257,29 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      await page.goto(server.EMPTY_PAGE);
-      server.setRoute('/post', (req, res) => res.end());
-      let request = null;
+      (await page.goto(server.EMPTY_PAGE))!;
+      server.setRoute('/post', (_req, res) => {
+        return res.end();
+      });
+      let request!: HTTPRequest;
       page.on('request', (r) => {
         if (!utils.isFavicon(r)) {
           request = r;
         }
       });
-      await page.evaluate(() =>
-        fetch('./post', {
+      await page.evaluate(() => {
+        return fetch('./post', {
           method: 'POST',
           body: JSON.stringify({ foo: 'bar' }),
-        })
-      );
+        });
+      });
       expect(request).toBeTruthy();
       expect(request.postData()).toBe('{"foo":"bar"}');
     });
     it('should be |undefined| when there is no post data', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.request().postData()).toBe(undefined);
     });
   });
@@ -280,7 +288,7 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/simple.json');
+      const response = (await page.goto(server.PREFIX + '/simple.json'))!;
       const responseText = (await response.text()).trimEnd();
       expect(responseText).toBe('{"foo": "bar"}');
     });
@@ -288,7 +296,7 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       server.enableGzip('/simple.json');
-      const response = await page.goto(server.PREFIX + '/simple.json');
+      const response = (await page.goto(server.PREFIX + '/simple.json'))!;
       expect(response.headers()['content-encoding']).toBe('gzip');
       const responseText = (await response.text()).trimEnd();
       expect(responseText).toBe('{"foo": "bar"}');
@@ -297,13 +305,15 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       server.setRedirect('/foo.html', '/empty.html');
-      const response = await page.goto(server.PREFIX + '/foo.html');
+      const response = (await page.goto(server.PREFIX + '/foo.html'))!;
       const redirectChain = response.request().redirectChain();
       expect(redirectChain.length).toBe(1);
-      const redirected = redirectChain[0].response();
+      const redirected = redirectChain[0]!.response()!;
       expect(redirected.status()).toBe(302);
-      let error = null;
-      await redirected.text().catch((error_) => (error = error_));
+      let error!: Error;
+      await redirected.text().catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain(
         'Response body is unavailable for redirect responses'
       );
@@ -311,10 +321,10 @@ describe('network', function () {
     it('should wait until response completes', async () => {
       const { page, server } = getTestState();
 
-      await page.goto(server.EMPTY_PAGE);
+      (await page.goto(server.EMPTY_PAGE))!;
       // Setup server to trap request.
-      let serverResponse = null;
-      server.setRoute('/get', (req, res) => {
+      let serverResponse!: ServerResponse;
+      server.setRoute('/get', (_req, res) => {
         serverResponse = res;
         // In Firefox, |fetch| will be hanging until it receives |Content-Type| header
         // from server.
@@ -323,14 +333,17 @@ describe('network', function () {
       });
       // Setup page to trap response.
       let requestFinished = false;
-      page.on(
-        'requestfinished',
-        (r) => (requestFinished = requestFinished || r.url().includes('/get'))
-      );
+      page.on('requestfinished', (r) => {
+        return (requestFinished = requestFinished || r.url().includes('/get'));
+      });
       // send request and wait for server response
       const [pageResponse] = await Promise.all([
-        page.waitForResponse((r) => !utils.isFavicon(r.request())),
-        page.evaluate(() => fetch('./get', { method: 'GET' })),
+        page.waitForResponse((r) => {
+          return !utils.isFavicon(r.request());
+        }),
+        page.evaluate(() => {
+          return fetch('./get', { method: 'GET' });
+        }),
         server.waitForRequest('/get'),
       ]);
 
@@ -341,9 +354,15 @@ describe('network', function () {
 
       const responseText = pageResponse.text();
       // Write part of the response and wait for it to be flushed.
-      await new Promise((x) => serverResponse.write('wor', x));
+      await new Promise((x) => {
+        return serverResponse.write('wor', x);
+      });
       // Finish response.
-      await new Promise((x) => serverResponse.end('ld!', x));
+      await new Promise<void>((x) => {
+        serverResponse.end('ld!', () => {
+          return x();
+        });
+      });
       expect(await responseText).toBe('hello world!');
     });
   });
@@ -352,7 +371,7 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/simple.json');
+      const response = (await page.goto(server.PREFIX + '/simple.json'))!;
       expect(await response.json()).toEqual({ foo: 'bar' });
     });
   });
@@ -361,7 +380,7 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      const response = await page.goto(server.PREFIX + '/pptr.png');
+      const response = (await page.goto(server.PREFIX + '/pptr.png'))!;
       const imageBuffer = fs.readFileSync(
         path.join(__dirname, '../assets', 'pptr.png')
       );
@@ -372,7 +391,7 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       server.enableGzip('/pptr.png');
-      const response = await page.goto(server.PREFIX + '/pptr.png');
+      const response = (await page.goto(server.PREFIX + '/pptr.png'))!;
       const imageBuffer = fs.readFileSync(
         path.join(__dirname, '../assets', 'pptr.png')
       );
@@ -384,7 +403,7 @@ describe('network', function () {
 
       await page.goto(server.PREFIX + '/empty.html');
 
-      server.setRoute('/test.html', (req, res) => {
+      server.setRoute('/test.html', (_req, res) => {
         res.setHeader('Access-Control-Allow-Origin', '*');
         res.setHeader('Access-Control-Allow-Headers', 'x-ping');
         res.end('Hello World');
@@ -422,22 +441,22 @@ describe('network', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      server.setRoute('/cool', (req, res) => {
+      server.setRoute('/cool', (_req, res) => {
         res.writeHead(200, 'cool!');
         res.end();
       });
-      const response = await page.goto(server.PREFIX + '/cool');
+      const response = (await page.goto(server.PREFIX + '/cool'))!;
       expect(response.statusText()).toBe('cool!');
     });
 
     it('handles missing status text', async () => {
       const { page, server } = getTestState();
 
-      server.setRoute('/nostatus', (req, res) => {
+      server.setRoute('/nostatus', (_req, res) => {
         res.writeHead(200, '');
         res.end();
       });
-      const response = await page.goto(server.PREFIX + '/nostatus');
+      const response = (await page.goto(server.PREFIX + '/nostatus'))!;
       expect(response.statusText()).toBe('');
     });
   });
@@ -445,11 +464,13 @@ describe('network', function () {
   describeFailsFirefox('Response.timing', function () {
     it('returns timing information', async () => {
       const { page, server } = getTestState();
-      const responses = [];
-      page.on('response', (response) => responses.push(response));
-      await page.goto(server.EMPTY_PAGE);
+      const responses: HTTPResponse[] = [];
+      page.on('response', (response) => {
+        return responses.push(response);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(responses.length).toBe(1);
-      expect(responses[0].timing().receiveHeadersEnd).toBeGreaterThan(0);
+      expect(responses[0]!.timing()!.receiveHeadersEnd).toBeGreaterThan(0);
     });
   });
 
@@ -457,24 +478,26 @@ describe('network', function () {
     it('Page.Events.Request', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on('request', (request) => requests.push(request));
-      await page.goto(server.EMPTY_PAGE);
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(server.EMPTY_PAGE);
-      expect(requests[0].resourceType()).toBe('document');
-      expect(requests[0].method()).toBe('GET');
-      expect(requests[0].response()).toBeTruthy();
-      expect(requests[0].frame() === page.mainFrame()).toBe(true);
-      expect(requests[0].frame().url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.resourceType()).toBe('document');
+      expect(requests[0]!.method()).toBe('GET');
+      expect(requests[0]!.response()).toBeTruthy();
+      expect(requests[0]!.frame() === page.mainFrame()).toBe(true);
+      expect(requests[0]!.frame()!.url()).toBe(server.EMPTY_PAGE);
     });
     it('Page.Events.RequestServedFromCache', async () => {
       const { page, server } = getTestState();
 
-      const cached = [];
-      page.on('requestservedfromcache', (r) =>
-        cached.push(r.url().split('/').pop())
-      );
+      const cached: string[] = [];
+      page.on('requestservedfromcache', (r) => {
+        return cached.push(r.url().split('/').pop()!);
+      });
 
       await page.goto(server.PREFIX + '/cached/one-style.html');
       expect(cached).toEqual([]);
@@ -485,18 +508,20 @@ describe('network', function () {
     it('Page.Events.Response', async () => {
       const { page, server } = getTestState();
 
-      const responses = [];
-      page.on('response', (response) => responses.push(response));
-      await page.goto(server.EMPTY_PAGE);
+      const responses: HTTPResponse[] = [];
+      page.on('response', (response) => {
+        return responses.push(response);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(responses.length).toBe(1);
-      expect(responses[0].url()).toBe(server.EMPTY_PAGE);
-      expect(responses[0].status()).toBe(200);
-      expect(responses[0].ok()).toBe(true);
-      expect(responses[0].request()).toBeTruthy();
-      const remoteAddress = responses[0].remoteAddress();
+      expect(responses[0]!.url()).toBe(server.EMPTY_PAGE);
+      expect(responses[0]!.status()).toBe(200);
+      expect(responses[0]!.ok()).toBe(true);
+      expect(responses[0]!.request()).toBeTruthy();
+      const remoteAddress = responses[0]!.remoteAddress();
       // Either IPv6 or IPv4, depending on environment.
       expect(
-        remoteAddress.ip.includes('::1') || remoteAddress.ip === '127.0.0.1'
+        remoteAddress.ip!.includes('::1') || remoteAddress.ip === '127.0.0.1'
       ).toBe(true);
       expect(remoteAddress.port).toBe(server.PORT);
     });
@@ -512,61 +537,73 @@ describe('network', function () {
           request.continue();
         }
       });
-      const failedRequests = [];
-      page.on('requestfailed', (request) => failedRequests.push(request));
+      const failedRequests: HTTPRequest[] = [];
+      page.on('requestfailed', (request) => {
+        return failedRequests.push(request);
+      });
       await page.goto(server.PREFIX + '/one-style.html');
       expect(failedRequests.length).toBe(1);
-      expect(failedRequests[0].url()).toContain('one-style.css');
-      expect(failedRequests[0].response()).toBe(null);
-      expect(failedRequests[0].resourceType()).toBe('stylesheet');
+      expect(failedRequests[0]!.url()).toContain('one-style.css');
+      expect(failedRequests[0]!.response()).toBe(null);
+      expect(failedRequests[0]!.resourceType()).toBe('stylesheet');
       if (isChrome) {
-        expect(failedRequests[0].failure().errorText).toBe('net::ERR_FAILED');
+        expect(failedRequests[0]!.failure()!.errorText).toBe('net::ERR_FAILED');
       } else {
-        expect(failedRequests[0].failure().errorText).toBe('NS_ERROR_FAILURE');
+        expect(failedRequests[0]!.failure()!.errorText).toBe(
+          'NS_ERROR_FAILURE'
+        );
       }
-      expect(failedRequests[0].frame()).toBeTruthy();
+      expect(failedRequests[0]!.frame()).toBeTruthy();
     });
     it('Page.Events.RequestFinished', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on('requestfinished', (request) => requests.push(request));
-      await page.goto(server.EMPTY_PAGE);
+      const requests: HTTPRequest[] = [];
+      page.on('requestfinished', (request) => {
+        return requests.push(request);
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(server.EMPTY_PAGE);
-      expect(requests[0].response()).toBeTruthy();
-      expect(requests[0].frame() === page.mainFrame()).toBe(true);
-      expect(requests[0].frame().url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.response()).toBeTruthy();
+      expect(requests[0]!.frame() === page.mainFrame()).toBe(true);
+      expect(requests[0]!.frame()!.url()).toBe(server.EMPTY_PAGE);
     });
     it('should fire events in proper order', async () => {
       const { page, server } = getTestState();
 
-      const events = [];
-      page.on('request', () => events.push('request'));
-      page.on('response', () => events.push('response'));
-      page.on('requestfinished', () => events.push('requestfinished'));
-      await page.goto(server.EMPTY_PAGE);
+      const events: string[] = [];
+      page.on('request', () => {
+        return events.push('request');
+      });
+      page.on('response', () => {
+        return events.push('response');
+      });
+      page.on('requestfinished', () => {
+        return events.push('requestfinished');
+      });
+      (await page.goto(server.EMPTY_PAGE))!;
       expect(events).toEqual(['request', 'response', 'requestfinished']);
     });
     it('should support redirects', async () => {
       const { page, server } = getTestState();
 
-      const events = [];
-      page.on('request', (request) =>
-        events.push(`${request.method()} ${request.url()}`)
-      );
-      page.on('response', (response) =>
-        events.push(`${response.status()} ${response.url()}`)
-      );
-      page.on('requestfinished', (request) =>
-        events.push(`DONE ${request.url()}`)
-      );
-      page.on('requestfailed', (request) =>
-        events.push(`FAIL ${request.url()}`)
-      );
+      const events: string[] = [];
+      page.on('request', (request) => {
+        return events.push(`${request.method()} ${request.url()}`);
+      });
+      page.on('response', (response) => {
+        return events.push(`${response.status()} ${response.url()}`);
+      });
+      page.on('requestfinished', (request) => {
+        return events.push(`DONE ${request.url()}`);
+      });
+      page.on('requestfailed', (request) => {
+        return events.push(`FAIL ${request.url()}`);
+      });
       server.setRedirect('/foo.html', '/empty.html');
       const FOO_URL = server.PREFIX + '/foo.html';
-      const response = await page.goto(FOO_URL);
+      const response = (await page.goto(FOO_URL))!;
       expect(events).toEqual([
         `GET ${FOO_URL}`,
         `302 ${FOO_URL}`,
@@ -579,8 +616,8 @@ describe('network', function () {
       // Check redirect chain
       const redirectChain = response.request().redirectChain();
       expect(redirectChain.length).toBe(1);
-      expect(redirectChain[0].url()).toContain('/foo.html');
-      expect(redirectChain[0].response().remoteAddress().port).toBe(
+      expect(redirectChain[0]!.url()).toContain('/foo.html');
+      expect(redirectChain[0]!.response()!.remoteAddress().port).toBe(
         server.PORT
       );
     });
@@ -591,9 +628,9 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       const requests = new Map();
-      page.on('request', (request) =>
-        requests.set(request.url().split('/').pop(), request)
-      );
+      page.on('request', (request) => {
+        return requests.set(request.url().split('/').pop(), request);
+      });
       server.setRedirect('/rrredirect', '/frames/one-frame.html');
       await page.goto(server.PREFIX + '/rrredirect');
       expect(requests.get('rrredirect').isNavigationRequest()).toBe(true);
@@ -622,10 +659,12 @@ describe('network', function () {
     itFailsFirefox('should work when navigating to image', async () => {
       const { page, server } = getTestState();
 
-      const requests = [];
-      page.on('request', (request) => requests.push(request));
-      await page.goto(server.PREFIX + '/pptr.png');
-      expect(requests[0].isNavigationRequest()).toBe(true);
+      const requests: HTTPRequest[] = [];
+      page.on('request', (request) => {
+        return requests.push(request);
+      });
+      (await page.goto(server.PREFIX + '/pptr.png'))!;
+      expect(requests[0]!.isNavigationRequest()).toBe(true);
     });
   });
 
@@ -645,12 +684,12 @@ describe('network', function () {
     it('should throw for non-string header values', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
         // @ts-expect-error purposeful bad input
         await page.setExtraHTTPHeaders({ foo: 1 });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toBe(
         'Expected value of header "foo" to be String, but "number" is found.'
@@ -665,11 +704,15 @@ describe('network', function () {
       server.setAuth('/empty.html', 'user', 'pass');
       let response;
       try {
-        response = await page.goto(server.EMPTY_PAGE);
+        response = (await page.goto(server.EMPTY_PAGE))!;
         expect(response.status()).toBe(401);
       } catch (error) {
         // In headful, an error is thrown instead of 401.
-        if (!error.message.startsWith('net::ERR_INVALID_AUTH_CREDENTIALS')) {
+        if (
+          !(error as Error).message.startsWith(
+            'net::ERR_INVALID_AUTH_CREDENTIALS'
+          )
+        ) {
           throw error;
         }
       }
@@ -677,7 +720,7 @@ describe('network', function () {
         username: 'user',
         password: 'pass',
       });
-      response = await page.reload();
+      response = (await page.reload())!;
       expect(response.status()).toBe(200);
     });
     it('should fail if wrong credentials', async () => {
@@ -689,7 +732,7 @@ describe('network', function () {
         username: 'foo',
         password: 'bar',
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.status()).toBe(401);
     });
     it('should allow disable authentication', async () => {
@@ -701,16 +744,25 @@ describe('network', function () {
         username: 'user3',
         password: 'pass3',
       });
-      let response = await page.goto(server.EMPTY_PAGE);
+      let response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.status()).toBe(200);
-      await page.authenticate(null);
+      await page.authenticate({
+        username: '',
+        password: '',
+      });
       // Navigate to a different origin to bust Chrome's credential caching.
       try {
-        response = await page.goto(server.CROSS_PROCESS_PREFIX + '/empty.html');
+        response = (await page.goto(
+          server.CROSS_PROCESS_PREFIX + '/empty.html'
+        ))!;
         expect(response.status()).toBe(401);
       } catch (error) {
         // In headful, an error is thrown instead of 401.
-        if (!error.message.startsWith('net::ERR_INVALID_AUTH_CREDENTIALS')) {
+        if (
+          !(error as Error).message.startsWith(
+            'net::ERR_INVALID_AUTH_CREDENTIALS'
+          )
+        ) {
           throw error;
         }
       }
@@ -727,7 +779,9 @@ describe('network', function () {
       });
 
       const responses = new Map();
-      page.on('response', (r) => responses.set(r.url().split('/').pop(), r));
+      page.on('response', (r) => {
+        return responses.set(r.url().split('/').pop(), r);
+      });
 
       // Load and re-load to make sure it's cached.
       await page.goto(server.PREFIX + '/cached/one-style.html');
@@ -745,27 +799,29 @@ describe('network', function () {
       const { page, server } = getTestState();
 
       const setCookieString = 'foo=bar';
-      server.setRoute('/empty.html', (req, res) => {
+      server.setRoute('/empty.html', (_req, res) => {
         res.setHeader('set-cookie', setCookieString);
         res.end('hello world');
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.headers()['set-cookie']).toBe(setCookieString);
     });
 
     it('Same-origin set-cookie subresource', async () => {
       const { page, server } = getTestState();
-      await page.goto(server.EMPTY_PAGE);
+      (await page.goto(server.EMPTY_PAGE))!;
 
       const setCookieString = 'foo=bar';
-      server.setRoute('/foo', (req, res) => {
+      server.setRoute('/foo', (_req, res) => {
         res.setHeader('set-cookie', setCookieString);
         res.end('hello world');
       });
 
-      const responsePromise = new Promise<HTTPResponse>((resolve) =>
-        page.on('response', (response) => resolve(response))
-      );
+      const responsePromise = new Promise<HTTPResponse>((resolve) => {
+        return page.on('response', (response) => {
+          return resolve(response);
+        });
+      });
       page.evaluate(() => {
         const xhr = new XMLHttpRequest();
         xhr.open('GET', '/foo');
@@ -789,7 +845,7 @@ describe('network', function () {
         await page.goto(httpsServer.PREFIX + '/empty.html');
 
         const setCookieString = 'hello=world';
-        httpsServer.setRoute('/setcookie.html', (req, res) => {
+        httpsServer.setRoute('/setcookie.html', (_req, res) => {
           res.setHeader('Access-Control-Allow-Origin', '*');
           res.setHeader('set-cookie', setCookieString);
           res.end();
diff --git a/test/src/oopif.spec.ts b/test/src/oopif.spec.ts
index 0a4d4a7a243ae..16461cb2c7a8c 100644
--- a/test/src/oopif.spec.ts
+++ b/test/src/oopif.spec.ts
@@ -20,7 +20,7 @@ import {
   getTestState,
   describeChromeOnly,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import {
   Browser,
   BrowserContext,
@@ -53,21 +53,19 @@ describeChromeOnly('OOPIF', function () {
 
   afterEach(async () => {
     await context.close();
-    page = null;
-    context = null;
   });
 
   after(async () => {
     await browser.close();
-    browser = null;
   });
+
   it('should treat OOP iframes and normal iframes the same', async () => {
     const { server } = getTestState();
 
     await page.goto(server.EMPTY_PAGE);
-    const framePromise = page.waitForFrame((frame) =>
-      frame.url().endsWith('/empty.html')
-    );
+    const framePromise = page.waitForFrame((frame) => {
+      return frame.url().endsWith('/empty.html');
+    });
     await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
     await utils.attachFrame(
       page,
@@ -137,12 +135,16 @@ describeChromeOnly('OOPIF', function () {
 
     const [frame1, frame2] = await Promise.all([frame1Promise, frame2Promise]);
 
-    expect(await frame1.evaluate(() => document.location.href)).toMatch(
-      /one-frame\.html$/
-    );
-    expect(await frame2.evaluate(() => document.location.href)).toMatch(
-      /frames\/frame\.html$/
-    );
+    expect(
+      await frame1.evaluate(() => {
+        return document.location.href;
+      })
+    ).toMatch(/one-frame\.html$/);
+    expect(
+      await frame2.evaluate(() => {
+        return document.location.href;
+      })
+    ).toMatch(/frames\/frame\.html$/);
   });
   it('should support OOP iframes getting detached', async () => {
     const { server } = getTestState();
@@ -279,9 +281,9 @@ describeChromeOnly('OOPIF', function () {
   it('should report oopif frames', async () => {
     const { server } = getTestState();
 
-    const frame = page.waitForFrame((frame) =>
-      frame.url().endsWith('/oopif.html')
-    );
+    const frame = page.waitForFrame((frame) => {
+      return frame.url().endsWith('/oopif.html');
+    });
     await page.goto(server.PREFIX + '/dynamic-oopif.html');
     await frame;
     expect(oopifs(context).length).toBe(1);
@@ -291,24 +293,32 @@ describeChromeOnly('OOPIF', function () {
   it('should wait for inner OOPIFs', async () => {
     const { server } = getTestState();
     await page.goto(`http://mainframe:${server.PORT}/main-frame.html`);
-    const frame2 = await page.waitForFrame((frame) =>
-      frame.url().endsWith('inner-frame2.html')
-    );
+    const frame2 = await page.waitForFrame((frame) => {
+      return frame.url().endsWith('inner-frame2.html');
+    });
     expect(oopifs(context).length).toBe(2);
-    expect(page.frames().filter((frame) => frame.isOOPFrame()).length).toBe(2);
     expect(
-      await frame2.evaluate(() => document.querySelectorAll('button').length)
+      page.frames().filter((frame) => {
+        return frame.isOOPFrame();
+      }).length
+    ).toBe(2);
+    expect(
+      await frame2.evaluate(() => {
+        return document.querySelectorAll('button').length;
+      })
     ).toStrictEqual(1);
   });
 
   it('should load oopif iframes with subresources and request interception', async () => {
     const { server } = getTestState();
 
-    const frame = page.waitForFrame((frame) =>
-      frame.url().endsWith('/oopif.html')
-    );
+    const frame = page.waitForFrame((frame) => {
+      return frame.url().endsWith('/oopif.html');
+    });
     await page.setRequestInterception(true);
-    page.on('request', (request) => request.continue());
+    page.on('request', (request) => {
+      return request.continue();
+    });
     await page.goto(server.PREFIX + '/dynamic-oopif.html');
     await frame;
     expect(oopifs(context).length).toBe(1);
@@ -327,7 +337,7 @@ describeChromeOnly('OOPIF', function () {
       server.CROSS_PROCESS_PREFIX + '/empty.html'
     );
 
-    const frame1 = oopIframe.childFrames()[0];
+    const frame1 = oopIframe.childFrames()[0]!;
     expect(frame1.url()).toMatch(/empty.html$/);
     await utils.navigateFrame(
       oopIframe,
@@ -367,13 +377,13 @@ describeChromeOnly('OOPIF', function () {
       button.innerText = 'click';
       document.body.appendChild(button);
     });
-    const button = await frame.waitForSelector('#test-button', {
+    const button = (await frame.waitForSelector('#test-button', {
       visible: true,
-    });
+    }))!;
     const result = await button.clickablePoint();
     expect(result.x).toBeGreaterThan(150); // padding + margin + border left
     expect(result.y).toBeGreaterThan(150); // padding + margin + border top
-    const resultBoxModel = await button.boxModel();
+    const resultBoxModel = (await button.boxModel())!;
     for (const quad of [
       resultBoxModel.content,
       resultBoxModel.border,
@@ -385,7 +395,7 @@ describeChromeOnly('OOPIF', function () {
         expect(part.y).toBeGreaterThan(150); // padding + margin + border top
       }
     }
-    const resultBoundingBox = await button.boundingBox();
+    const resultBoundingBox = (await button.boundingBox())!;
     expect(resultBoundingBox.x).toBeGreaterThan(150); // padding + margin + border left
     expect(resultBoundingBox.y).toBeGreaterThan(150); // padding + margin + border top
   });
@@ -393,9 +403,9 @@ describeChromeOnly('OOPIF', function () {
   it('should detect existing OOPIFs when Puppeteer connects to an existing page', async () => {
     const { server, puppeteer } = getTestState();
 
-    const frame = page.waitForFrame((frame) =>
-      frame.url().endsWith('/oopif.html')
-    );
+    const frame = page.waitForFrame((frame) => {
+      return frame.url().endsWith('/oopif.html');
+    });
     await page.goto(server.PREFIX + '/dynamic-oopif.html');
     await frame;
     expect(oopifs(context).length).toBe(1);
@@ -403,9 +413,9 @@ describeChromeOnly('OOPIF', function () {
 
     const browserURL = 'http://127.0.0.1:21222';
     const browser1 = await puppeteer.connect({ browserURL });
-    const target = await browser1.waitForTarget((target) =>
-      target.url().endsWith('dynamic-oopif.html')
-    );
+    const target = await browser1.waitForTarget((target) => {
+      return target.url().endsWith('dynamic-oopif.html');
+    });
     await target.page();
     browser1.disconnect();
   });
@@ -415,11 +425,11 @@ describeChromeOnly('OOPIF', function () {
     await page.goto(server.PREFIX + '/lazy-oopif-frame.html');
     await page.setViewport({ width: 1000, height: 1000 });
 
-    expect(page.frames().map((frame) => frame._hasStartedLoading)).toEqual([
-      true,
-      true,
-      false,
-    ]);
+    expect(
+      page.frames().map((frame) => {
+        return frame._hasStartedLoading;
+      })
+    ).toEqual([true, true, false]);
   });
 
   describe('waitForFrame', () => {
@@ -433,13 +443,15 @@ describeChromeOnly('OOPIF', function () {
         server.CROSS_PROCESS_PREFIX + '/empty.html'
       );
 
-      await page.waitForFrame((frame) => frame.url().endsWith('/empty.html'));
+      await page.waitForFrame((frame) => {
+        return frame.url().endsWith('/empty.html');
+      });
     });
   });
 });
 
 function oopifs(context: BrowserContext) {
-  return context
-    .targets()
-    .filter((target) => target._getTargetInfo().type === 'iframe');
+  return context.targets().filter((target) => {
+    return target._getTargetInfo().type === 'iframe';
+  });
 }
diff --git a/test/src/page.spec.ts b/test/src/page.spec.ts
index 37cb14aeb838d..dde97a23557ec 100644
--- a/test/src/page.spec.ts
+++ b/test/src/page.spec.ts
@@ -13,22 +13,23 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import expect from 'expect';
 import fs from 'fs';
+import { ServerResponse } from 'http';
 import path from 'path';
-import utils from './utils.js';
-const { waitEvent } = utils;
-import expect from 'expect';
 import sinon from 'sinon';
+import { CDPSession } from '../../lib/cjs/puppeteer/common/Connection.js';
+import { ConsoleMessage } from '../../lib/cjs/puppeteer/common/ConsoleMessage.js';
+import { JSHandle } from '../../lib/cjs/puppeteer/common/JSHandle.js';
+import { Metrics, Page } from '../../lib/cjs/puppeteer/common/Page.js';
 import {
+  describeFailsFirefox,
   getTestState,
+  itFailsFirefox,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-  itFailsFirefox,
-  describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import { Page, Metrics } from '../../lib/cjs/puppeteer/common/Page.js';
-import { CDPSession } from '../../lib/cjs/puppeteer/common/Connection.js';
-import { JSHandle } from '../../lib/cjs/puppeteer/common/JSHandle.js';
+} from './mocha-utils.js';
+import utils, { attachFrame, waitEvent } from './utils.js';
 
 describe('Page', function () {
   setupTestBrowserHooks();
@@ -38,11 +39,15 @@ describe('Page', function () {
       const { context } = getTestState();
 
       const newPage = await context.newPage();
-      let error = null;
+      let error!: Error;
       await Promise.all([
         newPage
-          .evaluate(() => new Promise(() => {}))
-          .catch((error_) => (error = error_)),
+          .evaluate(() => {
+            return new Promise(() => {});
+          })
+          .catch((error_) => {
+            return (error = error_);
+          }),
         newPage.close(),
       ]);
       expect(error.message).toContain('Protocol error');
@@ -98,8 +103,12 @@ describe('Page', function () {
 
       const newPage = await context.newPage();
       const results = await Promise.all([
-        newPage.waitForRequest(server.EMPTY_PAGE).catch((error) => error),
-        newPage.waitForResponse(server.EMPTY_PAGE).catch((error) => error),
+        newPage.waitForRequest(server.EMPTY_PAGE).catch((error) => {
+          return error;
+        }),
+        newPage.waitForResponse(server.EMPTY_PAGE).catch((error) => {
+          return error;
+        }),
         newPage.close(),
       ]);
       for (let i = 0; i < 2; i++) {
@@ -126,7 +135,7 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       const handler = sinon.spy();
-      const onResponse = (response) => {
+      const onResponse = (response: { url: () => string }) => {
         // Ignore default favicon requests.
         if (!response.url().endsWith('favicon.ico')) {
           handler();
@@ -149,7 +158,7 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       const handler = sinon.spy();
-      const onResponse = (response) => {
+      const onResponse = (response: { url: () => string }) => {
         // Ignore default favicon requests.
         if (!response.url().endsWith('favicon.ico')) {
           handler();
@@ -178,8 +187,10 @@ describe('Page', function () {
     it('should throw when page crashes', async () => {
       const { page } = getTestState();
 
-      let error = null;
-      page.on('error', (err) => (error = err));
+      let error!: Error;
+      page.on('error', (err) => {
+        return (error = err);
+      });
       page.goto('chrome://crash').catch(() => {});
       await waitEvent(page, 'error');
       expect(error.message).toBe('Page crashed!');
@@ -191,21 +202,45 @@ describe('Page', function () {
       const { page } = getTestState();
 
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
-        page.evaluate(() => window.open('about:blank')),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
+        page.evaluate(() => {
+          return window.open('about:blank');
+        }),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(true);
     });
     it('should work with noopener', async () => {
       const { page } = getTestState();
 
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
-        page.evaluate(() => window.open('about:blank', null, 'noopener')),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
+        page.evaluate(() => {
+          return window.open('about:blank', undefined, 'noopener');
+        }),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
     });
     it('should work with clicking target=_blank and without rel=opener', async () => {
       const { page, server } = getTestState();
@@ -213,11 +248,21 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       await page.setContent('<a target=_blank href="/one-style.html">yo</a>');
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
         page.click('a'),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
     });
     it('should work with clicking target=_blank and with rel=opener', async () => {
       const { page, server } = getTestState();
@@ -227,11 +272,21 @@ describe('Page', function () {
         '<a target=_blank rel=opener href="/one-style.html">yo</a>'
       );
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
         page.click('a'),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(true);
     });
     it('should work with fake-clicking target=_blank and rel=noopener', async () => {
       const { page, server } = getTestState();
@@ -241,11 +296,23 @@ describe('Page', function () {
         '<a target=_blank rel=noopener href="/one-style.html">yo</a>'
       );
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
-        page.$eval('a', (a: HTMLAnchorElement) => a.click()),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
+        page.$eval('a', (a) => {
+          return (a as HTMLAnchorElement).click();
+        }),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
     });
     it('should work with clicking target=_blank and rel=noopener', async () => {
       const { page, server } = getTestState();
@@ -255,21 +322,31 @@ describe('Page', function () {
         '<a target=_blank rel=noopener href="/one-style.html">yo</a>'
       );
       const [popup] = await Promise.all([
-        new Promise<Page>((x) => page.once('popup', x)),
+        new Promise<Page>((x) => {
+          return page.once('popup', x);
+        }),
         page.click('a'),
       ]);
-      expect(await page.evaluate(() => !!window.opener)).toBe(false);
-      expect(await popup.evaluate(() => !!window.opener)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
+      expect(
+        await popup.evaluate(() => {
+          return !!window.opener;
+        })
+      ).toBe(false);
     });
   });
 
   describe('BrowserContext.overridePermissions', function () {
-    function getPermission(page, name) {
-      return page.evaluate(
-        (name) =>
-          navigator.permissions.query({ name }).then((result) => result.state),
-        name
-      );
+    function getPermission(page: Page, name: string) {
+      return page.evaluate((name: PermissionName) => {
+        return navigator.permissions.query({ name }).then((result) => {
+          return result.state;
+        });
+      }, name);
     }
 
     it('should be prompt by default', async () => {
@@ -289,11 +366,13 @@ describe('Page', function () {
       const { page, server, context } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
-      let error = null;
+      let error!: Error;
       await context
         // @ts-expect-error purposeful bad input for test
         .overridePermissions(server.EMPTY_PAGE, ['foo'])
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toBe('Unknown permission: foo');
     });
     itFailsFirefox('should grant permission when listed', async () => {
@@ -322,35 +401,39 @@ describe('Page', function () {
 
       await page.goto(server.EMPTY_PAGE);
       await page.evaluate(() => {
-        globalThis.events = [];
+        (globalThis as any).events = [];
         return navigator.permissions
           .query({ name: 'geolocation' })
           .then(function (result) {
-            globalThis.events.push(result.state);
+            (globalThis as any).events.push(result.state);
             result.onchange = function () {
-              globalThis.events.push(result.state);
+              (globalThis as any).events.push(result.state);
             };
           });
       });
-      expect(await page.evaluate(() => globalThis.events)).toEqual(['prompt']);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).events;
+        })
+      ).toEqual(['prompt']);
       await context.overridePermissions(server.EMPTY_PAGE, []);
-      expect(await page.evaluate(() => globalThis.events)).toEqual([
-        'prompt',
-        'denied',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).events;
+        })
+      ).toEqual(['prompt', 'denied']);
       await context.overridePermissions(server.EMPTY_PAGE, ['geolocation']);
-      expect(await page.evaluate(() => globalThis.events)).toEqual([
-        'prompt',
-        'denied',
-        'granted',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).events;
+        })
+      ).toEqual(['prompt', 'denied', 'granted']);
       await context.clearPermissionOverrides();
-      expect(await page.evaluate(() => globalThis.events)).toEqual([
-        'prompt',
-        'denied',
-        'granted',
-        'prompt',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).events;
+        })
+      ).toEqual(['prompt', 'denied', 'granted', 'prompt']);
     });
     itFailsFirefox(
       'should isolate permissions between browser contexts',
@@ -399,17 +482,16 @@ describe('Page', function () {
       await context.overridePermissions(server.PREFIX, ['geolocation']);
       await page.goto(server.EMPTY_PAGE);
       await page.setGeolocation({ longitude: 10, latitude: 10 });
-      const geolocation = await page.evaluate(
-        () =>
-          new Promise((resolve) =>
-            navigator.geolocation.getCurrentPosition((position) => {
-              resolve({
-                latitude: position.coords.latitude,
-                longitude: position.coords.longitude,
-              });
-            })
-          )
-      );
+      const geolocation = await page.evaluate(() => {
+        return new Promise((resolve) => {
+          return navigator.geolocation.getCurrentPosition((position) => {
+            resolve({
+              latitude: position.coords.latitude,
+              longitude: position.coords.longitude,
+            });
+          });
+        });
+      });
       expect(geolocation).toEqual({
         latitude: 10,
         longitude: 10,
@@ -418,11 +500,11 @@ describe('Page', function () {
     it('should throw when invalid longitude', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
         await page.setGeolocation({ longitude: 200, latitude: 10 });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toContain('Invalid longitude "200"');
     });
@@ -433,21 +515,35 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.setOfflineMode(true);
-      let error = null;
-      await page.goto(server.EMPTY_PAGE).catch((error_) => (error = error_));
+      let error!: Error;
+      await page.goto(server.EMPTY_PAGE).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeTruthy();
       await page.setOfflineMode(false);
-      const response = await page.reload();
+      const response = (await page.reload())!;
       expect(response.status()).toBe(200);
     });
     it('should emulate navigator.onLine', async () => {
       const { page } = getTestState();
 
-      expect(await page.evaluate(() => window.navigator.onLine)).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return window.navigator.onLine;
+        })
+      ).toBe(true);
       await page.setOfflineMode(true);
-      expect(await page.evaluate(() => window.navigator.onLine)).toBe(false);
+      expect(
+        await page.evaluate(() => {
+          return window.navigator.onLine;
+        })
+      ).toBe(false);
       await page.setOfflineMode(false);
-      expect(await page.evaluate(() => window.navigator.onLine)).toBe(true);
+      expect(
+        await page.evaluate(() => {
+          return window.navigator.onLine;
+        })
+      ).toBe(true);
     });
   });
 
@@ -456,18 +552,20 @@ describe('Page', function () {
       const { page } = getTestState();
 
       // Instantiate an object
-      await page.evaluate(() => (globalThis.set = new Set(['hello', 'world'])));
-      const prototypeHandle = await page.evaluateHandle(() => Set.prototype);
+      await page.evaluate(() => {
+        return ((globalThis as any).set = new Set(['hello', 'world']));
+      });
+      const prototypeHandle = await page.evaluateHandle(() => {
+        return Set.prototype;
+      });
       const objectsHandle = await page.queryObjects(prototypeHandle);
-      const count = await page.evaluate(
-        (objects: JSHandle[]) => objects.length,
-        objectsHandle
-      );
+      const count = await page.evaluate((objects: JSHandle[]) => {
+        return objects.length;
+      }, objectsHandle);
       expect(count).toBe(1);
-      const values = await page.evaluate(
-        (objects) => Array.from(objects[0].values()),
-        objectsHandle
-      );
+      const values = await page.evaluate((objects) => {
+        return Array.from(objects[0]!.values());
+      }, objectsHandle);
       expect(values).toEqual(['hello', 'world']);
     });
     itFailsFirefox('should work for non-blank page', async () => {
@@ -475,36 +573,41 @@ describe('Page', function () {
 
       // Instantiate an object
       await page.goto(server.EMPTY_PAGE);
-      await page.evaluate(() => (globalThis.set = new Set(['hello', 'world'])));
-      const prototypeHandle = await page.evaluateHandle(() => Set.prototype);
+      await page.evaluate(() => {
+        return ((globalThis as any).set = new Set(['hello', 'world']));
+      });
+      const prototypeHandle = await page.evaluateHandle(() => {
+        return Set.prototype;
+      });
       const objectsHandle = await page.queryObjects(prototypeHandle);
-      const count = await page.evaluate(
-        (objects: JSHandle[]) => objects.length,
-        objectsHandle
-      );
+      const count = await page.evaluate((objects: JSHandle[]) => {
+        return objects.length;
+      }, objectsHandle);
       expect(count).toBe(1);
     });
     it('should fail for disposed handles', async () => {
       const { page } = getTestState();
 
-      const prototypeHandle = await page.evaluateHandle(
-        () => HTMLBodyElement.prototype
-      );
+      const prototypeHandle = await page.evaluateHandle(() => {
+        return HTMLBodyElement.prototype;
+      });
       await prototypeHandle.dispose();
-      let error = null;
-      await page
-        .queryObjects(prototypeHandle)
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await page.queryObjects(prototypeHandle).catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe('Prototype JSHandle is disposed!');
     });
     it('should fail primitive values as prototypes', async () => {
       const { page } = getTestState();
 
-      const prototypeHandle = await page.evaluateHandle(() => 42);
-      let error = null;
-      await page
-        .queryObjects(prototypeHandle)
-        .catch((error_) => (error = error_));
+      const prototypeHandle = await page.evaluateHandle(() => {
+        return 42;
+      });
+      let error!: Error;
+      await page.queryObjects(prototypeHandle).catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toBe(
         'Prototype JSHandle must not be referencing primitive value'
       );
@@ -515,10 +618,14 @@ describe('Page', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      let message = null;
-      page.once('console', (m) => (message = m));
+      let message!: ConsoleMessage;
+      page.once('console', (m) => {
+        return (message = m);
+      });
       await Promise.all([
-        page.evaluate(() => console.log('hello', 5, { foo: 'bar' })),
+        page.evaluate(() => {
+          return console.log('hello', 5, { foo: 'bar' });
+        }),
         waitEvent(page, 'console'),
       ]);
       expect(message.text()).toEqual('hello 5 JSHandle@object');
@@ -530,15 +637,17 @@ describe('Page', function () {
         columnNumber: expect.any(Number),
       });
 
-      expect(await message.args()[0].jsonValue()).toEqual('hello');
-      expect(await message.args()[1].jsonValue()).toEqual(5);
-      expect(await message.args()[2].jsonValue()).toEqual({ foo: 'bar' });
+      expect(await message.args()[0]!.jsonValue()).toEqual('hello');
+      expect(await message.args()[1]!.jsonValue()).toEqual(5);
+      expect(await message.args()[2]!.jsonValue()).toEqual({ foo: 'bar' });
     });
     it('should work for different console API calls', async () => {
       const { page } = getTestState();
 
-      const messages = [];
-      page.on('console', (msg) => messages.push(msg));
+      const messages: any[] = [];
+      page.on('console', (msg) => {
+        return messages.push(msg);
+      });
       // All console events will be reported before `page.evaluate` is finished.
       await page.evaluate(() => {
         // A pair of time/timeEnd generates only one Console API call.
@@ -550,16 +659,17 @@ describe('Page', function () {
         console.error('calling console.error');
         console.log(Promise.resolve('should not wait until resolved!'));
       });
-      expect(messages.map((msg) => msg.type())).toEqual([
-        'timeEnd',
-        'trace',
-        'dir',
-        'warning',
-        'error',
-        'log',
-      ]);
-      expect(messages[0].text()).toContain('calling console.time');
-      expect(messages.slice(1).map((msg) => msg.text())).toEqual([
+      expect(
+        messages.map((msg) => {
+          return msg.type();
+        })
+      ).toEqual(['timeEnd', 'trace', 'dir', 'warning', 'error', 'log']);
+      expect(messages[0]!.text()).toContain('calling console.time');
+      expect(
+        messages.slice(1).map((msg) => {
+          return msg.text();
+        })
+      ).toEqual([
         'calling console.trace',
         'calling console.dir',
         'calling console.warn',
@@ -570,10 +680,14 @@ describe('Page', function () {
     it('should not fail for window object', async () => {
       const { page } = getTestState();
 
-      let message = null;
-      page.once('console', (msg) => (message = msg));
+      let message!: ConsoleMessage;
+      page.once('console', (msg) => {
+        return (message = msg);
+      });
       await Promise.all([
-        page.evaluate(() => console.error(window)),
+        page.evaluate(() => {
+          return console.error(window);
+        }),
         waitEvent(page, 'console'),
       ]);
       expect(message.text()).toBe('JSHandle@object');
@@ -584,10 +698,9 @@ describe('Page', function () {
       await page.goto('about:blank');
       const [message] = await Promise.all([
         waitEvent(page, 'console'),
-        page.evaluate(
-          async (url: string) => fetch(url).catch(() => {}),
-          server.EMPTY_PAGE
-        ),
+        page.evaluate(async (url: string) => {
+          return fetch(url).catch(() => {});
+        }, server.EMPTY_PAGE),
       ]);
       expect(message.text()).toContain('Access-Control-Allow-Origin');
       if (isChrome) {
@@ -657,19 +770,25 @@ describe('Page', function () {
           window.location.href,
           'Title',
           'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,width=780,height=200,top=0,left=0'
-        );
-        await new Promise((x) => (win.onload = x));
+        )!;
+        await new Promise((x) => {
+          return (win.onload = x);
+        });
         // 2. In this popup, create an iframe that console.logs a message.
         win.document.body.innerHTML = `<iframe src='/consolelog.html'></iframe>`;
-        const frame = win.document.querySelector('iframe');
-        await new Promise((x) => (frame.onload = x));
+        const frame = win.document.querySelector('iframe')!;
+        await new Promise((x) => {
+          return (frame.onload = x);
+        });
         // 3. After that, remove the iframe.
         frame.remove();
       });
       const popupTarget = page
         .browserContext()
         .targets()
-        .find((target) => target !== page.target());
+        .find((target) => {
+          return target !== page.target();
+        })!;
       // 4. Connect to the popup and make sure it doesn't throw.
       await popupTarget.page();
     });
@@ -696,14 +815,18 @@ describe('Page', function () {
       const { page } = getTestState();
 
       const metricsPromise = new Promise<{ metrics: Metrics; title: string }>(
-        (fulfill) => page.once('metrics', fulfill)
+        (fulfill) => {
+          return page.once('metrics', fulfill);
+        }
       );
-      await page.evaluate(() => console.timeStamp('test42'));
+      await page.evaluate(() => {
+        return console.timeStamp('test42');
+      });
       const metrics = await metricsPromise;
       expect(metrics.title).toBe('test42');
       checkMetrics(metrics.metrics);
     });
-    function checkMetrics(metrics) {
+    function checkMetrics(metrics: Metrics) {
       const metricsToCheck = new Set([
         'Timestamp',
         'Documents',
@@ -721,7 +844,7 @@ describe('Page', function () {
       ]);
       for (const name in metrics) {
         expect(metricsToCheck.has(name)).toBeTruthy();
-        expect(metrics[name]).toBeGreaterThanOrEqual(0);
+        expect(metrics[name as keyof Metrics]).toBeGreaterThanOrEqual(0);
         metricsToCheck.delete(name);
       }
       expect(metricsToCheck.size).toBe(0);
@@ -748,9 +871,9 @@ describe('Page', function () {
 
       await page.goto(server.EMPTY_PAGE);
       const [request] = await Promise.all([
-        page.waitForRequest(
-          (request) => request.url() === server.PREFIX + '/digits/2.png'
-        ),
+        page.waitForRequest((request) => {
+          return request.url() === server.PREFIX + '/digits/2.png';
+        }),
         page.evaluate(() => {
           fetch('/digits/1.png');
           fetch('/digits/2.png');
@@ -762,20 +885,31 @@ describe('Page', function () {
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
-        .waitForRequest(() => false, { timeout: 1 })
-        .catch((error_) => (error = error_));
+        .waitForRequest(
+          () => {
+            return false;
+          },
+          { timeout: 1 }
+        )
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should respect default timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       page.setDefaultTimeout(1);
       await page
-        .waitForRequest(() => false)
-        .catch((error_) => (error = error_));
+        .waitForRequest(() => {
+          return false;
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should work with no timeout', async () => {
@@ -784,13 +918,13 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       const [request] = await Promise.all([
         page.waitForRequest(server.PREFIX + '/digits/2.png', { timeout: 0 }),
-        page.evaluate(() =>
-          setTimeout(() => {
+        page.evaluate(() => {
+          return setTimeout(() => {
             fetch('/digits/1.png');
             fetch('/digits/2.png');
             fetch('/digits/3.png');
-          }, 50)
-        ),
+          }, 50);
+        }),
       ]);
       expect(request.url()).toBe(server.PREFIX + '/digits/2.png');
     });
@@ -814,20 +948,31 @@ describe('Page', function () {
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
-        .waitForResponse(() => false, { timeout: 1 })
-        .catch((error_) => (error = error_));
+        .waitForResponse(
+          () => {
+            return false;
+          },
+          { timeout: 1 }
+        )
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should respect default timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       page.setDefaultTimeout(1);
       await page
-        .waitForResponse(() => false)
-        .catch((error_) => (error = error_));
+        .waitForResponse(() => {
+          return false;
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should work with predicate', async () => {
@@ -835,9 +980,9 @@ describe('Page', function () {
 
       await page.goto(server.EMPTY_PAGE);
       const [response] = await Promise.all([
-        page.waitForResponse(
-          (response) => response.url() === server.PREFIX + '/digits/2.png'
-        ),
+        page.waitForResponse((response) => {
+          return response.url() === server.PREFIX + '/digits/2.png';
+        }),
         page.evaluate(() => {
           fetch('/digits/1.png');
           fetch('/digits/2.png');
@@ -867,13 +1012,13 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       const [response] = await Promise.all([
         page.waitForResponse(server.PREFIX + '/digits/2.png', { timeout: 0 }),
-        page.evaluate(() =>
-          setTimeout(() => {
+        page.evaluate(() => {
+          return setTimeout(() => {
             fetch('/digits/1.png');
             fetch('/digits/2.png');
             fetch('/digits/3.png');
-          }, 50)
-        ),
+          }, 50);
+        }),
       ]);
       expect(response.url()).toBe(server.PREFIX + '/digits/2.png');
     });
@@ -890,19 +1035,25 @@ describe('Page', function () {
           return Date.now();
         }),
         page
-          .evaluate(() =>
-            (async () => {
+          .evaluate(() => {
+            return (async () => {
               await Promise.all([
                 fetch('/digits/1.png'),
                 fetch('/digits/2.png'),
               ]);
-              await new Promise((resolve) => setTimeout(resolve, 200));
+              await new Promise((resolve) => {
+                return setTimeout(resolve, 200);
+              });
               await fetch('/digits/3.png');
-              await new Promise((resolve) => setTimeout(resolve, 200));
+              await new Promise((resolve) => {
+                return setTimeout(resolve, 200);
+              });
               await fetch('/digits/4.png');
-            })()
-          )
-          .then(() => Date.now()),
+            })();
+          })
+          .then(() => {
+            return Date.now();
+          }),
       ]);
       expect(res).toBe(undefined);
       expect(t1).toBeGreaterThan(t2);
@@ -910,28 +1061,34 @@ describe('Page', function () {
     });
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
-      let error = null;
-      await page
-        .waitForNetworkIdle({ timeout: 1 })
-        .catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitForNetworkIdle({ timeout: 1 }).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should respect idleTime', async () => {
       const { page, server } = getTestState();
       await page.goto(server.EMPTY_PAGE);
       const [t1, t2] = await Promise.all([
-        page.waitForNetworkIdle({ idleTime: 10 }).then(() => Date.now()),
+        page.waitForNetworkIdle({ idleTime: 10 }).then(() => {
+          return Date.now();
+        }),
         page
-          .evaluate(() =>
-            (async () => {
+          .evaluate(() => {
+            return (async () => {
               await Promise.all([
                 fetch('/digits/1.png'),
                 fetch('/digits/2.png'),
               ]);
-              await new Promise((resolve) => setTimeout(resolve, 250));
-            })()
-          )
-          .then(() => Date.now()),
+              await new Promise((resolve) => {
+                return setTimeout(resolve, 250);
+              });
+            })();
+          })
+          .then(() => {
+            return Date.now();
+          }),
       ]);
       expect(t2).toBeGreaterThan(t1);
     });
@@ -940,13 +1097,13 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       const [result] = await Promise.all([
         page.waitForNetworkIdle({ timeout: 0 }),
-        page.evaluate(() =>
-          setTimeout(() => {
+        page.evaluate(() => {
+          return setTimeout(() => {
             fetch('/digits/1.png');
             fetch('/digits/2.png');
             fetch('/digits/3.png');
-          }, 50)
-        ),
+          }, 50);
+        }),
       ]);
       expect(result).toBe(undefined);
     });
@@ -956,11 +1113,11 @@ describe('Page', function () {
     it('should work', async () => {
       const { page } = getTestState();
 
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return a * b;
       });
       const result = await page.evaluate(async function () {
-        return await globalThis.compute(9, 4);
+        return await (globalThis as any).compute(9, 4);
       });
       expect(result).toBe(36);
     });
@@ -972,9 +1129,12 @@ describe('Page', function () {
       });
       const { message, stack } = await page.evaluate(async () => {
         try {
-          await globalThis.woof();
+          return await (globalThis as any).woof();
         } catch (error) {
-          return { message: error.message, stack: error.stack };
+          return {
+            message: (error as Error).message,
+            stack: (error as Error).stack,
+          };
         }
       });
       expect(message).toBe('WOOF WOOF');
@@ -988,7 +1148,7 @@ describe('Page', function () {
       });
       const thrown = await page.evaluate(async () => {
         try {
-          await globalThis.woof();
+          await (globalThis as any).woof();
         } catch (error) {
           return error;
         }
@@ -1002,46 +1162,48 @@ describe('Page', function () {
       await page.exposeFunction('woof', function () {
         called = true;
       });
-      await page.evaluateOnNewDocument(() => globalThis.woof());
+      await page.evaluateOnNewDocument(() => {
+        return (globalThis as any).woof();
+      });
       await page.reload();
       expect(called).toBe(true);
     });
     it('should survive navigation', async () => {
       const { page, server } = getTestState();
 
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return a * b;
       });
 
       await page.goto(server.EMPTY_PAGE);
       const result = await page.evaluate(async function () {
-        return await globalThis.compute(9, 4);
+        return await (globalThis as any).compute(9, 4);
       });
       expect(result).toBe(36);
     });
     it('should await returned promise', async () => {
       const { page } = getTestState();
 
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return Promise.resolve(a * b);
       });
 
       const result = await page.evaluate(async function () {
-        return await globalThis.compute(3, 5);
+        return await (globalThis as any).compute(3, 5);
       });
       expect(result).toBe(15);
     });
     it('should work on frames', async () => {
       const { page, server } = getTestState();
 
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return Promise.resolve(a * b);
       });
 
       await page.goto(server.PREFIX + '/frames/nested-frames.html');
-      const frame = page.frames()[1];
+      const frame = page.frames()[1]!;
       const result = await frame.evaluate(async function () {
-        return await globalThis.compute(3, 5);
+        return await (globalThis as any).compute(3, 5);
       });
       expect(result).toBe(15);
     });
@@ -1049,13 +1211,13 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/frames/nested-frames.html');
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return Promise.resolve(a * b);
       });
 
-      const frame = page.frames()[1];
+      const frame = page.frames()[1]!;
       const result = await frame.evaluate(async function () {
-        return await globalThis.compute(3, 5);
+        return await (globalThis as any).compute(3, 5);
       });
       expect(result).toBe(15);
     });
@@ -1064,39 +1226,44 @@ describe('Page', function () {
 
       await page.goto(server.EMPTY_PAGE);
       await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      await page.exposeFunction('compute', function (a, b) {
+      await page.exposeFunction('compute', function (a: number, b: number) {
         return Promise.resolve(a * b);
       });
       await utils.detachFrame(page, 'frame1');
 
       await expect(
         page.evaluate(async function () {
-          return await globalThis.compute(3, 5);
+          return await (globalThis as any).compute(3, 5);
         })
       ).resolves.toEqual(15);
     });
     it('should work with complex objects', async () => {
       const { page } = getTestState();
 
-      await page.exposeFunction('complexObject', function (a, b) {
-        return { x: a.x + b.x };
-      });
+      await page.exposeFunction(
+        'complexObject',
+        function (a: { x: any }, b: { x: any }) {
+          return { x: a.x + b.x };
+        }
+      );
       const result = await page.evaluate<() => Promise<{ x: number }>>(
-        async () => globalThis.complexObject({ x: 5 }, { x: 2 })
+        async () => {
+          return (globalThis as any).complexObject({ x: 5 }, { x: 2 });
+        }
       );
       expect(result.x).toBe(7);
     });
     it('should fallback to default export when passed a module object', async () => {
       const { page, server } = getTestState();
       const moduleObject = {
-        default: function (a, b) {
+        default: function (a: number, b: number) {
           return a * b;
         },
       };
       await page.goto(server.EMPTY_PAGE);
       await page.exposeFunction('compute', moduleObject);
       const result = await page.evaluate(async function () {
-        return await globalThis.compute(9, 4);
+        return await (globalThis as any).compute(9, 4);
       });
       expect(result).toBe(36);
     });
@@ -1106,8 +1273,10 @@ describe('Page', function () {
     it('should fire', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
-      page.once('pageerror', (e) => (error = e));
+      let error!: Error;
+      page.once('pageerror', (e) => {
+        return (error = e);
+      });
       await Promise.all([
         page.goto(server.PREFIX + '/error.html'),
         waitEvent(page, 'pageerror'),
@@ -1120,9 +1289,11 @@ describe('Page', function () {
     it('should work', async () => {
       const { page, server } = getTestState();
 
-      expect(await page.evaluate(() => navigator.userAgent)).toContain(
-        'Mozilla'
-      );
+      expect(
+        await page.evaluate(() => {
+          return navigator.userAgent;
+        })
+      ).toContain('Mozilla');
       await page.setUserAgent('foobar');
       const [request] = await Promise.all([
         server.waitForRequest('/empty.html'),
@@ -1133,9 +1304,11 @@ describe('Page', function () {
     it('should work for subframes', async () => {
       const { page, server } = getTestState();
 
-      expect(await page.evaluate(() => navigator.userAgent)).toContain(
-        'Mozilla'
-      );
+      expect(
+        await page.evaluate(() => {
+          return navigator.userAgent;
+        })
+      ).toContain('Mozilla');
       await page.setUserAgent('foobar');
       const [request] = await Promise.all([
         server.waitForRequest('/empty.html'),
@@ -1147,13 +1320,17 @@ describe('Page', function () {
       const { page, server, puppeteer } = getTestState();
 
       await page.goto(server.PREFIX + '/mobile.html');
-      expect(await page.evaluate(() => navigator.userAgent)).not.toContain(
-        'iPhone'
-      );
-      await page.setUserAgent(puppeteer.devices['iPhone 6'].userAgent);
-      expect(await page.evaluate(() => navigator.userAgent)).toContain(
-        'iPhone'
-      );
+      expect(
+        await page.evaluate(() => {
+          return navigator.userAgent;
+        })
+      ).not.toContain('iPhone');
+      await page.setUserAgent(puppeteer.devices['iPhone 6']!.userAgent);
+      expect(
+        await page.evaluate(() => {
+          return navigator.userAgent;
+        })
+      ).toContain('iPhone');
     });
     itFailsFirefox('should work with additional userAgentMetdata', async () => {
       const { page, server } = getTestState();
@@ -1229,12 +1406,14 @@ describe('Page', function () {
       const imgPath = '/img.png';
       // stall for image
       server.setRoute(imgPath, () => {});
-      let error = null;
+      let error!: Error;
       await page
         .setContent(`<img src="${server.PREFIX + imgPath}"></img>`, {
           timeout: 1,
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should respect default navigation timeout', async () => {
@@ -1244,22 +1423,28 @@ describe('Page', function () {
       const imgPath = '/img.png';
       // stall for image
       server.setRoute(imgPath, () => {});
-      let error = null;
+      let error!: Error;
       await page
         .setContent(`<img src="${server.PREFIX + imgPath}"></img>`)
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should await resources to load', async () => {
       const { page, server } = getTestState();
 
       const imgPath = '/img.png';
-      let imgResponse = null;
-      server.setRoute(imgPath, (req, res) => (imgResponse = res));
+      let imgResponse!: ServerResponse;
+      server.setRoute(imgPath, (_req, res) => {
+        return (imgResponse = res);
+      });
       let loaded = false;
       const contentPromise = page
         .setContent(`<img src="${server.PREFIX + imgPath}"></img>`)
-        .then(() => (loaded = true));
+        .then(() => {
+          return (loaded = true);
+        });
       await server.waitForRequest(imgPath);
       expect(loaded).toBe(false);
       imgResponse.end();
@@ -1276,29 +1461,41 @@ describe('Page', function () {
       const { page } = getTestState();
 
       await page.setContent('<div>hello world</div>' + '\x7F');
-      expect(await page.$eval('div', (div) => div.textContent)).toBe(
-        'hello world'
-      );
+      expect(
+        await page.$eval('div', (div) => {
+          return div.textContent;
+        })
+      ).toBe('hello world');
     });
     it('should work with accents', async () => {
       const { page } = getTestState();
 
       await page.setContent('<div>aberración</div>');
-      expect(await page.$eval('div', (div) => div.textContent)).toBe(
-        'aberración'
-      );
+      expect(
+        await page.$eval('div', (div) => {
+          return div.textContent;
+        })
+      ).toBe('aberración');
     });
     it('should work with emojis', async () => {
       const { page } = getTestState();
 
       await page.setContent('<div>🐥</div>');
-      expect(await page.$eval('div', (div) => div.textContent)).toBe('🐥');
+      expect(
+        await page.$eval('div', (div) => {
+          return div.textContent;
+        })
+      ).toBe('🐥');
     });
     it('should work with newline', async () => {
       const { page } = getTestState();
 
       await page.setContent('<div>\n</div>');
-      expect(await page.$eval('div', (div) => div.textContent)).toBe('\n');
+      expect(
+        await page.$eval('div', (div) => {
+          return div.textContent;
+        })
+      ).toBe('\n');
     });
   });
 
@@ -1310,14 +1507,24 @@ describe('Page', function () {
       await page.goto(server.PREFIX + '/csp.html');
       await page
         .addScriptTag({ content: 'window.__injected = 42;' })
-        .catch((error) => void error);
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(undefined);
+        .catch((error) => {
+          return void error;
+        });
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(undefined);
 
       // By-pass CSP and try one more time.
       await page.setBypassCSP(true);
       await page.reload();
       await page.addScriptTag({ content: 'window.__injected = 42;' });
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
     });
 
     it('should bypass CSP header', async () => {
@@ -1328,14 +1535,24 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       await page
         .addScriptTag({ content: 'window.__injected = 42;' })
-        .catch((error) => void error);
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(undefined);
+        .catch((error) => {
+          return void error;
+        });
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(undefined);
 
       // By-pass CSP and try one more time.
       await page.setBypassCSP(true);
       await page.reload();
       await page.addScriptTag({ content: 'window.__injected = 42;' });
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
     });
 
     it('should bypass after cross-process navigation', async () => {
@@ -1344,11 +1561,19 @@ describe('Page', function () {
       await page.setBypassCSP(true);
       await page.goto(server.PREFIX + '/csp.html');
       await page.addScriptTag({ content: 'window.__injected = 42;' });
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
 
       await page.goto(server.CROSS_PROCESS_PREFIX + '/csp.html');
       await page.addScriptTag({ content: 'window.__injected = 42;' });
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
     });
     it('should bypass CSP in iframes as well', async () => {
       const { page, server } = getTestState();
@@ -1356,17 +1581,21 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       {
         // Make sure CSP prohibits addScriptTag in an iframe.
-        const frame = await utils.attachFrame(
+        const frame = (await attachFrame(
           page,
           'frame1',
           server.PREFIX + '/csp.html'
-        );
+        ))!;
         await frame
           .addScriptTag({ content: 'window.__injected = 42;' })
-          .catch((error) => void error);
-        expect(await frame.evaluate(() => globalThis.__injected)).toBe(
-          undefined
-        );
+          .catch((error) => {
+            return void error;
+          });
+        expect(
+          await frame.evaluate(() => {
+            return (globalThis as any).__injected;
+          })
+        ).toBe(undefined);
       }
 
       // By-pass CSP and try one more time.
@@ -1374,15 +1603,21 @@ describe('Page', function () {
       await page.reload();
 
       {
-        const frame = await utils.attachFrame(
+        const frame = (await attachFrame(
           page,
           'frame1',
           server.PREFIX + '/csp.html'
-        );
+        ))!;
         await frame
           .addScriptTag({ content: 'window.__injected = 42;' })
-          .catch((error) => void error);
-        expect(await frame.evaluate(() => globalThis.__injected)).toBe(42);
+          .catch((error) => {
+            return void error;
+          });
+        expect(
+          await frame.evaluate(() => {
+            return (globalThis as any).__injected;
+          })
+        ).toBe(42);
       }
     });
   });
@@ -1391,12 +1626,12 @@ describe('Page', function () {
     it('should throw an error if no options are provided', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
         // @ts-expect-error purposefully passing bad options
         await page.addScriptTag('/injectedfile.js');
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toBe(
         'Provide an object with a `url`, `path` or `content` property'
@@ -1409,7 +1644,11 @@ describe('Page', function () {
       await page.goto(server.EMPTY_PAGE);
       const scriptHandle = await page.addScriptTag({ url: '/injectedfile.js' });
       expect(scriptHandle.asElement()).not.toBeNull();
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
     });
 
     it('should work with a url and type=module', async () => {
@@ -1417,7 +1656,11 @@ describe('Page', function () {
 
       await page.goto(server.EMPTY_PAGE);
       await page.addScriptTag({ url: '/es6/es6import.js', type: 'module' });
-      expect(await page.evaluate(() => globalThis.__es6injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__es6injected;
+        })
+      ).toBe(42);
     });
 
     it('should work with a path and type=module', async () => {
@@ -1429,7 +1672,11 @@ describe('Page', function () {
         type: 'module',
       });
       await page.waitForFunction('window.__es6injected');
-      expect(await page.evaluate(() => globalThis.__es6injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__es6injected;
+        })
+      ).toBe(42);
     });
 
     it('should work with a content and type=module', async () => {
@@ -1441,18 +1688,22 @@ describe('Page', function () {
         type: 'module',
       });
       await page.waitForFunction('window.__es6injected');
-      expect(await page.evaluate(() => globalThis.__es6injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__es6injected;
+        })
+      ).toBe(42);
     });
 
     it('should throw an error if loading from url fail', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
-      let error = null;
+      let error!: Error;
       try {
         await page.addScriptTag({ url: '/nonexistfile.js' });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toBe('Loading script from /nonexistfile.js failed');
     });
@@ -1465,7 +1716,11 @@ describe('Page', function () {
         path: path.join(__dirname, '../assets/injectedfile.js'),
       });
       expect(scriptHandle.asElement()).not.toBeNull();
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(42);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(42);
     });
 
     it('should include sourcemap when path is provided', async () => {
@@ -1475,9 +1730,9 @@ describe('Page', function () {
       await page.addScriptTag({
         path: path.join(__dirname, '../assets/injectedfile.js'),
       });
-      const result = await page.evaluate(
-        () => globalThis.__injectedError.stack
-      );
+      const result = await page.evaluate(() => {
+        return (globalThis as any).__injectedError.stack;
+      });
       expect(result).toContain(path.join('assets', 'injectedfile.js'));
     });
 
@@ -1489,7 +1744,11 @@ describe('Page', function () {
         content: 'window.__injected = 35;',
       });
       expect(scriptHandle.asElement()).not.toBeNull();
-      expect(await page.evaluate(() => globalThis.__injected)).toBe(35);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).__injected;
+        })
+      ).toBe(35);
     });
 
     it('should add id when provided', async () => {
@@ -1506,10 +1765,12 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/csp.html');
-      let error = null;
+      let error!: Error;
       await page
         .addScriptTag({ content: 'window.__injected = 35;' })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
     });
 
@@ -1517,10 +1778,12 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/csp.html');
-      let error = null;
+      let error!: Error;
       await page
         .addScriptTag({ url: server.CROSS_PROCESS_PREFIX + '/injectedfile.js' })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
     });
   });
@@ -1529,12 +1792,12 @@ describe('Page', function () {
     it('should throw an error if no options are provided', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
         // @ts-expect-error purposefully passing bad input
         await page.addStyleTag('/injectedstyle.css');
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toBe(
         'Provide an object with a `url`, `path` or `content` property'
@@ -1558,11 +1821,11 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
-      let error = null;
+      let error!: Error;
       try {
         await page.addStyleTag({ url: '/nonexistfile.js' });
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toBe('Loading style from /nonexistfile.js failed');
     });
@@ -1589,11 +1852,10 @@ describe('Page', function () {
       await page.addStyleTag({
         path: path.join(__dirname, '../assets/injectedstyle.css'),
       });
-      const styleHandle = await page.$('style');
-      const styleContent = await page.evaluate(
-        (style: HTMLStyleElement) => style.innerHTML,
-        styleHandle
-      );
+      const styleHandle = (await page.$('style'))!;
+      const styleContent = await page.evaluate((style: HTMLStyleElement) => {
+        return style.innerHTML;
+      }, styleHandle);
       expect(styleContent).toContain(path.join('assets', 'injectedstyle.css'));
     });
 
@@ -1618,10 +1880,12 @@ describe('Page', function () {
         const { page, server } = getTestState();
 
         await page.goto(server.PREFIX + '/csp.html');
-        let error = null;
+        let error!: Error;
         await page
           .addStyleTag({ content: 'body { background-color: green; }' })
-          .catch((error_) => (error = error_));
+          .catch((error_) => {
+            return (error = error_);
+          });
         expect(error).toBeTruthy();
       }
     );
@@ -1630,12 +1894,14 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/csp.html');
-      let error = null;
+      let error!: Error;
       await page
         .addStyleTag({
           url: server.CROSS_PROCESS_PREFIX + '/injectedstyle.css',
         })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
     });
   });
@@ -1658,8 +1924,10 @@ describe('Page', function () {
       await page.goto(
         'data:text/html, <script>var something = "forbidden"</script>'
       );
-      let error = null;
-      await page.evaluate('something').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.evaluate('something').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain('something is not defined');
 
       await page.setJavaScriptEnabled(true);
@@ -1747,8 +2015,10 @@ describe('Page', function () {
 
       await page.goto(server.PREFIX + '/pdf.html');
 
-      let error = null;
-      await page.pdf({ timeout: 1 }).catch((_error) => (error = _error));
+      let error!: Error;
+      await page.pdf({ timeout: 1 }).catch((_error) => {
+        return (error = _error);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
   });
@@ -1768,35 +2038,42 @@ describe('Page', function () {
 
       await page.goto(server.PREFIX + '/input/select.html');
       await page.select('select', 'blue');
-      expect(await page.evaluate(() => globalThis.result.onInput)).toEqual([
-        'blue',
-      ]);
-      expect(await page.evaluate(() => globalThis.result.onChange)).toEqual([
-        'blue',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onInput;
+        })
+      ).toEqual(['blue']);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onChange;
+        })
+      ).toEqual(['blue']);
     });
     it('should select only first option', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/select.html');
       await page.select('select', 'blue', 'green', 'red');
-      expect(await page.evaluate(() => globalThis.result.onInput)).toEqual([
-        'blue',
-      ]);
-      expect(await page.evaluate(() => globalThis.result.onChange)).toEqual([
-        'blue',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onInput;
+        })
+      ).toEqual(['blue']);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onChange;
+        })
+      ).toEqual(['blue']);
     });
     it('should not throw when select causes navigation', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/select.html');
-      await page.$eval('select', (select) =>
-        select.addEventListener(
-          'input',
-          () => ((window as any).location = '/empty.html')
-        )
-      );
+      await page.$eval('select', (select) => {
+        return select.addEventListener('input', () => {
+          return ((window as any).location = '/empty.html');
+        });
+      });
       await Promise.all([
         page.select('select', 'blue'),
         page.waitForNavigation(),
@@ -1807,18 +2084,20 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/select.html');
-      await page.evaluate(() => globalThis.makeMultiple());
+      await page.evaluate(() => {
+        return (globalThis as any).makeMultiple();
+      });
       await page.select('select', 'blue', 'green', 'red');
-      expect(await page.evaluate(() => globalThis.result.onInput)).toEqual([
-        'blue',
-        'green',
-        'red',
-      ]);
-      expect(await page.evaluate(() => globalThis.result.onChange)).toEqual([
-        'blue',
-        'green',
-        'red',
-      ]);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onInput;
+        })
+      ).toEqual(['blue', 'green', 'red']);
+      expect(
+        await page.evaluate(() => {
+          return (globalThis as any).result.onChange;
+        })
+      ).toEqual(['blue', 'green', 'red']);
     });
     it('should respect event bubbling', async () => {
       const { page, server } = getTestState();
@@ -1826,18 +2105,24 @@ describe('Page', function () {
       await page.goto(server.PREFIX + '/input/select.html');
       await page.select('select', 'blue');
       expect(
-        await page.evaluate(() => globalThis.result.onBubblingInput)
+        await page.evaluate(() => {
+          return (globalThis as any).result.onBubblingInput;
+        })
       ).toEqual(['blue']);
       expect(
-        await page.evaluate(() => globalThis.result.onBubblingChange)
+        await page.evaluate(() => {
+          return (globalThis as any).result.onBubblingChange;
+        })
       ).toEqual(['blue']);
     });
     it('should throw when element is not a <select>', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page.goto(server.PREFIX + '/input/select.html');
-      await page.select('body', '').catch((error_) => (error = error_));
+      await page.select('body', '').catch((error_) => {
+        return (error = error_);
+      });
       expect(error.message).toContain('Element is not a <select> element.');
     });
     it('should return [] on no matched values', async () => {
@@ -1851,14 +2136,14 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/select.html');
-      await page.evaluate(() => globalThis.makeMultiple());
+      await page.evaluate(() => {
+        return (globalThis as any).makeMultiple();
+      });
       const result = await page.select('select', 'blue', 'black', 'magenta');
       expect(
-        result.reduce(
-          (accumulator, current) =>
-            ['blue', 'black', 'magenta'].includes(current) && accumulator,
-          true
-        )
+        result.reduce((accumulator, current) => {
+          return ['blue', 'black', 'magenta'].includes(current) && accumulator;
+        }, true)
       ).toEqual(true);
     });
     it('should return an array of one element when multiple is not set', async () => {
@@ -1885,15 +2170,19 @@ describe('Page', function () {
       const { page, server } = getTestState();
 
       await page.goto(server.PREFIX + '/input/select.html');
-      await page.evaluate(() => globalThis.makeMultiple());
+      await page.evaluate(() => {
+        return (globalThis as any).makeMultiple();
+      });
       await page.select('select', 'blue', 'black', 'magenta');
       await page.select('select');
       expect(
-        await page.$eval('select', (select: HTMLSelectElement) =>
-          Array.from(select.options).every(
-            (option: HTMLOptionElement) => !option.selected
-          )
-        )
+        await page.$eval('select', (select) => {
+          return Array.from((select as HTMLSelectElement).options).every(
+            (option) => {
+              return !option.selected;
+            }
+          );
+        })
       ).toEqual(true);
     });
     it('should deselect all options when passed no values for a select without multiple', async () => {
@@ -1903,24 +2192,25 @@ describe('Page', function () {
       await page.select('select', 'blue', 'black', 'magenta');
       await page.select('select');
       expect(
-        await page.$eval(
-          'select',
-          (select: HTMLSelectElement) =>
-            Array.from(select.options).filter((option) => option.selected)[0]
-              .value
-        )
+        await page.$eval('select', (select) => {
+          return Array.from((select as HTMLSelectElement).options).filter(
+            (option) => {
+              return option.selected;
+            }
+          )[0]!.value;
+        })
       ).toEqual('');
     });
     it('should throw if passed in non-strings', async () => {
       const { page } = getTestState();
 
       await page.setContent('<select><option value="12"/></select>');
-      let error = null;
+      let error!: Error;
       try {
         // @ts-expect-error purposefully passing bad input
         await page.select('select', 12);
       } catch (error_) {
-        error = error_;
+        error = error_ as Error;
       }
       expect(error.message).toContain('Values must be strings');
     });
@@ -1931,14 +2221,21 @@ describe('Page', function () {
         const { page, server } = getTestState();
 
         await page.goto(server.PREFIX + '/input/select.html');
-        await page.evaluate(() => (window.Event = null));
+        await page.evaluate(() => {
+          // @ts-expect-error Expected.
+          return (window.Event = undefined);
+        });
         await page.select('select', 'blue');
-        expect(await page.evaluate(() => globalThis.result.onInput)).toEqual([
-          'blue',
-        ]);
-        expect(await page.evaluate(() => globalThis.result.onChange)).toEqual([
-          'blue',
-        ]);
+        expect(
+          await page.evaluate(() => {
+            return (globalThis as any).result.onInput;
+          })
+        ).toEqual(['blue']);
+        expect(
+          await page.evaluate(() => {
+            return (globalThis as any).result.onChange;
+          })
+        ).toEqual(['blue']);
       }
     );
   });
@@ -1947,22 +2244,30 @@ describe('Page', function () {
     itFailsFirefox('should work with window.close', async () => {
       const { page, context } = getTestState();
 
-      const newPagePromise = new Promise<Page>((fulfill) =>
-        context.once('targetcreated', (target) => fulfill(target.page()))
-      );
-      await page.evaluate(
-        () => (window['newPage'] = window.open('about:blank'))
-      );
+      const newPagePromise = new Promise<Page>((fulfill) => {
+        return context.once('targetcreated', (target) => {
+          return fulfill(target.page());
+        });
+      });
+      await page.evaluate(() => {
+        return ((window as any)['newPage'] = window.open('about:blank'));
+      });
       const newPage = await newPagePromise;
-      const closedPromise = new Promise((x) => newPage.on('close', x));
-      await page.evaluate(() => window['newPage'].close());
+      const closedPromise = new Promise((x) => {
+        return newPage.on('close', x);
+      });
+      await page.evaluate(() => {
+        return (window as any)['newPage'].close();
+      });
       await closedPromise;
     });
     it('should work with page.close', async () => {
       const { context } = getTestState();
 
       const newPage = await context.newPage();
-      const closedPromise = new Promise((x) => newPage.on('close', x));
+      const closedPromise = new Promise((x) => {
+        return newPage.on('close', x);
+      });
       await newPage.close();
       await closedPromise;
     });
diff --git a/test/src/proxy.spec.ts b/test/src/proxy.spec.ts
index 3c102fe6cb225..f6ddaf79aedc6 100644
--- a/test/src/proxy.spec.ts
+++ b/test/src/proxy.spec.ts
@@ -21,7 +21,7 @@ import {
   getTestState,
   describeFailsFirefox,
   itFailsWindows,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import type { Server, IncomingMessage, ServerResponse } from 'http';
 import type { Browser } from '../../lib/cjs/puppeteer/common/Browser.js';
 import type { AddressInfo } from 'net';
@@ -108,7 +108,7 @@ describeFailsFirefox('request proxy', () => {
     });
 
     const page = await browser.newPage();
-    const response = await page.goto(emptyPageUrl);
+    const response = (await page.goto(emptyPageUrl))!;
 
     expect(response.ok()).toBe(true);
 
@@ -129,7 +129,7 @@ describeFailsFirefox('request proxy', () => {
     });
 
     const page = await browser.newPage();
-    const response = await page.goto(emptyPageUrl);
+    const response = (await page.goto(emptyPageUrl))!;
 
     expect(response.ok()).toBe(true);
 
@@ -148,7 +148,7 @@ describeFailsFirefox('request proxy', () => {
 
       const context = await browser.createIncognitoBrowserContext();
       const page = await context.newPage();
-      const response = await page.goto(emptyPageUrl);
+      const response = (await page.goto(emptyPageUrl))!;
 
       expect(response.ok()).toBe(true);
 
@@ -170,7 +170,7 @@ describeFailsFirefox('request proxy', () => {
 
       const context = await browser.createIncognitoBrowserContext();
       const page = await context.newPage();
-      const response = await page.goto(emptyPageUrl);
+      const response = (await page.goto(emptyPageUrl))!;
 
       expect(response.ok()).toBe(true);
 
@@ -195,7 +195,7 @@ describeFailsFirefox('request proxy', () => {
           proxyServer: proxyServerUrl,
         });
         const page = await context.newPage();
-        const response = await page.goto(emptyPageUrl);
+        const response = (await page.goto(emptyPageUrl))!;
 
         expect(response.ok()).toBe(true);
 
@@ -217,7 +217,7 @@ describeFailsFirefox('request proxy', () => {
         proxyBypassList: [new URL(emptyPageUrl).host],
       });
       const page = await context.newPage();
-      const response = await page.goto(emptyPageUrl);
+      const response = (await page.goto(emptyPageUrl))!;
 
       expect(response.ok()).toBe(true);
 
diff --git a/test/src/queryselector.spec.ts b/test/src/queryselector.spec.ts
index 08f9363e72bd2..cdf3fa74848f7 100644
--- a/test/src/queryselector.spec.ts
+++ b/test/src/queryselector.spec.ts
@@ -18,7 +18,7 @@ import {
   getTestState,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 import { CustomQueryHandler } from '../../lib/cjs/puppeteer/common/QueryHandler.js';
 
 describe('querySelector', function () {
@@ -29,7 +29,9 @@ describe('querySelector', function () {
       const { page } = getTestState();
 
       await page.setContent('<section id="testAttribute">43543</section>');
-      const idAttribute = await page.$eval('section', (e) => e.id);
+      const idAttribute = await page.$eval('section', (e) => {
+        return e.id;
+      });
       expect(idAttribute).toBe('testAttribute');
     });
     it('should accept arguments', async () => {
@@ -38,7 +40,9 @@ describe('querySelector', function () {
       await page.setContent('<section>hello</section>');
       const text = await page.$eval(
         'section',
-        (e, suffix) => e.textContent + suffix,
+        (e, suffix) => {
+          return e.textContent! + suffix;
+        },
         ' world!'
       );
       expect(text).toBe('hello world!');
@@ -47,10 +51,12 @@ describe('querySelector', function () {
       const { page } = getTestState();
 
       await page.setContent('<section>hello</section><div> world</div>');
-      const divHandle = await page.$('div');
+      const divHandle = (await page.$('div'))!;
       const text = await page.$eval(
         'section',
-        (e, div: HTMLElement) => e.textContent + div.textContent,
+        (e, div) => {
+          return e.textContent! + (div as HTMLElement).textContent!;
+        },
         divHandle
       );
       expect(text).toBe('hello world');
@@ -58,10 +64,14 @@ describe('querySelector', function () {
     it('should throw error if no element is found', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await page
-        .$eval('section', (e) => e.id)
-        .catch((error_) => (error = error_));
+        .$eval('section', (e) => {
+          return e.id;
+        })
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error.message).toContain(
         'failed to find element matching selector "section"'
       );
@@ -89,39 +99,43 @@ describe('querySelector', function () {
     });
     it('should find first element in shadow', async () => {
       const { page } = getTestState();
-      const div = await page.$('pierce/.foo');
-      const text = await div.evaluate(
-        (element: Element) => element.textContent
-      );
+      const div = (await page.$('pierce/.foo'))!;
+      const text = await div.evaluate((element: Element) => {
+        return element.textContent;
+      });
       expect(text).toBe('Hello');
     });
     it('should find all elements in shadow', async () => {
       const { page } = getTestState();
       const divs = await page.$$('pierce/.foo');
       const text = await Promise.all(
-        divs.map((div) =>
-          div.evaluate((element: Element) => element.textContent)
-        )
+        divs.map((div) => {
+          return div.evaluate((element: Element) => {
+            return element.textContent;
+          });
+        })
       );
       expect(text.join(' ')).toBe('Hello World');
     });
     it('should find first child element', async () => {
       const { page } = getTestState();
-      const parentElement = await page.$('html > div');
-      const childElement = await parentElement.$('pierce/div');
-      const text = await childElement.evaluate(
-        (element: Element) => element.textContent
-      );
+      const parentElement = (await page.$('html > div'))!;
+      const childElement = (await parentElement.$('pierce/div'))!;
+      const text = await childElement.evaluate((element: Element) => {
+        return element.textContent;
+      });
       expect(text).toBe('Hello');
     });
     it('should find all child elements', async () => {
       const { page } = getTestState();
-      const parentElement = await page.$('html > div');
+      const parentElement = (await page.$('html > div'))!;
       const childElements = await parentElement.$$('pierce/div');
       const text = await Promise.all(
-        childElements.map((div) =>
-          div.evaluate((element: Element) => element.textContent)
-        )
+        childElements.map((div) => {
+          return div.evaluate((element: Element) => {
+            return element.textContent;
+          });
+        })
       );
       expect(text.join(' ')).toBe('Hello World');
     });
@@ -137,7 +151,9 @@ describe('querySelector', function () {
       await page.setContent(
         '<div>hello</div><div>beautiful</div><div>world!</div>'
       );
-      const divsCount = await page.$$eval('div', (divs) => divs.length);
+      const divsCount = await page.$$eval('div', (divs) => {
+        return divs.length;
+      });
       expect(divsCount).toBe(3);
     });
     it('should accept extra arguments', async () => {
@@ -147,7 +163,9 @@ describe('querySelector', function () {
       );
       const divsCountPlus5 = await page.$$eval(
         'div',
-        (divs, two: number, three: number) => divs.length + two + three,
+        (divs, two, three) => {
+          return divs.length + (two as number) + (three as number);
+        },
         2,
         3
       );
@@ -158,14 +176,16 @@ describe('querySelector', function () {
       await page.setContent(
         '<section>2</section><section>2</section><section>1</section><div>3</div>'
       );
-      const divHandle = await page.$('div');
+      const divHandle = (await page.$('div'))!;
       const sum = await page.$$eval(
         'section',
-        (sections, div: HTMLElement) =>
-          sections.reduce(
-            (acc, section) => acc + Number(section.textContent),
-            0
-          ) + Number(div.textContent),
+        (sections, div) => {
+          return (
+            sections.reduce((acc, section) => {
+              return acc + Number(section.textContent);
+            }, 0) + Number((div as HTMLElement).textContent)
+          );
+        },
         divHandle
       );
       expect(sum).toBe(8);
@@ -181,9 +201,11 @@ describe('querySelector', function () {
         }
         `
       );
-      const sum = await page.$$eval('section', (sections) =>
-        sections.reduce((acc, section) => acc + Number(section.textContent), 0)
-      );
+      const sum = await page.$$eval('section', (sections) => {
+        return sections.reduce((acc, section) => {
+          return acc + Number(section.textContent);
+        }, 0);
+      });
       expect(sum).toBe(500500);
     });
   });
@@ -193,13 +215,13 @@ describe('querySelector', function () {
       const { page } = getTestState();
 
       await page.setContent('<section>test</section>');
-      const element = await page.$('section');
+      const element = (await page.$('section'))!;
       expect(element).toBeTruthy();
     });
     it('should return null for non-existing element', async () => {
       const { page } = getTestState();
 
-      const element = await page.$('non-existing-element');
+      const element = (await page.$('non-existing-element'))!;
       expect(element).toBe(null);
     });
   });
@@ -211,9 +233,11 @@ describe('querySelector', function () {
       await page.setContent('<div>A</div><br/><div>B</div>');
       const elements = await page.$$('div');
       expect(elements.length).toBe(2);
-      const promises = elements.map((element) =>
-        page.evaluate((e: HTMLElement) => e.textContent, element)
-      );
+      const promises = elements.map((element) => {
+        return page.evaluate((e: HTMLElement) => {
+          return e.textContent;
+        }, element);
+      });
       expect(await Promise.all(promises)).toEqual(['A', 'B']);
     });
     it('should return empty array if nothing is found', async () => {
@@ -231,7 +255,7 @@ describe('querySelector', function () {
 
       await page.setContent('<section>test</section>');
       const elements = await page.$x('/html/body/section');
-      expect(elements[0]).toBeTruthy();
+      expect(elements[0]!).toBeTruthy();
       expect(elements.length).toBe(1);
     });
     it('should return empty array for non-existing element', async () => {
@@ -257,13 +281,12 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="second"><div class="inner">A</div></div></body></html>'
       );
-      const html = await page.$('html');
-      const second = await html.$('.second');
+      const html = (await page.$('html'))!;
+      const second = (await html.$('.second'))!;
       const inner = await second.$('.inner');
-      const content = await page.evaluate(
-        (e: HTMLElement) => e.textContent,
-        inner
-      );
+      const content = await page.evaluate((e: HTMLElement) => {
+        return e.textContent;
+      }, inner);
       expect(content).toBe('A');
     });
 
@@ -273,7 +296,7 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="second"><div class="inner">B</div></div></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const second = await html.$('.third');
       expect(second).toBe(null);
     });
@@ -285,11 +308,10 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="tweet"><div class="like">100</div><div class="retweets">10</div></div></body></html>'
       );
-      const tweet = await page.$('.tweet');
-      const content = await tweet.$eval(
-        '.like',
-        (node: HTMLElement) => node.innerText
-      );
+      const tweet = (await page.$('.tweet'))!;
+      const content = await tweet.$eval('.like', (node) => {
+        return (node as HTMLElement).innerText;
+      });
       expect(content).toBe('100');
     });
 
@@ -299,11 +321,10 @@ describe('querySelector', function () {
       const htmlContent =
         '<div class="a">not-a-child-div</div><div id="myId"><div class="a">a-child-div</div></div>';
       await page.setContent(htmlContent);
-      const elementHandle = await page.$('#myId');
-      const content = await elementHandle.$eval(
-        '.a',
-        (node: HTMLElement) => node.innerText
-      );
+      const elementHandle = (await page.$('#myId'))!;
+      const content = await elementHandle.$eval('.a', (node) => {
+        return (node as HTMLElement).innerText;
+      });
       expect(content).toBe('a-child-div');
     });
 
@@ -313,10 +334,14 @@ describe('querySelector', function () {
       const htmlContent =
         '<div class="a">not-a-child-div</div><div id="myId"></div>';
       await page.setContent(htmlContent);
-      const elementHandle = await page.$('#myId');
+      const elementHandle = (await page.$('#myId'))!;
       const errorMessage = await elementHandle
-        .$eval('.a', (node: HTMLElement) => node.innerText)
-        .catch((error) => error.message);
+        .$eval('.a', (node) => {
+          return (node as HTMLElement).innerText;
+        })
+        .catch((error) => {
+          return error.message;
+        });
       expect(errorMessage).toBe(
         `Error: failed to find element matching selector ".a"`
       );
@@ -329,10 +354,12 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="tweet"><div class="like">100</div><div class="like">10</div></div></body></html>'
       );
-      const tweet = await page.$('.tweet');
-      const content = await tweet.$$eval('.like', (nodes: HTMLElement[]) =>
-        nodes.map((n) => n.innerText)
-      );
+      const tweet = (await page.$('.tweet'))!;
+      const content = await tweet.$$eval('.like', (nodes) => {
+        return (nodes as HTMLElement[]).map((n) => {
+          return n.innerText;
+        });
+      });
       expect(content).toEqual(['100', '10']);
     });
 
@@ -342,10 +369,12 @@ describe('querySelector', function () {
       const htmlContent =
         '<div class="a">not-a-child-div</div><div id="myId"><div class="a">a1-child-div</div><div class="a">a2-child-div</div></div>';
       await page.setContent(htmlContent);
-      const elementHandle = await page.$('#myId');
-      const content = await elementHandle.$$eval('.a', (nodes: HTMLElement[]) =>
-        nodes.map((n) => n.innerText)
-      );
+      const elementHandle = (await page.$('#myId'))!;
+      const content = await elementHandle.$$eval('.a', (nodes) => {
+        return (nodes as HTMLElement[]).map((n) => {
+          return n.innerText;
+        });
+      });
       expect(content).toEqual(['a1-child-div', 'a2-child-div']);
     });
 
@@ -355,11 +384,10 @@ describe('querySelector', function () {
       const htmlContent =
         '<div class="a">not-a-child-div</div><div id="myId"></div>';
       await page.setContent(htmlContent);
-      const elementHandle = await page.$('#myId');
-      const nodesLength = await elementHandle.$$eval(
-        '.a',
-        (nodes) => nodes.length
-      );
+      const elementHandle = (await page.$('#myId'))!;
+      const nodesLength = await elementHandle.$$eval('.a', (nodes) => {
+        return nodes.length;
+      });
       expect(nodesLength).toBe(0);
     });
   });
@@ -371,12 +399,14 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div>A</div><br/><div>B</div></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const elements = await html.$$('div');
       expect(elements.length).toBe(2);
-      const promises = elements.map((element) =>
-        page.evaluate((e: HTMLElement) => e.textContent, element)
-      );
+      const promises = elements.map((element) => {
+        return page.evaluate((e: HTMLElement) => {
+          return e.textContent;
+        }, element);
+      });
       expect(await Promise.all(promises)).toEqual(['A', 'B']);
     });
 
@@ -386,7 +416,7 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><span>A</span><br/><span>B</span></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const elements = await html.$$('div');
       expect(elements.length).toBe(0);
     });
@@ -400,13 +430,12 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="second"><div class="inner">A</div></div></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const second = await html.$x(`./body/div[contains(@class, 'second')]`);
-      const inner = await second[0].$x(`./div[contains(@class, 'inner')]`);
-      const content = await page.evaluate(
-        (e: HTMLElement) => e.textContent,
-        inner[0]
-      );
+      const inner = await second[0]!.$x(`./div[contains(@class, 'inner')]`);
+      const content = await page.evaluate((e: HTMLElement) => {
+        return e.textContent;
+      }, inner[0]!);
       expect(content).toBe('A');
     });
 
@@ -416,7 +445,7 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div class="second"><div class="inner">B</div></div></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const second = await html.$x(`/div[contains(@class, 'third')]`);
       expect(second).toEqual([]);
     });
@@ -426,8 +455,9 @@ describe('querySelector', function () {
   // handler that returns an array instead of a list of nodes.
   describe('QueryAll', function () {
     const handler: CustomQueryHandler = {
-      queryAll: (element: Element, selector: string) =>
-        Array.from(element.querySelectorAll(selector)),
+      queryAll: (element, selector) => {
+        return Array.from(element.querySelectorAll(selector));
+      },
     };
     before(() => {
       const { puppeteer } = getTestState();
@@ -446,12 +476,14 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><div>A</div><br/><div>B</div></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const elements = await html.$$('allArray/div');
       expect(elements.length).toBe(2);
-      const promises = elements.map((element) =>
-        page.evaluate((e: HTMLElement) => e.textContent, element)
-      );
+      const promises = elements.map((element) => {
+        return page.evaluate((e: HTMLElement) => {
+          return e.textContent;
+        }, element);
+      });
       expect(await Promise.all(promises)).toEqual(['A', 'B']);
     });
 
@@ -461,7 +493,7 @@ describe('querySelector', function () {
       await page.setContent(
         '<html><body><span>A</span><br/><span>B</span></body></html>'
       );
-      const html = await page.$('html');
+      const html = (await page.$('html'))!;
       const elements = await html.$$('allArray/div');
       expect(elements.length).toBe(0);
     });
@@ -471,10 +503,9 @@ describe('querySelector', function () {
       await page.setContent(
         '<div>hello</div><div>beautiful</div><div>world!</div>'
       );
-      const divsCount = await page.$$eval(
-        'allArray/div',
-        (divs) => divs.length
-      );
+      const divsCount = await page.$$eval('allArray/div', (divs) => {
+        return divs.length;
+      });
       expect(divsCount).toBe(3);
     });
     it('$$eval should accept extra arguments', async () => {
@@ -484,7 +515,9 @@ describe('querySelector', function () {
       );
       const divsCountPlus5 = await page.$$eval(
         'allArray/div',
-        (divs, two: number, three: number) => divs.length + two + three,
+        (divs, two, three) => {
+          return divs.length + (two as number) + (three as number);
+        },
         2,
         3
       );
@@ -495,14 +528,16 @@ describe('querySelector', function () {
       await page.setContent(
         '<section>2</section><section>2</section><section>1</section><div>3</div>'
       );
-      const divHandle = await page.$('div');
+      const divHandle = (await page.$('div'))!;
       const sum = await page.$$eval(
         'allArray/section',
-        (sections, div: HTMLElement) =>
-          sections.reduce(
-            (acc, section) => acc + Number(section.textContent),
-            0
-          ) + Number(div.textContent),
+        (sections, div) => {
+          return (
+            sections.reduce((acc, section) => {
+              return acc + Number(section.textContent);
+            }, 0) + Number((div as HTMLElement).textContent)
+          );
+        },
         divHandle
       );
       expect(sum).toBe(8);
@@ -518,9 +553,11 @@ describe('querySelector', function () {
         }
         `
       );
-      const sum = await page.$$eval('allArray/section', (sections) =>
-        sections.reduce((acc, section) => acc + Number(section.textContent), 0)
-      );
+      const sum = await page.$$eval('allArray/section', (sections) => {
+        return sections.reduce((acc, section) => {
+          return acc + Number(section.textContent);
+        }, 0);
+      });
       expect(sum).toBe(500500);
     });
   });
diff --git a/test/src/requestinterception-experimental.spec.ts b/test/src/requestinterception-experimental.spec.ts
index 2365764bac118..50015a1559f8b 100644
--- a/test/src/requestinterception-experimental.spec.ts
+++ b/test/src/requestinterception-experimental.spec.ts
@@ -23,9 +23,13 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import { ActionResult } from '../../lib/cjs/puppeteer/api-docs-entry.js';
-import { InterceptResolutionAction } from '../../lib/cjs/puppeteer/common/HTTPRequest.js';
+} from './mocha-utils.js';
+import { ConsoleMessage } from '../../lib/cjs/puppeteer/common/ConsoleMessage.js';
+import {
+  ActionResult,
+  HTTPRequest,
+  InterceptResolutionAction,
+} from '../../lib/cjs/puppeteer/common/HTTPRequest.js';
 
 describe('request interception', function () {
   setupTestBrowserHooks();
@@ -67,8 +71,8 @@ describe('request interception', function () {
           }
         });
         page.on('response', (response) => {
-          const { xaction } = response.headers();
-          if (response.url().endsWith('.css') && !!xaction) {
+          const { xaction } = response!.headers();
+          if (response!.url().endsWith('.css') && !!xaction) {
             actionResults.push(xaction as ActionResult);
           }
         });
@@ -78,22 +82,24 @@ describe('request interception', function () {
           }
         });
 
-        const response = await (async () => {
+        const response = (await (async () => {
           if (expectedAction === 'continue') {
             const [serverRequest, response] = await Promise.all([
               server.waitForRequest('/one-style.css'),
               page.goto(server.PREFIX + '/one-style.html'),
             ]);
-            actionResults.push(serverRequest.headers.xaction as ActionResult);
+            actionResults.push(
+              serverRequest.headers['xaction'] as ActionResult
+            );
             return response;
           } else {
             return await page.goto(server.PREFIX + '/one-style.html');
           }
-        })();
+        })())!;
 
         expect(actionResults.length).toBe(1);
-        expect(actionResults[0]).toBe(expectedAction);
-        expect(response.ok()).toBe(true);
+        expect(actionResults[0]!).toBe(expectedAction);
+        expect(response!.ok()).toBe(true);
       });
     }
 
@@ -113,12 +119,12 @@ describe('request interception', function () {
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.resourceType()).toBe('document');
         expect(request.frame() === page.mainFrame()).toBe(true);
-        expect(request.frame().url()).toBe('about:blank');
+        expect(request.frame()!.url()).toBe('about:blank');
         request.continue({}, 0);
       });
-      const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.ok()).toBe(true);
-      expect(response.remoteAddress().port).toBe(server.PORT);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
+      expect(response!.ok()).toBe(true);
+      expect(response!.remoteAddress().port).toBe(server.PORT);
     });
     // @see https://github.com/puppeteer/puppeteer/pull/3105
     it('should work when POST is redirected with 302', async () => {
@@ -127,14 +133,18 @@ describe('request interception', function () {
       server.setRedirect('/rredirect', '/empty.html');
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       await page.setContent(`
         <form action='/rredirect' method='post'>
           <input type="hidden" id="foo" name="foo" value="FOOBAR">
         </form>
       `);
       await Promise.all([
-        page.$eval('form', (form: HTMLFormElement) => form.submit()),
+        page.$eval('form', (form) => {
+          return (form as HTMLFormElement).submit();
+        }),
         page.waitForNavigation(),
       ]);
     });
@@ -179,7 +189,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         if (!utils.isFavicon(request)) {
           requests.push(request);
@@ -187,8 +197,8 @@ describe('request interception', function () {
         request.continue({}, 0);
       });
       await page.goto(server.PREFIX + '/one-style.html');
-      expect(requests[1].url()).toContain('/one-style.css');
-      expect(requests[1].headers().referer).toContain('/one-style.html');
+      expect(requests[1]!.url()).toContain('/one-style.css');
+      expect(requests[1]!.headers()['referer']).toContain('/one-style.html');
     });
     it('should properly return navigation response when URL has cookies', async () => {
       const { page, server } = getTestState();
@@ -199,15 +209,19 @@ describe('request interception', function () {
 
       // Setup request interception.
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       const response = await page.reload();
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
     });
     it('should stop intercepting', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      page.once('request', (request) => request.continue({}, 0));
+      page.once('request', (request) => {
+        return request.continue({}, 0);
+      });
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(false);
       await page.goto(server.EMPTY_PAGE);
@@ -224,7 +238,7 @@ describe('request interception', function () {
         request.continue({}, 0);
       });
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.ok()).toBe(true);
+      expect(response!.ok()).toBe(true);
     });
     // @see https://github.com/puppeteer/puppeteer/issues/4337
     it('should work with redirect inside sync XHR', async () => {
@@ -233,7 +247,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       server.setRedirect('/logo.png', '/pptr.png');
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       const status = await page.evaluate(async () => {
         const request = new XMLHttpRequest();
         request.open('GET', '/logo.png', false); // `false` makes the request synchronous
@@ -252,7 +268,7 @@ describe('request interception', function () {
         request.continue({}, 0);
       });
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.ok()).toBe(true);
+      expect(response!.ok()).toBe(true);
     });
     it('should be abortable', async () => {
       const { page, server } = getTestState();
@@ -266,10 +282,12 @@ describe('request interception', function () {
         }
       });
       let failedRequests = 0;
-      page.on('requestfailed', () => ++failedRequests);
+      page.on('requestfailed', () => {
+        return ++failedRequests;
+      });
       const response = await page.goto(server.PREFIX + '/one-style.html');
-      expect(response.ok()).toBe(true);
-      expect(response.request().failure()).toBe(null);
+      expect(response!.ok()).toBe(true);
+      expect(response!.request().failure()).toBe(null);
       expect(failedRequests).toBe(1);
     });
     it('should be able to access the error reason', async () => {
@@ -294,11 +312,13 @@ describe('request interception', function () {
       page.on('request', (request) => {
         request.abort('internetdisconnected', 0);
       });
-      let failedRequest = null;
-      page.on('requestfailed', (request) => (failedRequest = request));
+      let failedRequest!: HTTPRequest;
+      page.on('requestfailed', (request) => {
+        return (failedRequest = request);
+      });
       await page.goto(server.EMPTY_PAGE).catch(() => {});
       expect(failedRequest).toBeTruthy();
-      expect(failedRequest.failure().errorText).toBe(
+      expect(failedRequest.failure()!.errorText).toBe(
         'net::ERR_INTERNET_DISCONNECTED'
       );
     });
@@ -309,7 +329,9 @@ describe('request interception', function () {
         referer: 'http://google.com/',
       });
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       const [request] = await Promise.all([
         server.waitForRequest('/grid.html'),
         page.goto(server.PREFIX + '/grid.html'),
@@ -320,9 +342,13 @@ describe('request interception', function () {
       const { page, server, isChrome } = getTestState();
 
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.abort('failed', 0));
-      let error = null;
-      await page.goto(server.EMPTY_PAGE).catch((error_) => (error = error_));
+      page.on('request', (request) => {
+        return request.abort('failed', 0);
+      });
+      let error!: Error;
+      await page.goto(server.EMPTY_PAGE).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeTruthy();
       if (isChrome) {
         expect(error.message).toContain('net::ERR_FAILED');
@@ -334,7 +360,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue({}, 0);
         requests.push(request);
@@ -355,17 +381,17 @@ describe('request interception', function () {
       const response = await page.goto(
         server.PREFIX + '/non-existing-page.html'
       );
-      expect(response.status()).toBe(200);
-      expect(response.url()).toContain('empty.html');
+      expect(response!.status()).toBe(200);
+      expect(response!.url()).toContain('empty.html');
       expect(requests.length).toBe(5);
-      expect(requests[2].resourceType()).toBe('document');
+      expect(requests[2]!.resourceType()).toBe('document');
       // Check redirect chain
-      const redirectChain = response.request().redirectChain();
+      const redirectChain = response!.request().redirectChain();
       expect(redirectChain.length).toBe(4);
-      expect(redirectChain[0].url()).toContain('/non-existing-page.html');
-      expect(redirectChain[2].url()).toContain('/non-existing-page-3.html');
+      expect(redirectChain[0]!.url()).toContain('/non-existing-page.html');
+      expect(redirectChain[2]!.url()).toContain('/non-existing-page-3.html');
       for (let i = 0; i < redirectChain.length; ++i) {
-        const request = redirectChain[i];
+        const request = redirectChain[i]!;
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.redirectChain().indexOf(request)).toBe(i);
       }
@@ -374,7 +400,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue({}, 0);
         if (!utils.isFavicon(request)) {
@@ -384,21 +410,21 @@ describe('request interception', function () {
       server.setRedirect('/one-style.css', '/two-style.css');
       server.setRedirect('/two-style.css', '/three-style.css');
       server.setRedirect('/three-style.css', '/four-style.css');
-      server.setRoute('/four-style.css', (req, res) =>
-        res.end('body {box-sizing: border-box; }')
-      );
+      server.setRoute('/four-style.css', (_req, res) => {
+        return res.end('body {box-sizing: border-box; }');
+      });
 
       const response = await page.goto(server.PREFIX + '/one-style.html');
-      expect(response.status()).toBe(200);
-      expect(response.url()).toContain('one-style.html');
+      expect(response!.status()).toBe(200);
+      expect(response!.url()).toContain('one-style.html');
       expect(requests.length).toBe(5);
-      expect(requests[0].resourceType()).toBe('document');
-      expect(requests[1].resourceType()).toBe('stylesheet');
+      expect(requests[0]!.resourceType()).toBe('document');
+      expect(requests[1]!.resourceType()).toBe('stylesheet');
       // Check redirect chain
-      const redirectChain = requests[1].redirectChain();
+      const redirectChain = requests[1]!.redirectChain();
       expect(redirectChain.length).toBe(3);
-      expect(redirectChain[0].url()).toContain('/one-style.css');
-      expect(redirectChain[2].url()).toContain('/three-style.css');
+      expect(redirectChain[0]!.url()).toContain('/one-style.css');
+      expect(redirectChain[2]!.url()).toContain('/three-style.css');
     });
     it('should be able to abort redirects', async () => {
       const { page, server, isChrome } = getTestState();
@@ -416,9 +442,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       const result = await page.evaluate(async () => {
         try {
-          await fetch('/non-existing.json');
+          return await fetch('/non-existing.json');
         } catch (error) {
-          return error.message;
+          return (error as Error).message;
         }
       });
       if (isChrome) {
@@ -432,7 +458,9 @@ describe('request interception', function () {
 
       await page.goto(server.EMPTY_PAGE);
       let responseCount = 1;
-      server.setRoute('/zzz', (req, res) => res.end(responseCount++ * 11 + ''));
+      server.setRoute('/zzz', (_req, res) => {
+        return res.end(responseCount++ * 11 + '');
+      });
       await page.setRequestInterception(true);
 
       let spinner = false;
@@ -445,69 +473,82 @@ describe('request interception', function () {
         spinner ? request.abort('failed', 0) : request.continue({}, 0);
         spinner = !spinner;
       });
-      const results = await page.evaluate(() =>
-        Promise.all([
+      const results = await page.evaluate(() => {
+        return Promise.all([
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
+            .then((response) => {
+              return response!.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
+            .then((response) => {
+              return response!.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
-        ])
-      );
+            .then((response) => {
+              return response!.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
+        ]);
+      });
       expect(results).toEqual(['11', 'FAILED', '22']);
     });
     it('should navigate to dataURL and fire dataURL requests', async () => {
       const { page } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         requests.push(request);
         request.continue({}, 0);
       });
       const dataURL = 'data:text/html,<div>yo</div>';
       const response = await page.goto(dataURL);
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(dataURL);
+      expect(requests[0]!.url()).toBe(dataURL);
     });
     it('should be able to fetch dataURL and fire dataURL requests', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         !utils.isFavicon(request) && requests.push(request);
         request.continue({}, 0);
       });
       const dataURL = 'data:text/html,<div>yo</div>';
-      const text = await page.evaluate(
-        (url: string) => fetch(url).then((r) => r.text()),
-        dataURL
-      );
+      const text = await page.evaluate((url: string) => {
+        return fetch(url).then((r) => {
+          return r.text();
+        });
+      }, dataURL);
       expect(text).toBe('<div>yo</div>');
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(dataURL);
+      expect(requests[0]!.url()).toBe(dataURL);
     });
     it('should navigate to URL with hash and fire requests without hash', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         requests.push(request);
         request.continue({}, 0);
       });
       const response = await page.goto(server.EMPTY_PAGE + '#hash');
-      expect(response.status()).toBe(200);
-      expect(response.url()).toBe(server.EMPTY_PAGE);
+      expect(response!.status()).toBe(200);
+      expect(response!.url()).toBe(server.EMPTY_PAGE);
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.url()).toBe(server.EMPTY_PAGE);
     });
     it('should work with encoded server', async () => {
       const { page, server } = getTestState();
@@ -515,20 +556,26 @@ describe('request interception', function () {
       // The requestWillBeSent will report encoded URL, whereas interception will
       // report URL as-is. @see crbug.com/759388
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       const response = await page.goto(
         server.PREFIX + '/some nonexisting page'
       );
-      expect(response.status()).toBe(404);
+      expect(response!.status()).toBe(404);
     });
     it('should work with badly encoded server', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      server.setRoute('/malformed?rnd=%911', (req, res) => res.end());
-      page.on('request', (request) => request.continue({}, 0));
+      server.setRoute('/malformed?rnd=%911', (_req, res) => {
+        return res.end();
+      });
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       const response = await page.goto(server.PREFIX + '/malformed?rnd=%911');
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
     });
     it('should work with encoded server - 2', async () => {
       const { page, server } = getTestState();
@@ -536,7 +583,7 @@ describe('request interception', function () {
       // The requestWillBeSent will report URL as-is, whereas interception will
       // report encoded URL for stylesheet. @see crbug.com/759388
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue({}, 0);
         requests.push(request);
@@ -544,39 +591,47 @@ describe('request interception', function () {
       const response = await page.goto(
         `data:text/html,<link rel="stylesheet" href="${server.PREFIX}/fonts?helvetica|arial"/>`
       );
-      expect(response.status()).toBe(200);
+      expect(response!.status()).toBe(200);
       expect(requests.length).toBe(2);
-      expect(requests[1].response().status()).toBe(404);
+      expect(requests[1]!.response()!.status()).toBe(404);
     });
     it('should not throw "Invalid Interception Id" if the request was cancelled', async () => {
       const { page, server } = getTestState();
 
       await page.setContent('<iframe></iframe>');
       await page.setRequestInterception(true);
-      let request = null;
-      page.on('request', async (r) => (request = r));
+      let request!: HTTPRequest;
+      page.on('request', async (r) => {
+        return (request = r);
+      });
       page.$eval(
         'iframe',
-        (frame: HTMLIFrameElement, url: string) => (frame.src = url),
+        (frame, url) => {
+          return ((frame as HTMLIFrameElement).src = url as string);
+        },
         server.EMPTY_PAGE
       ),
         // Wait for request interception.
         await utils.waitEvent(page, 'request');
       // Delete frame to cause request to be canceled.
-      await page.$eval('iframe', (frame) => frame.remove());
-      let error = null;
-      await request.continue({}, 0).catch((error_) => (error = error_));
-      expect(error).toBe(null);
+      await page.$eval('iframe', (frame) => {
+        return frame.remove();
+      });
+      let error!: Error;
+      await request.continue({}, 0).catch((error_) => {
+        return (error = error_);
+      });
+      expect(error).toBeUndefined();
     });
     it('should throw if interception is not enabled', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       page.on('request', async (request) => {
         try {
           await request.continue({}, 0);
         } catch (error_) {
-          error = error_;
+          error = error_ as Error;
         }
       });
       await page.goto(server.EMPTY_PAGE);
@@ -606,10 +661,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(false);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
 
       const cached = [];
-      page.on('requestservedfromcache', (r) => cached.push(r));
+      page.on('requestservedfromcache', (r) => {
+        return cached.push(r);
+      });
 
       await page.reload();
       expect(cached.length).toBe(0);
@@ -622,10 +681,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
 
       const cached = [];
-      page.on('requestservedfromcache', (r) => cached.push(r));
+      page.on('requestservedfromcache', (r) => {
+        return cached.push(r);
+      });
 
       await page.reload();
       expect(cached.length).toBe(1);
@@ -635,10 +698,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
 
       await page.goto(server.PREFIX + '/cached/one-style-font.html');
-      await page.waitForResponse((r) => r.url().endsWith('/one-style.woff'));
+      await page.waitForResponse((r) => {
+        return r.url().endsWith('/one-style.woff');
+      });
     });
   });
 
@@ -647,7 +714,9 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue({}, 0));
+      page.on('request', (request) => {
+        return request.continue({}, 0);
+      });
       await page.goto(server.EMPTY_PAGE);
     });
     it('should amend HTTP headers', async () => {
@@ -662,7 +731,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       const [request] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() => fetch('/sleep.zzz')),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz');
+        }),
       ]);
       expect(request.headers['foo']).toBe('bar');
     });
@@ -676,8 +747,10 @@ describe('request interception', function () {
           : undefined;
         request.continue({ url: redirectURL }, 0);
       });
-      let consoleMessage = null;
-      page.on('console', (msg) => (consoleMessage = msg));
+      let consoleMessage!: ConsoleMessage;
+      page.on('console', (msg) => {
+        return (consoleMessage = msg);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(page.url()).toBe(server.EMPTY_PAGE);
       expect(consoleMessage.text()).toBe('yellow');
@@ -693,7 +766,9 @@ describe('request interception', function () {
       });
       const [request] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() => fetch('/sleep.zzz')),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz');
+        }),
       ]);
       expect(request.method).toBe('POST');
     });
@@ -708,9 +783,9 @@ describe('request interception', function () {
       });
       const [serverRequest] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() =>
-          fetch('/sleep.zzz', { method: 'POST', body: 'birdy' })
-        ),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz', { method: 'POST', body: 'birdy' });
+        }),
       ]);
       expect(await serverRequest.postBody).toBe('doggo');
     });
@@ -748,11 +823,13 @@ describe('request interception', function () {
         );
       });
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.status()).toBe(201);
-      expect(response.headers().foo).toBe('bar');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(response!.status()).toBe(201);
+      expect(response!.headers()['foo']).toBe('bar');
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should be able to access the response', async () => {
       const { page, server } = getTestState();
@@ -789,11 +866,13 @@ describe('request interception', function () {
         );
       });
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.status()).toBe(422);
-      expect(response.statusText()).toBe('Unprocessable Entity');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(response!.status()).toBe(422);
+      expect(response!.statusText()).toBe('Unprocessable Entity');
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should redirect', async () => {
       const { page, server } = getTestState();
@@ -815,11 +894,11 @@ describe('request interception', function () {
         );
       });
       const response = await page.goto(server.PREFIX + '/rrredirect');
-      expect(response.request().redirectChain().length).toBe(1);
-      expect(response.request().redirectChain()[0].url()).toBe(
+      expect(response!.request().redirectChain().length).toBe(1);
+      expect(response!.request().redirectChain()[0]!.url()).toBe(
         server.PREFIX + '/rrredirect'
       );
-      expect(response.url()).toBe(server.EMPTY_PAGE);
+      expect(response!.url()).toBe(server.EMPTY_PAGE);
     });
     it('should allow mocking binary responses', async () => {
       const { page, server } = getTestState();
@@ -841,10 +920,12 @@ describe('request interception', function () {
         const img = document.createElement('img');
         img.src = PREFIX + '/does-not-exist.png';
         document.body.appendChild(img);
-        return new Promise((fulfill) => (img.onload = fulfill));
+        return new Promise((fulfill) => {
+          return (img.onload = fulfill);
+        });
       }, server.PREFIX);
-      const img = await page.$('img');
-      expect(await img.screenshot()).toBeGolden('mock-binary-response.png');
+      const img = (await page.$('img'))!;
+      expect(await img.screenshot()).toBeGolden('mock-binary-response!.png');
     });
     it('should stringify intercepted request response headers', async () => {
       const { page, server } = getTestState();
@@ -863,12 +944,14 @@ describe('request interception', function () {
         );
       });
       const response = await page.goto(server.EMPTY_PAGE);
-      expect(response.status()).toBe(200);
-      const headers = response.headers();
-      expect(headers.foo).toBe('true');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(response!.status()).toBe(200);
+      const headers = response!.headers();
+      expect(headers['foo']).toBe('true');
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should indicate already-handled if an intercept has been handled', async () => {
       const { page, server } = getTestState();
diff --git a/test/src/requestinterception.spec.ts b/test/src/requestinterception.spec.ts
index f2bd9bf5cb686..c07fe93866589 100644
--- a/test/src/requestinterception.spec.ts
+++ b/test/src/requestinterception.spec.ts
@@ -23,7 +23,9 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
+import { HTTPRequest } from '../../lib/cjs/puppeteer/common/HTTPRequest.js';
+import { ConsoleMessage } from '../../lib/cjs/puppeteer/common/ConsoleMessage.js';
 
 describe('request interception', function () {
   setupTestBrowserHooks();
@@ -46,10 +48,10 @@ describe('request interception', function () {
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.resourceType()).toBe('document');
         expect(request.frame() === page.mainFrame()).toBe(true);
-        expect(request.frame().url()).toBe('about:blank');
+        expect(request.frame()!.url()).toBe('about:blank');
         request.continue();
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.ok()).toBe(true);
       expect(response.remoteAddress().port).toBe(server.PORT);
     });
@@ -60,14 +62,18 @@ describe('request interception', function () {
       server.setRedirect('/rredirect', '/empty.html');
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
       await page.setContent(`
         <form action='/rredirect' method='post'>
           <input type="hidden" id="foo" name="foo" value="FOOBAR">
         </form>
       `);
       await Promise.all([
-        page.$eval('form', (form: HTMLFormElement) => form.submit()),
+        page.$eval('form', (form) => {
+          return (form as HTMLFormElement).submit();
+        }),
         page.waitForNavigation(),
       ]);
     });
@@ -109,7 +115,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         if (!utils.isFavicon(request)) {
           requests.push(request);
@@ -117,8 +123,8 @@ describe('request interception', function () {
         request.continue();
       });
       await page.goto(server.PREFIX + '/one-style.html');
-      expect(requests[1].url()).toContain('/one-style.css');
-      expect(requests[1].headers().referer).toContain('/one-style.html');
+      expect(requests[1]!.url()).toContain('/one-style.css');
+      expect(requests[1]!.headers()['referer']).toContain('/one-style.html');
     });
     it('should properly return navigation response when URL has cookies', async () => {
       const { page, server } = getTestState();
@@ -129,15 +135,19 @@ describe('request interception', function () {
 
       // Setup request interception.
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
-      const response = await page.reload();
+      page.on('request', (request) => {
+        return request.continue();
+      });
+      const response = (await page.reload())!;
       expect(response.status()).toBe(200);
     });
     it('should stop intercepting', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      page.once('request', (request) => request.continue());
+      page.once('request', (request) => {
+        return request.continue();
+      });
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(false);
       await page.goto(server.EMPTY_PAGE);
@@ -153,7 +163,7 @@ describe('request interception', function () {
         expect(request.headers()['foo']).toBe('bar');
         request.continue();
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.ok()).toBe(true);
     });
     // @see https://github.com/puppeteer/puppeteer/issues/4337
@@ -163,7 +173,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       server.setRedirect('/logo.png', '/pptr.png');
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
       const status = await page.evaluate(async () => {
         const request = new XMLHttpRequest();
         request.open('GET', '/logo.png', false); // `false` makes the request synchronous
@@ -181,7 +193,7 @@ describe('request interception', function () {
         expect(request.headers()['referer']).toBe(server.EMPTY_PAGE);
         request.continue();
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.ok()).toBe(true);
     });
     it('should be abortable', async () => {
@@ -196,8 +208,10 @@ describe('request interception', function () {
         }
       });
       let failedRequests = 0;
-      page.on('requestfailed', () => ++failedRequests);
-      const response = await page.goto(server.PREFIX + '/one-style.html');
+      page.on('requestfailed', () => {
+        return ++failedRequests;
+      });
+      const response = (await page.goto(server.PREFIX + '/one-style.html'))!;
       expect(response.ok()).toBe(true);
       expect(response.request().failure()).toBe(null);
       expect(failedRequests).toBe(1);
@@ -209,11 +223,13 @@ describe('request interception', function () {
       page.on('request', (request) => {
         request.abort('internetdisconnected');
       });
-      let failedRequest = null;
-      page.on('requestfailed', (request) => (failedRequest = request));
+      let failedRequest!: HTTPRequest;
+      page.on('requestfailed', (request) => {
+        return (failedRequest = request);
+      });
       await page.goto(server.EMPTY_PAGE).catch(() => {});
       expect(failedRequest).toBeTruthy();
-      expect(failedRequest.failure().errorText).toBe(
+      expect(failedRequest.failure()!.errorText).toBe(
         'net::ERR_INTERNET_DISCONNECTED'
       );
     });
@@ -224,7 +240,9 @@ describe('request interception', function () {
         referer: 'http://google.com/',
       });
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
       const [request] = await Promise.all([
         server.waitForRequest('/grid.html'),
         page.goto(server.PREFIX + '/grid.html'),
@@ -235,9 +253,13 @@ describe('request interception', function () {
       const { page, server, isChrome } = getTestState();
 
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.abort());
-      let error = null;
-      await page.goto(server.EMPTY_PAGE).catch((error_) => (error = error_));
+      page.on('request', (request) => {
+        return request.abort();
+      });
+      let error!: Error;
+      await page.goto(server.EMPTY_PAGE).catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeTruthy();
       if (isChrome) {
         expect(error.message).toContain('net::ERR_FAILED');
@@ -249,7 +271,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue();
         requests.push(request);
@@ -267,20 +289,20 @@ describe('request interception', function () {
         '/non-existing-page-4.html'
       );
       server.setRedirect('/non-existing-page-4.html', '/empty.html');
-      const response = await page.goto(
+      const response = (await page.goto(
         server.PREFIX + '/non-existing-page.html'
-      );
+      ))!;
       expect(response.status()).toBe(200);
       expect(response.url()).toContain('empty.html');
       expect(requests.length).toBe(5);
-      expect(requests[2].resourceType()).toBe('document');
+      expect(requests[2]!.resourceType()).toBe('document');
       // Check redirect chain
       const redirectChain = response.request().redirectChain();
       expect(redirectChain.length).toBe(4);
-      expect(redirectChain[0].url()).toContain('/non-existing-page.html');
-      expect(redirectChain[2].url()).toContain('/non-existing-page-3.html');
+      expect(redirectChain[0]!.url()).toContain('/non-existing-page.html');
+      expect(redirectChain[2]!.url()).toContain('/non-existing-page-3.html');
       for (let i = 0; i < redirectChain.length; ++i) {
-        const request = redirectChain[i];
+        const request = redirectChain[i]!;
         expect(request.isNavigationRequest()).toBe(true);
         expect(request.redirectChain().indexOf(request)).toBe(i);
       }
@@ -289,7 +311,7 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue();
         if (!utils.isFavicon(request)) {
@@ -299,21 +321,21 @@ describe('request interception', function () {
       server.setRedirect('/one-style.css', '/two-style.css');
       server.setRedirect('/two-style.css', '/three-style.css');
       server.setRedirect('/three-style.css', '/four-style.css');
-      server.setRoute('/four-style.css', (req, res) =>
-        res.end('body {box-sizing: border-box; }')
-      );
+      server.setRoute('/four-style.css', (_req, res) => {
+        return res.end('body {box-sizing: border-box; }');
+      });
 
-      const response = await page.goto(server.PREFIX + '/one-style.html');
+      const response = (await page.goto(server.PREFIX + '/one-style.html'))!;
       expect(response.status()).toBe(200);
       expect(response.url()).toContain('one-style.html');
       expect(requests.length).toBe(5);
-      expect(requests[0].resourceType()).toBe('document');
-      expect(requests[1].resourceType()).toBe('stylesheet');
+      expect(requests[0]!.resourceType()).toBe('document');
+      expect(requests[1]!.resourceType()).toBe('stylesheet');
       // Check redirect chain
-      const redirectChain = requests[1].redirectChain();
+      const redirectChain = requests[1]!.redirectChain();
       expect(redirectChain.length).toBe(3);
-      expect(redirectChain[0].url()).toContain('/one-style.css');
-      expect(redirectChain[2].url()).toContain('/three-style.css');
+      expect(redirectChain[0]!.url()).toContain('/one-style.css');
+      expect(redirectChain[2]!.url()).toContain('/three-style.css');
     });
     it('should be able to abort redirects', async () => {
       const { page, server, isChrome } = getTestState();
@@ -331,9 +353,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       const result = await page.evaluate(async () => {
         try {
-          await fetch('/non-existing.json');
+          return await fetch('/non-existing.json');
         } catch (error) {
-          return error.message;
+          return (error as Error).message;
         }
       });
       if (isChrome) {
@@ -347,7 +369,9 @@ describe('request interception', function () {
 
       await page.goto(server.EMPTY_PAGE);
       let responseCount = 1;
-      server.setRoute('/zzz', (req, res) => res.end(responseCount++ * 11 + ''));
+      server.setRoute('/zzz', (_req, res) => {
+        return res.end(responseCount++ * 11 + '');
+      });
       await page.setRequestInterception(true);
 
       let spinner = false;
@@ -360,69 +384,82 @@ describe('request interception', function () {
         spinner ? request.abort() : request.continue();
         spinner = !spinner;
       });
-      const results = await page.evaluate(() =>
-        Promise.all([
+      const results = await page.evaluate(() => {
+        return Promise.all([
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
+            .then((response) => {
+              return response.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
+            .then((response) => {
+              return response.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
           fetch('/zzz')
-            .then((response) => response.text())
-            .catch(() => 'FAILED'),
-        ])
-      );
+            .then((response) => {
+              return response.text();
+            })
+            .catch(() => {
+              return 'FAILED';
+            }),
+        ]);
+      });
       expect(results).toEqual(['11', 'FAILED', '22']);
     });
     it('should navigate to dataURL and fire dataURL requests', async () => {
       const { page } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         requests.push(request);
         request.continue();
       });
       const dataURL = 'data:text/html,<div>yo</div>';
-      const response = await page.goto(dataURL);
+      const response = (await page.goto(dataURL))!;
       expect(response.status()).toBe(200);
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(dataURL);
+      expect(requests[0]!.url()).toBe(dataURL);
     });
     it('should be able to fetch dataURL and fire dataURL requests', async () => {
       const { page, server } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         !utils.isFavicon(request) && requests.push(request);
         request.continue();
       });
       const dataURL = 'data:text/html,<div>yo</div>';
-      const text = await page.evaluate(
-        (url: string) => fetch(url).then((r) => r.text()),
-        dataURL
-      );
+      const text = await page.evaluate((url: string) => {
+        return fetch(url).then((r) => {
+          return r.text();
+        });
+      }, dataURL);
       expect(text).toBe('<div>yo</div>');
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(dataURL);
+      expect(requests[0]!.url()).toBe(dataURL);
     });
     it('should navigate to URL with hash and fire requests without hash', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         requests.push(request);
         request.continue();
       });
-      const response = await page.goto(server.EMPTY_PAGE + '#hash');
+      const response = (await page.goto(server.EMPTY_PAGE + '#hash'))!;
       expect(response.status()).toBe(200);
       expect(response.url()).toBe(server.EMPTY_PAGE);
       expect(requests.length).toBe(1);
-      expect(requests[0].url()).toBe(server.EMPTY_PAGE);
+      expect(requests[0]!.url()).toBe(server.EMPTY_PAGE);
     });
     it('should work with encoded server', async () => {
       const { page, server } = getTestState();
@@ -430,19 +467,27 @@ describe('request interception', function () {
       // The requestWillBeSent will report encoded URL, whereas interception will
       // report URL as-is. @see crbug.com/759388
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
-      const response = await page.goto(
+      page.on('request', (request) => {
+        return request.continue();
+      });
+      const response = (await page.goto(
         server.PREFIX + '/some nonexisting page'
-      );
+      ))!;
       expect(response.status()).toBe(404);
     });
     it('should work with badly encoded server', async () => {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      server.setRoute('/malformed?rnd=%911', (req, res) => res.end());
-      page.on('request', (request) => request.continue());
-      const response = await page.goto(server.PREFIX + '/malformed?rnd=%911');
+      server.setRoute('/malformed?rnd=%911', (_req, res) => {
+        return res.end();
+      });
+      page.on('request', (request) => {
+        return request.continue();
+      });
+      const response = (await page.goto(
+        server.PREFIX + '/malformed?rnd=%911'
+      ))!;
       expect(response.status()).toBe(200);
     });
     it('should work with encoded server - 2', async () => {
@@ -451,47 +496,55 @@ describe('request interception', function () {
       // The requestWillBeSent will report URL as-is, whereas interception will
       // report encoded URL for stylesheet. @see crbug.com/759388
       await page.setRequestInterception(true);
-      const requests = [];
+      const requests: HTTPRequest[] = [];
       page.on('request', (request) => {
         request.continue();
         requests.push(request);
       });
-      const response = await page.goto(
+      const response = (await page.goto(
         `data:text/html,<link rel="stylesheet" href="${server.PREFIX}/fonts?helvetica|arial"/>`
-      );
+      ))!;
       expect(response.status()).toBe(200);
       expect(requests.length).toBe(2);
-      expect(requests[1].response().status()).toBe(404);
+      expect(requests[1]!.response()!.status()).toBe(404);
     });
     it('should not throw "Invalid Interception Id" if the request was cancelled', async () => {
       const { page, server } = getTestState();
 
       await page.setContent('<iframe></iframe>');
       await page.setRequestInterception(true);
-      let request = null;
-      page.on('request', async (r) => (request = r));
+      let request!: HTTPRequest;
+      page.on('request', async (r) => {
+        return (request = r);
+      });
       page.$eval(
         'iframe',
-        (frame: HTMLIFrameElement, url: string) => (frame.src = url),
+        (frame, url) => {
+          return ((frame as HTMLIFrameElement).src = url as string);
+        },
         server.EMPTY_PAGE
       ),
         // Wait for request interception.
         await utils.waitEvent(page, 'request');
       // Delete frame to cause request to be canceled.
-      await page.$eval('iframe', (frame) => frame.remove());
-      let error = null;
-      await request.continue().catch((error_) => (error = error_));
-      expect(error).toBe(null);
+      await page.$eval('iframe', (frame) => {
+        return frame.remove();
+      });
+      let error!: Error;
+      await request.continue().catch((error_) => {
+        return (error = error_);
+      });
+      expect(error).toBeUndefined();
     });
     it('should throw if interception is not enabled', async () => {
       const { page, server } = getTestState();
 
-      let error = null;
+      let error!: Error;
       page.on('request', async (request) => {
         try {
           await request.continue();
         } catch (error_) {
-          error = error_;
+          error = error_ as Error;
         }
       });
       await page.goto(server.EMPTY_PAGE);
@@ -521,10 +574,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(false);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
 
       const cached = [];
-      page.on('requestservedfromcache', (r) => cached.push(r));
+      page.on('requestservedfromcache', (r) => {
+        return cached.push(r);
+      });
 
       await page.reload();
       expect(cached.length).toBe(0);
@@ -537,10 +594,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
 
       const cached = [];
-      page.on('requestservedfromcache', (r) => cached.push(r));
+      page.on('requestservedfromcache', (r) => {
+        return cached.push(r);
+      });
 
       await page.reload();
       expect(cached.length).toBe(1);
@@ -550,10 +611,14 @@ describe('request interception', function () {
 
       await page.setRequestInterception(true);
       await page.setCacheEnabled(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
 
       await page.goto(server.PREFIX + '/cached/one-style-font.html');
-      await page.waitForResponse((r) => r.url().endsWith('/one-style.woff'));
+      await page.waitForResponse((r) => {
+        return r.url().endsWith('/one-style.woff');
+      });
     });
   });
 
@@ -562,7 +627,9 @@ describe('request interception', function () {
       const { page, server } = getTestState();
 
       await page.setRequestInterception(true);
-      page.on('request', (request) => request.continue());
+      page.on('request', (request) => {
+        return request.continue();
+      });
       await page.goto(server.EMPTY_PAGE);
     });
     it('should amend HTTP headers', async () => {
@@ -577,7 +644,9 @@ describe('request interception', function () {
       await page.goto(server.EMPTY_PAGE);
       const [request] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() => fetch('/sleep.zzz')),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz');
+        }),
       ]);
       expect(request.headers['foo']).toBe('bar');
     });
@@ -591,8 +660,10 @@ describe('request interception', function () {
           : undefined;
         request.continue({ url: redirectURL });
       });
-      let consoleMessage = null;
-      page.on('console', (msg) => (consoleMessage = msg));
+      let consoleMessage!: ConsoleMessage;
+      page.on('console', (msg) => {
+        return (consoleMessage = msg);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(page.url()).toBe(server.EMPTY_PAGE);
       expect(consoleMessage.text()).toBe('yellow');
@@ -608,7 +679,9 @@ describe('request interception', function () {
       });
       const [request] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() => fetch('/sleep.zzz')),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz');
+        }),
       ]);
       expect(request.method).toBe('POST');
     });
@@ -623,9 +696,9 @@ describe('request interception', function () {
       });
       const [serverRequest] = await Promise.all([
         server.waitForRequest('/sleep.zzz'),
-        page.evaluate(() =>
-          fetch('/sleep.zzz', { method: 'POST', body: 'birdy' })
-        ),
+        page.evaluate(() => {
+          return fetch('/sleep.zzz', { method: 'POST', body: 'birdy' });
+        }),
       ]);
       expect(await serverRequest.postBody).toBe('doggo');
     });
@@ -646,7 +719,7 @@ describe('request interception', function () {
     it('should fail if the header value is invalid', async () => {
       const { page, server } = getTestState();
 
-      let error;
+      let error!: Error;
       await page.setRequestInterception(true);
       page.on('request', async (request) => {
         await request
@@ -656,7 +729,7 @@ describe('request interception', function () {
             },
           })
           .catch((error_) => {
-            error = error_;
+            error = error_ as Error;
           });
         await request.continue();
       });
@@ -679,12 +752,14 @@ describe('request interception', function () {
           body: 'Yo, page!',
         });
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.status()).toBe(201);
-      expect(response.headers().foo).toBe('bar');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(response.headers()['foo']).toBe('bar');
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should work with status code 422', async () => {
       const { page, server } = getTestState();
@@ -696,12 +771,14 @@ describe('request interception', function () {
           body: 'Yo, page!',
         });
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.status()).toBe(422);
       expect(response.statusText()).toBe('Unprocessable Entity');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should redirect', async () => {
       const { page, server } = getTestState();
@@ -719,9 +796,9 @@ describe('request interception', function () {
           },
         });
       });
-      const response = await page.goto(server.PREFIX + '/rrredirect');
+      const response = (await page.goto(server.PREFIX + '/rrredirect'))!;
       expect(response.request().redirectChain().length).toBe(1);
-      expect(response.request().redirectChain()[0].url()).toBe(
+      expect(response.request().redirectChain()[0]!.url()).toBe(
         server.PREFIX + '/rrredirect'
       );
       expect(response.url()).toBe(server.EMPTY_PAGE);
@@ -741,13 +818,17 @@ describe('request interception', function () {
           body: 'Hello world',
         });
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       const cookies = await page.cookies();
-      const firstCookie = cookies.find((cookie) => cookie.name === 'first');
-      const secondCookie = cookies.find((cookie) => cookie.name === 'second');
+      const firstCookie = cookies.find((cookie) => {
+        return cookie.name === 'first';
+      });
+      const secondCookie = cookies.find((cookie) => {
+        return cookie.name === 'second';
+      });
       expect(response.status()).toBe(200);
-      expect(response.headers().foo).toBe('bar');
-      expect(response.headers().arr).toBe('1\n2');
+      expect(response.headers()['foo']).toBe('bar');
+      expect(response.headers()['arr']).toBe('1\n2');
       // request.respond() will not trigger Network.responseReceivedExtraInfo
       // fail to get 'set-cookie' header from response
       expect(firstCookie?.value).toBe('1');
@@ -770,9 +851,11 @@ describe('request interception', function () {
         const img = document.createElement('img');
         img.src = PREFIX + '/does-not-exist.png';
         document.body.appendChild(img);
-        return new Promise((fulfill) => (img.onload = fulfill));
+        return new Promise((fulfill) => {
+          return (img.onload = fulfill);
+        });
       }, server.PREFIX);
-      const img = await page.$('img');
+      const img = (await page.$('img'))!;
       expect(await img.screenshot()).toBeGolden('mock-binary-response.png');
     });
     it('should stringify intercepted request response headers', async () => {
@@ -788,18 +871,20 @@ describe('request interception', function () {
           body: 'Yo, page!',
         });
       });
-      const response = await page.goto(server.EMPTY_PAGE);
+      const response = (await page.goto(server.EMPTY_PAGE))!;
       expect(response.status()).toBe(200);
       const headers = response.headers();
-      expect(headers.foo).toBe('true');
-      expect(await page.evaluate(() => document.body.textContent)).toBe(
-        'Yo, page!'
-      );
+      expect(headers['foo']).toBe('true');
+      expect(
+        await page.evaluate(() => {
+          return document.body.textContent;
+        })
+      ).toBe('Yo, page!');
     });
     it('should fail if the header value is invalid', async () => {
       const { page, server } = getTestState();
 
-      let error;
+      let error!: Error;
       await page.setRequestInterception(true);
       page.on('request', async (request) => {
         await request
@@ -809,7 +894,7 @@ describe('request interception', function () {
             },
           })
           .catch((error_) => {
-            error = error_;
+            error = error_ as Error;
           });
         await request.respond({
           status: 200,
diff --git a/test/src/screenshot.spec.ts b/test/src/screenshot.spec.ts
index 42ffad744a0b5..0507bbb4b3e2b 100644
--- a/test/src/screenshot.spec.ts
+++ b/test/src/screenshot.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describe('Screenshots', function () {
   setupTestBrowserHooks();
@@ -86,7 +86,7 @@ describe('Screenshots', function () {
         );
       }
       const screenshots = await Promise.all(promises);
-      expect(screenshots[1]).toBeGolden('grid-cell-1.png');
+      expect(screenshots[1]!).toBeGolden('grid-cell-1.png');
     });
     itFailsFirefox('should take fullPage screenshots', async () => {
       const { page, server } = getTestState();
@@ -114,7 +114,7 @@ describe('Screenshots', function () {
       const promises = [];
       for (let i = 0; i < N; ++i) {
         promises.push(
-          pages[i].screenshot({
+          pages[i]!.screenshot({
             clip: { x: 50 * i, y: 0, width: 50, height: 50 },
           })
         );
@@ -123,7 +123,11 @@ describe('Screenshots', function () {
       for (let i = 0; i < N; ++i) {
         expect(screenshots[i]).toBeGolden(`grid-cell-${i}.png`);
       }
-      await Promise.all(pages.map((page) => page.close()));
+      await Promise.all(
+        pages.map((page) => {
+          return page.close();
+        })
+      );
     });
     itFailsFirefox('should allow transparency', async () => {
       const { page, server } = getTestState();
@@ -192,8 +196,10 @@ describe('Screenshots', function () {
 
       await page.setViewport({ width: 500, height: 500 });
       await page.goto(server.PREFIX + '/grid.html');
-      await page.evaluate(() => window.scrollBy(50, 100));
-      const elementHandle = await page.$('.box:nth-of-type(3)');
+      await page.evaluate(() => {
+        return window.scrollBy(50, 100);
+      });
+      const elementHandle = (await page.$('.box:nth-of-type(3)'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden('screenshot-element-bounding-box.png');
     });
@@ -212,7 +218,7 @@ describe('Screenshots', function () {
         </style>
         <div></div>
       `);
-      const elementHandle = await page.$('div');
+      const elementHandle = (await page.$('div'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden('screenshot-element-padding-border.png');
     });
@@ -238,17 +244,19 @@ describe('Screenshots', function () {
           </style>
           <div class="to-screenshot"></div>
         `);
-        const elementHandle = await page.$('div.to-screenshot');
+        const elementHandle = (await page.$('div.to-screenshot'))!;
         const screenshot = await elementHandle.screenshot();
         expect(screenshot).toBeGolden(
           'screenshot-element-larger-than-viewport.png'
         );
 
         expect(
-          await page.evaluate(() => ({
-            w: window.innerWidth,
-            h: window.innerHeight,
-          }))
+          await page.evaluate(() => {
+            return {
+              w: window.innerWidth,
+              h: window.innerHeight,
+            };
+          })
         ).toEqual({ w: 500, h: 500 });
       }
     );
@@ -273,7 +281,7 @@ describe('Screenshots', function () {
         <div class="above"></div>
         <div class="to-screenshot"></div>
       `);
-      const elementHandle = await page.$('div.to-screenshot');
+      const elementHandle = (await page.$('div.to-screenshot'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden(
         'screenshot-element-scrolled-into-view.png'
@@ -290,7 +298,7 @@ describe('Screenshots', function () {
                                         height: 100px;
                                         background: green;
                                         transform: rotateZ(200deg);">&nbsp;</div>`);
-      const elementHandle = await page.$('div');
+      const elementHandle = (await page.$('div'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden('screenshot-element-rotate.png');
     });
@@ -298,14 +306,15 @@ describe('Screenshots', function () {
       const { page } = getTestState();
 
       await page.setContent('<h1>remove this</h1>');
-      const elementHandle = await page.$('h1');
-      await page.evaluate(
-        (element: HTMLElement) => element.remove(),
-        elementHandle
-      );
+      const elementHandle = (await page.$('h1'))!;
+      await page.evaluate((element: HTMLElement) => {
+        return element.remove();
+      }, elementHandle);
       const screenshotError = await elementHandle
         .screenshot()
-        .catch((error) => error);
+        .catch((error) => {
+          return error;
+        });
       expect(screenshotError.message).toBe(
         'Node is either not visible or not an HTMLElement'
       );
@@ -314,8 +323,10 @@ describe('Screenshots', function () {
       const { page } = getTestState();
 
       await page.setContent('<div style="width: 50px; height: 0"></div>');
-      const div = await page.$('div');
-      const error = await div.screenshot().catch((error_) => error_);
+      const div = (await page.$('div'))!;
+      const error = await div.screenshot().catch((error_) => {
+        return error_;
+      });
       expect(error.message).toBe('Node has 0 height.');
     });
     it('should work for an element with fractional dimensions', async () => {
@@ -324,7 +335,7 @@ describe('Screenshots', function () {
       await page.setContent(
         '<div style="width:48.51px;height:19.8px;border:1px solid black;"></div>'
       );
-      const elementHandle = await page.$('div');
+      const elementHandle = (await page.$('div'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden('screenshot-element-fractional.png');
     });
@@ -334,7 +345,7 @@ describe('Screenshots', function () {
       await page.setContent(
         '<div style="position:absolute; top: 10.3px; left: 20.4px;width:50.3px;height:20.2px;border:1px solid black;"></div>'
       );
-      const elementHandle = await page.$('div');
+      const elementHandle = (await page.$('div'))!;
       const screenshot = await elementHandle.screenshot();
       expect(screenshot).toBeGolden('screenshot-element-fractional-offset.png');
     });
diff --git a/test/src/target.spec.ts b/test/src/target.spec.ts
index b1656a642d376..04249aae43ba7 100644
--- a/test/src/target.spec.ts
+++ b/test/src/target.spec.ts
@@ -14,16 +14,18 @@
  * limitations under the License.
  */
 
-import utils from './utils.js';
-const { waitEvent } = utils;
 import expect from 'expect';
+import { ServerResponse } from 'http';
+import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
+import { Target } from '../../lib/cjs/puppeteer/common/Target.js';
 import {
   getTestState,
+  itFailsFirefox,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-  itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import { Target } from '../../lib/cjs/puppeteer/common/Target.js';
+} from './mocha-utils.js';
+import utils from './utils.js';
+const { waitEvent } = utils;
 
 describe('Target', function () {
   setupTestBrowserHooks();
@@ -35,11 +37,15 @@ describe('Target', function () {
     // The pages will be the testing page and the original newtab page
     const targets = browser.targets();
     expect(
-      targets.some(
-        (target) => target.type() === 'page' && target.url() === 'about:blank'
-      )
+      targets.some((target) => {
+        return target.type() === 'page' && target.url() === 'about:blank';
+      })
+    ).toBeTruthy();
+    expect(
+      targets.some((target) => {
+        return target.type() === 'browser';
+      })
     ).toBeTruthy();
-    expect(targets.some((target) => target.type() === 'browser')).toBeTruthy();
   });
   it('Browser.pages should return all of the pages', async () => {
     const { page, context } = getTestState();
@@ -53,7 +59,9 @@ describe('Target', function () {
     const { browser } = getTestState();
 
     const targets = browser.targets();
-    const browserTarget = targets.find((target) => target.type() === 'browser');
+    const browserTarget = targets.find((target) => {
+      return target.type() === 'browser';
+    });
     expect(browserTarget).toBeTruthy();
   });
   it('should be able to use the default page in the browser', async () => {
@@ -61,9 +69,13 @@ describe('Target', function () {
 
     // The pages will be the testing page and the original newtab page
     const allPages = await browser.pages();
-    const originalPage = allPages.find((p) => p !== page);
+    const originalPage = allPages.find((p) => {
+      return p !== page;
+    })!;
     expect(
-      await originalPage.evaluate(() => ['Hello', 'world'].join(' '))
+      await originalPage.evaluate(() => {
+        return ['Hello', 'world'].join(' ');
+      })
     ).toBe('Hello world');
     expect(await originalPage.$('body')).toBeTruthy();
   });
@@ -72,21 +84,19 @@ describe('Target', function () {
 
     const [otherPage] = await Promise.all([
       context
-        .waitForTarget((target) =>
-          target
-            .page()
-            .then(
-              (page) =>
-                page.url() === server.CROSS_PROCESS_PREFIX + '/empty.html'
-            )
-        )
-        .then((target) => target.page()),
-      page.evaluate(
-        (url: string) => window.open(url),
-        server.CROSS_PROCESS_PREFIX + '/empty.html'
-      ),
+        .waitForTarget((target) => {
+          return target.page().then((page) => {
+            return page!.url() === server.CROSS_PROCESS_PREFIX + '/empty.html';
+          });
+        })
+        .then((target) => {
+          return target.page();
+        }),
+      page.evaluate((url: string) => {
+        return window.open(url);
+      }, server.CROSS_PROCESS_PREFIX + '/empty.html'),
     ]);
-    expect(otherPage.url()).toEqual(
+    expect(otherPage!.url()).toEqual(
       server.CROSS_PROCESS_PREFIX + '/empty.html'
     );
     expect(page).not.toEqual(otherPage);
@@ -98,35 +108,41 @@ describe('Target', function () {
 
       const [otherPage] = await Promise.all([
         context
-          .waitForTarget(
-            (target) =>
-              target.url() === server.CROSS_PROCESS_PREFIX + '/empty.html'
-          )
-          .then((target) => target.page()),
-        page.evaluate(
-          (url: string) => window.open(url),
-          server.CROSS_PROCESS_PREFIX + '/empty.html'
-        ),
+          .waitForTarget((target) => {
+            return target.url() === server.CROSS_PROCESS_PREFIX + '/empty.html';
+          })
+          .then((target) => {
+            return target.page();
+          }),
+        page.evaluate((url: string) => {
+          return window.open(url);
+        }, server.CROSS_PROCESS_PREFIX + '/empty.html'),
       ]);
-      expect(otherPage.url()).toContain(server.CROSS_PROCESS_PREFIX);
-      expect(await otherPage.evaluate(() => ['Hello', 'world'].join(' '))).toBe(
-        'Hello world'
-      );
-      expect(await otherPage.$('body')).toBeTruthy();
+      expect(otherPage!.url()).toContain(server.CROSS_PROCESS_PREFIX);
+      expect(
+        await otherPage!.evaluate(() => {
+          return ['Hello', 'world'].join(' ');
+        })
+      ).toBe('Hello world');
+      expect(await otherPage!.$('body')).toBeTruthy();
 
       let allPages = await context.pages();
       expect(allPages).toContain(page);
       expect(allPages).toContain(otherPage);
 
-      const closePagePromise = new Promise((fulfill) =>
-        context.once('targetdestroyed', (target) => fulfill(target.page()))
-      );
-      await otherPage.close();
+      const closePagePromise = new Promise((fulfill) => {
+        return context.once('targetdestroyed', (target) => {
+          return fulfill(target.page());
+        });
+      });
+      await otherPage!.close();
       expect(await closePagePromise).toBe(otherPage);
 
-      allPages = await Promise.all(
-        context.targets().map((target) => target.page())
-      );
+      allPages = (await Promise.all(
+        context.targets().map((target) => {
+          return target.page();
+        })
+      )) as Page[];
       expect(allPages).toContain(page);
       expect(allPages).not.toContain(otherPage);
     }
@@ -137,9 +153,11 @@ describe('Target', function () {
       const { page, server, context } = getTestState();
 
       await page.goto(server.EMPTY_PAGE);
-      const createdTarget = new Promise<Target>((fulfill) =>
-        context.once('targetcreated', (target) => fulfill(target))
-      );
+      const createdTarget = new Promise<Target>((fulfill) => {
+        return context.once('targetcreated', (target) => {
+          return fulfill(target);
+        });
+      });
 
       await page.goto(server.PREFIX + '/serviceworkers/empty/sw.html');
 
@@ -148,14 +166,18 @@ describe('Target', function () {
         server.PREFIX + '/serviceworkers/empty/sw.js'
       );
 
-      const destroyedTarget = new Promise((fulfill) =>
-        context.once('targetdestroyed', (target) => fulfill(target))
-      );
-      await page.evaluate(() =>
-        globalThis.registrationPromise.then((registration) =>
-          registration.unregister()
-        )
-      );
+      const destroyedTarget = new Promise((fulfill) => {
+        return context.once('targetdestroyed', (target) => {
+          return fulfill(target);
+        });
+      });
+      await page.evaluate(() => {
+        return (globalThis as any).registrationPromise.then(
+          (registration: { unregister: () => any }) => {
+            return registration.unregister();
+          }
+        );
+      });
       expect(await destroyedTarget).toBe(await createdTarget);
     }
   );
@@ -164,13 +186,15 @@ describe('Target', function () {
 
     await page.goto(server.PREFIX + '/serviceworkers/empty/sw.html');
 
-    const target = await context.waitForTarget(
-      (target) => target.type() === 'service_worker'
-    );
-    const worker = await target.worker();
-    expect(await worker.evaluate(() => self.toString())).toBe(
-      '[object ServiceWorkerGlobalScope]'
-    );
+    const target = await context.waitForTarget((target) => {
+      return target.type() === 'service_worker';
+    });
+    const worker = (await target.worker())!;
+    expect(
+      await worker.evaluate(() => {
+        return self.toString();
+      })
+    ).toBe('[object ServiceWorkerGlobalScope]');
   });
   itFailsFirefox('should create a worker from a shared worker', async () => {
     const { page, server, context } = getTestState();
@@ -179,27 +203,33 @@ describe('Target', function () {
     await page.evaluate(() => {
       new SharedWorker('data:text/javascript,console.log("hi")');
     });
-    const target = await context.waitForTarget(
-      (target) => target.type() === 'shared_worker'
-    );
-    const worker = await target.worker();
-    expect(await worker.evaluate(() => self.toString())).toBe(
-      '[object SharedWorkerGlobalScope]'
-    );
+    const target = await context.waitForTarget((target) => {
+      return target.type() === 'shared_worker';
+    });
+    const worker = (await target.worker())!;
+    expect(
+      await worker.evaluate(() => {
+        return self.toString();
+      })
+    ).toBe('[object SharedWorkerGlobalScope]');
   });
   itFailsFirefox('should report when a target url changes', async () => {
     const { page, server, context } = getTestState();
 
     await page.goto(server.EMPTY_PAGE);
-    let changedTarget = new Promise<Target>((fulfill) =>
-      context.once('targetchanged', (target) => fulfill(target))
-    );
+    let changedTarget = new Promise<Target>((fulfill) => {
+      return context.once('targetchanged', (target) => {
+        return fulfill(target);
+      });
+    });
     await page.goto(server.CROSS_PROCESS_PREFIX + '/');
     expect((await changedTarget).url()).toBe(server.CROSS_PROCESS_PREFIX + '/');
 
-    changedTarget = new Promise((fulfill) =>
-      context.once('targetchanged', (target) => fulfill(target))
-    );
+    changedTarget = new Promise((fulfill) => {
+      return context.once('targetchanged', (target) => {
+        return fulfill(target);
+      });
+    });
     await page.goto(server.EMPTY_PAGE);
     expect((await changedTarget).url()).toBe(server.EMPTY_PAGE);
   });
@@ -207,20 +237,28 @@ describe('Target', function () {
     const { context } = getTestState();
 
     let targetChanged = false;
-    const listener = () => (targetChanged = true);
+    const listener = () => {
+      return (targetChanged = true);
+    };
     context.on('targetchanged', listener);
-    const targetPromise = new Promise<Target>((fulfill) =>
-      context.once('targetcreated', (target) => fulfill(target))
-    );
+    const targetPromise = new Promise<Target>((fulfill) => {
+      return context.once('targetcreated', (target) => {
+        return fulfill(target);
+      });
+    });
     const newPagePromise = context.newPage();
     const target = await targetPromise;
     expect(target.url()).toBe('about:blank');
 
     const newPage = await newPagePromise;
-    const targetPromise2 = new Promise<Target>((fulfill) =>
-      context.once('targetcreated', (target) => fulfill(target))
-    );
-    const evaluatePromise = newPage.evaluate(() => window.open('about:blank'));
+    const targetPromise2 = new Promise<Target>((fulfill) => {
+      return context.once('targetcreated', (target) => {
+        return fulfill(target);
+      });
+    });
+    const evaluatePromise = newPage.evaluate(() => {
+      return window.open('about:blank');
+    });
     const target2 = await targetPromise2;
     expect(target2.url()).toBe('about:blank');
     await evaluatePromise;
@@ -233,21 +271,22 @@ describe('Target', function () {
     async () => {
       const { page, server, context } = getTestState();
 
-      let serverResponse = null;
-      server.setRoute('/one-style.css', (req, res) => (serverResponse = res));
+      let serverResponse!: ServerResponse;
+      server.setRoute('/one-style.css', (_req, res) => {
+        return (serverResponse = res);
+      });
       // Open a new page. Use window.open to connect to the page later.
       await Promise.all([
-        page.evaluate(
-          (url: string) => window.open(url),
-          server.PREFIX + '/one-style.html'
-        ),
+        page.evaluate((url: string) => {
+          return window.open(url);
+        }, server.PREFIX + '/one-style.html'),
         server.waitForRequest('/one-style.css'),
       ]);
       // Connect to the opened page.
-      const target = await context.waitForTarget((target) =>
-        target.url().includes('one-style.html')
-      );
-      const newPage = await target.page();
+      const target = await context.waitForTarget((target) => {
+        return target.url().includes('one-style.html');
+      });
+      const newPage = (await target.page())!;
       // Issue a redirect.
       serverResponse.writeHead(302, { location: '/injectedstyle.css' });
       serverResponse.end();
@@ -262,12 +301,14 @@ describe('Target', function () {
 
     await page.goto(server.EMPTY_PAGE);
     const [createdTarget] = await Promise.all([
-      new Promise<Target>((fulfill) =>
-        context.once('targetcreated', (target) => fulfill(target))
-      ),
+      new Promise<Target>((fulfill) => {
+        return context.once('targetcreated', (target) => {
+          return fulfill(target);
+        });
+      }),
       page.goto(server.PREFIX + '/popup/window-open.html'),
     ]);
-    expect((await createdTarget.page()).url()).toBe(
+    expect((await createdTarget.page())!.url()).toBe(
       server.PREFIX + '/popup/popup.html'
     );
     expect(createdTarget.opener()).toBe(page.target());
@@ -279,11 +320,13 @@ describe('Target', function () {
       const { browser, puppeteer, server } = getTestState();
 
       let resolved = false;
-      const targetPromise = browser.waitForTarget(
-        (target) => target.url() === server.EMPTY_PAGE
-      );
+      const targetPromise = browser.waitForTarget((target) => {
+        return target.url() === server.EMPTY_PAGE;
+      });
       targetPromise
-        .then(() => (resolved = true))
+        .then(() => {
+          return (resolved = true);
+        })
         .catch((error) => {
           resolved = true;
           if (error instanceof puppeteer.errors.TimeoutError) {
@@ -310,12 +353,19 @@ describe('Target', function () {
     it('should timeout waiting for a non-existent target', async () => {
       const { browser, server, puppeteer } = getTestState();
 
-      let error = null;
+      let error!: Error;
       await browser
-        .waitForTarget((target) => target.url() === server.EMPTY_PAGE, {
-          timeout: 1,
-        })
-        .catch((error_) => (error = error_));
+        .waitForTarget(
+          (target) => {
+            return target.url() === server.EMPTY_PAGE;
+          },
+          {
+            timeout: 1,
+          }
+        )
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
   });
diff --git a/test/src/touchscreen.spec.ts b/test/src/touchscreen.spec.ts
index 36931d5852699..f94f396ee34af 100644
--- a/test/src/touchscreen.spec.ts
+++ b/test/src/touchscreen.spec.ts
@@ -20,7 +20,7 @@ import {
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
   describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
 
 describeFailsFirefox('Touchscreen', function () {
   setupTestBrowserHooks();
@@ -28,22 +28,27 @@ describeFailsFirefox('Touchscreen', function () {
 
   it('should tap the button', async () => {
     const { puppeteer, page, server } = getTestState();
-    const iPhone = puppeteer.devices['iPhone 6'];
+    const iPhone = puppeteer.devices['iPhone 6']!;
     await page.emulate(iPhone);
     await page.goto(server.PREFIX + '/input/button.html');
     await page.tap('button');
-    expect(await page.evaluate(() => globalThis.result)).toBe('Clicked');
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).result;
+      })
+    ).toBe('Clicked');
   });
   it('should report touches', async () => {
     const { puppeteer, page, server } = getTestState();
-    const iPhone = puppeteer.devices['iPhone 6'];
+    const iPhone = puppeteer.devices['iPhone 6']!;
     await page.emulate(iPhone);
     await page.goto(server.PREFIX + '/input/touches.html');
-    const button = await page.$('button');
+    const button = (await page.$('button'))!;
     await button.tap();
-    expect(await page.evaluate(() => globalThis.getResult())).toEqual([
-      'Touchstart: 0',
-      'Touchend: 0',
-    ]);
+    expect(
+      await page.evaluate(() => {
+        return (globalThis as any).getResult();
+      })
+    ).toEqual(['Touchstart: 0', 'Touchend: 0']);
   });
 });
diff --git a/test/src/tracing.spec.ts b/test/src/tracing.spec.ts
index 39907e8905b7e..e830cd07a27e1 100644
--- a/test/src/tracing.spec.ts
+++ b/test/src/tracing.spec.ts
@@ -17,12 +17,14 @@
 import fs from 'fs';
 import path from 'path';
 import expect from 'expect';
-import { getTestState, describeChromeOnly } from './mocha-utils'; // eslint-disable-line import/extensions
+import { getTestState, describeChromeOnly } from './mocha-utils.js';
+import { Browser } from '../../lib/cjs/puppeteer/common/Browser.js';
+import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
 
 describeChromeOnly('Tracing', function () {
-  let outputFile;
-  let browser;
-  let page;
+  let outputFile!: string;
+  let browser!: Browser;
+  let page!: Page;
 
   /* we manually manage the browser here as we want a new browser for each
    * individual test, which isn't the default behaviour of getTestState()
@@ -37,11 +39,8 @@ describeChromeOnly('Tracing', function () {
 
   afterEach(async () => {
     await browser.close();
-    browser = null;
-    page = null;
     if (fs.existsSync(outputFile)) {
       fs.unlinkSync(outputFile);
-      outputFile = null;
     }
   });
   it('should output a trace', async () => {
@@ -93,10 +92,10 @@ describeChromeOnly('Tracing', function () {
   it('should throw if tracing on two pages', async () => {
     await page.tracing.start({ path: outputFile });
     const newPage = await browser.newPage();
-    let error = null;
-    await newPage.tracing
-      .start({ path: outputFile })
-      .catch((error_) => (error = error_));
+    let error!: Error;
+    await newPage.tracing.start({ path: outputFile }).catch((error_) => {
+      return (error = error_);
+    });
     await newPage.close();
     expect(error).toBeTruthy();
     await page.tracing.stop();
@@ -106,7 +105,7 @@ describeChromeOnly('Tracing', function () {
 
     await page.tracing.start({ screenshots: true, path: outputFile });
     await page.goto(server.PREFIX + '/grid.html');
-    const trace = await page.tracing.stop();
+    const trace = (await page.tracing.stop())!;
     const buf = fs.readFileSync(outputFile);
     expect(trace.toString()).toEqual(buf.toString());
   });
@@ -138,18 +137,18 @@ describeChromeOnly('Tracing', function () {
 
     await page.tracing.start({ screenshots: true });
     await page.goto(server.PREFIX + '/grid.html');
-    const trace = await page.tracing.stop();
+    const trace = (await page.tracing.stop())!;
     expect(trace.toString()).toContain('screenshot');
   });
 
   it('should properly fail if readProtocolStream errors out', async () => {
     await page.tracing.start({ path: __dirname });
 
-    let error: Error = null;
+    let error!: Error;
     try {
       await page.tracing.stop();
     } catch (error_) {
-      error = error_;
+      error = error_ as Error;
     }
     expect(error).toBeDefined();
   });
diff --git a/test/src/utils.js b/test/src/utils.js
deleted file mode 100644
index 60271411a62bb..0000000000000
--- a/test/src/utils.js
+++ /dev/null
@@ -1,137 +0,0 @@
-/**
- * Copyright 2017 Google Inc. All rights reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-// TODO (@jackfranklin): convert to TS and enable type checking.
-
-// @ts-nocheck
-const path = require('path');
-const expect = require('expect');
-const GoldenUtils = require('./golden-utils.js');
-const PROJECT_ROOT = path.join(__dirname, '..', '..');
-
-const utils = (module.exports = {
-  extendExpectWithToBeGolden: function (goldenDir, outputDir) {
-    expect.extend({
-      toBeGolden: (testScreenshot, goldenFilePath) => {
-        const result = GoldenUtils.compare(
-          goldenDir,
-          outputDir,
-          testScreenshot,
-          goldenFilePath
-        );
-
-        return {
-          message: () => result.message,
-          pass: result.pass,
-        };
-      },
-    });
-  },
-
-  /**
-   * @returns {string}
-   */
-  projectRoot: function () {
-    return PROJECT_ROOT;
-  },
-
-  /**
-   * @param {!Page} page
-   * @param {string} frameId
-   * @param {string} url
-   * @returns {!Frame}
-   */
-  attachFrame: async function (page, frameId, url) {
-    const handle = await page.evaluateHandle(attachFrame, frameId, url);
-    return await handle.asElement().contentFrame();
-
-    async function attachFrame(frameId, url) {
-      const frame = document.createElement('iframe');
-      frame.src = url;
-      frame.id = frameId;
-      document.body.appendChild(frame);
-      await new Promise((x) => (frame.onload = x));
-      return frame;
-    }
-  },
-
-  isFavicon: function (request) {
-    return request.url().includes('favicon.ico');
-  },
-
-  /**
-   * @param {!Page} page
-   * @param {string} frameId
-   */
-  detachFrame: async function (page, frameId) {
-    await page.evaluate(detachFrame, frameId);
-
-    function detachFrame(frameId) {
-      const frame = document.getElementById(frameId);
-      frame.remove();
-    }
-  },
-
-  /**
-   * @param {!Page} page
-   * @param {string} frameId
-   * @param {string} url
-   */
-  navigateFrame: async function (page, frameId, url) {
-    await page.evaluate(navigateFrame, frameId, url);
-
-    function navigateFrame(frameId, url) {
-      const frame = document.getElementById(frameId);
-      frame.src = url;
-      return new Promise((x) => (frame.onload = x));
-    }
-  },
-
-  /**
-   * @param {!Frame} frame
-   * @param {string=} indentation
-   * @returns {Array<string>}
-   */
-  dumpFrames: function (frame, indentation) {
-    indentation = indentation || '';
-    let description = frame.url().replace(/:\d{4}\//, ':<PORT>/');
-    if (frame.name()) {
-      description += ' (' + frame.name() + ')';
-    }
-    const result = [indentation + description];
-    for (const child of frame.childFrames()) {
-      result.push(...utils.dumpFrames(child, '    ' + indentation));
-    }
-    return result;
-  },
-
-  /**
-   * @param {!EventEmitter} emitter
-   * @param {string} eventName
-   * @returns {!Promise<!Object>}
-   */
-  waitEvent: function (emitter, eventName, predicate = () => true) {
-    return new Promise((fulfill) => {
-      emitter.on(eventName, function listener(event) {
-        if (!predicate(event)) {
-          return;
-        }
-        emitter.removeListener(eventName, listener);
-        fulfill(event);
-      });
-    });
-  },
-});
diff --git a/test/src/utils.ts b/test/src/utils.ts
new file mode 100644
index 0000000000000..121c2dbe50d0f
--- /dev/null
+++ b/test/src/utils.ts
@@ -0,0 +1,162 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import expect from 'expect';
+import path from 'path';
+import { Frame } from '../../lib/cjs/puppeteer/common/FrameManager.js';
+import { Page } from '../../lib/cjs/puppeteer/common/Page.js';
+import { EventEmitter } from '../../lib/cjs/puppeteer/common/EventEmitter.js';
+import { compare } from './golden-utils.js';
+
+const PROJECT_ROOT = path.join(__dirname, '..', '..');
+
+export const extendExpectWithToBeGolden = (
+  goldenDir: string,
+  outputDir: string
+): void => {
+  expect.extend({
+    toBeGolden: (testScreenshot: string | Buffer, goldenFilePath: string) => {
+      const result = compare(
+        goldenDir,
+        outputDir,
+        testScreenshot,
+        goldenFilePath
+      );
+
+      if (result.pass) {
+        return {
+          pass: true,
+          message: () => {
+            return void 0;
+          },
+        };
+      } else {
+        return {
+          pass: false,
+          message: () => {
+            return result.message;
+          },
+        };
+      }
+    },
+  });
+};
+
+export const projectRoot = (): string => {
+  return PROJECT_ROOT;
+};
+
+export const attachFrame = async (
+  pageOrFrame: Page | Frame,
+  frameId: string,
+  url: string
+): Promise<Frame | undefined> => {
+  const handle = await pageOrFrame.evaluateHandle(attachFrame, frameId, url);
+  return (await handle.asElement()?.contentFrame()) ?? undefined;
+
+  async function attachFrame(frameId: string, url: string) {
+    const frame = document.createElement('iframe');
+    frame.src = url;
+    frame.id = frameId;
+    document.body.appendChild(frame);
+    await new Promise((x) => {
+      return (frame.onload = x);
+    });
+    return frame;
+  }
+};
+
+export const isFavicon = (request: {
+  url: () => string | string[];
+}): boolean => {
+  return request.url().includes('favicon.ico');
+};
+
+export async function detachFrame(
+  pageOrFrame: Page | Frame,
+  frameId: string
+): Promise<void> {
+  await pageOrFrame.evaluate(detachFrame, frameId);
+
+  function detachFrame(frameId: string) {
+    const frame = document.getElementById(frameId) as HTMLIFrameElement;
+    frame.remove();
+  }
+}
+
+export async function navigateFrame(
+  pageOrFrame: Page | Frame,
+  frameId: string,
+  url: string
+): Promise<void> {
+  await pageOrFrame.evaluate(navigateFrame, frameId, url);
+
+  function navigateFrame(frameId: string, url: any) {
+    const frame = document.getElementById(frameId) as HTMLIFrameElement;
+    frame.src = url;
+    return new Promise((x) => {
+      return (frame.onload = x);
+    });
+  }
+}
+
+export const dumpFrames = (
+  frame: Frame,
+  indentation?: string
+): Array<string> => {
+  indentation = indentation || '';
+  let description = frame.url().replace(/:\d{4}\//, ':<PORT>/');
+  if (frame.name()) {
+    description += ' (' + frame.name() + ')';
+  }
+  const result = [indentation + description];
+  for (const child of frame.childFrames()) {
+    result.push(...dumpFrames(child, '    ' + indentation));
+  }
+  return result;
+};
+
+export const waitEvent = (
+  emitter: EventEmitter,
+  eventName: string,
+  predicate: (event: any) => boolean = () => {
+    return true;
+  }
+): Promise<any> => {
+  return new Promise((fulfill) => {
+    emitter.on(eventName, function listener(event: any) {
+      if (!predicate(event)) {
+        return;
+      }
+      emitter.off(eventName, listener);
+      fulfill(event);
+    });
+  });
+};
+
+/**
+ * @deprecated Use exports directly.
+ */
+export default {
+  extendExpectWithToBeGolden,
+  waitEvent,
+  dumpFrames,
+  navigateFrame,
+  isFavicon,
+  attachFrame,
+  projectRoot,
+  detachFrame,
+};
diff --git a/test/src/waittask.spec.ts b/test/src/waittask.spec.ts
index 641e06d4cca21..8b77c6fe2d587 100644
--- a/test/src/waittask.spec.ts
+++ b/test/src/waittask.spec.ts
@@ -14,15 +14,16 @@
  * limitations under the License.
  */
 
-import utils from './utils.js';
-import sinon from 'sinon';
 import expect from 'expect';
+import sinon from 'sinon';
+import { isErrorLike } from '../../lib/cjs/puppeteer/common/util.js';
 import {
   getTestState,
+  itFailsFirefox,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-  itFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
+} from './mocha-utils.js';
+import { attachFrame, detachFrame } from './utils.js';
 
 describe('waittask specs', function () {
   setupTestBrowserHooks();
@@ -33,13 +34,17 @@ describe('waittask specs', function () {
      * tests. Until we remove this method we still want to ensure we don't break
      * it.
      */
-    beforeEach(() => sinon.stub(console, 'warn').callsFake(() => {}));
+    beforeEach(() => {
+      return sinon.stub(console, 'warn').callsFake(() => {});
+    });
 
     it('should wait for selector', async () => {
       const { page, server } = getTestState();
 
       let found = false;
-      const waitFor = page.waitFor('div').then(() => (found = true));
+      const waitFor = page.waitForSelector('div').then(() => {
+        return (found = true);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(found).toBe(false);
       await page.goto(server.PREFIX + '/grid.html');
@@ -51,7 +56,9 @@ describe('waittask specs', function () {
       const { page, server } = getTestState();
 
       let found = false;
-      const waitFor = page.waitFor('//div').then(() => (found = true));
+      const waitFor = page.waitFor('//div').then(() => {
+        return (found = true);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(found).toBe(false);
       await page.goto(server.PREFIX + '/grid.html');
@@ -74,8 +81,10 @@ describe('waittask specs', function () {
       const { page } = getTestState();
 
       await page.setContent(`<div>some text</div>`);
-      let error = null;
-      await page.waitFor('/html/body/div').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitFor('/html/body/div').catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeTruthy();
     });
     it('should timeout', async () => {
@@ -98,28 +107,31 @@ describe('waittask specs', function () {
       const { page } = getTestState();
 
       await Promise.all([
-        page.waitFor(() => window.innerWidth < 100),
+        page.waitFor(() => {
+          return window.innerWidth < 100;
+        }),
         page.setViewport({ width: 10, height: 10 }),
       ]);
     });
-    it('should throw when unknown type', async () => {
-      const { page } = getTestState();
-
-      let error = null;
-      // @ts-expect-error purposefully passing bad type for test
-      await page.waitFor({ foo: 'bar' }).catch((error_) => (error = error_));
-      expect(error.message).toContain('Unsupported target type');
-    });
     it('should wait for predicate with arguments', async () => {
       const { page } = getTestState();
 
-      await page.waitFor((arg1, arg2) => arg1 !== arg2, {}, 1, 2);
+      await page.waitFor(
+        (arg1: number, arg2: number) => {
+          return arg1 !== arg2;
+        },
+        {},
+        1,
+        2
+      );
     });
 
     it('should log a deprecation warning', async () => {
       const { page } = getTestState();
 
-      await page.waitFor(() => true);
+      await page.waitFor(() => {
+        return true;
+      });
 
       const consoleWarnStub = console.warn as sinon.SinonSpy;
 
@@ -138,15 +150,19 @@ describe('waittask specs', function () {
       const { page } = getTestState();
 
       const watchdog = page.waitForFunction('window.__FOO === 1');
-      await page.evaluate(() => (globalThis.__FOO = 1));
+      await page.evaluate(() => {
+        return ((globalThis as any).__FOO = 1);
+      });
       await watchdog;
     });
     it('should work when resolved right before execution context disposal', async () => {
       const { page } = getTestState();
 
-      await page.evaluateOnNewDocument(() => (globalThis.__RELOADED = true));
+      await page.evaluateOnNewDocument(() => {
+        return ((globalThis as any).__RELOADED = true);
+      });
       await page.waitForFunction(() => {
-        if (!globalThis.__RELOADED) {
+        if (!(globalThis as any).__RELOADED) {
           window.location.reload();
         }
         return true;
@@ -159,13 +175,24 @@ describe('waittask specs', function () {
       const startTime = Date.now();
       const polling = 100;
       const watchdog = page
-        .waitForFunction(() => globalThis.__FOO === 'hit', { polling })
-        .then(() => (success = true));
-      await page.evaluate(() => (globalThis.__FOO = 'hit'));
+        .waitForFunction(
+          () => {
+            return (globalThis as any).__FOO === 'hit';
+          },
+          {
+            polling,
+          }
+        )
+        .then(() => {
+          return (success = true);
+        });
+      await page.evaluate(() => {
+        return ((globalThis as any).__FOO = 'hit');
+      });
       expect(success).toBe(false);
-      await page.evaluate(() =>
-        document.body.appendChild(document.createElement('div'))
-      );
+      await page.evaluate(() => {
+        return document.body.appendChild(document.createElement('div'));
+      });
       await watchdog;
       expect(Date.now() - startTime).not.toBeLessThan(polling / 2);
     });
@@ -175,13 +202,24 @@ describe('waittask specs', function () {
       const startTime = Date.now();
       const polling = 100;
       const watchdog = page
-        .waitForFunction(async () => globalThis.__FOO === 'hit', { polling })
-        .then(() => (success = true));
-      await page.evaluate(async () => (globalThis.__FOO = 'hit'));
+        .waitForFunction(
+          async () => {
+            return (globalThis as any).__FOO === 'hit';
+          },
+          {
+            polling,
+          }
+        )
+        .then(() => {
+          return (success = true);
+        });
+      await page.evaluate(async () => {
+        return ((globalThis as any).__FOO = 'hit');
+      });
       expect(success).toBe(false);
-      await page.evaluate(async () =>
-        document.body.appendChild(document.createElement('div'))
-      );
+      await page.evaluate(async () => {
+        return document.body.appendChild(document.createElement('div'));
+      });
       await watchdog;
       expect(Date.now() - startTime).not.toBeLessThan(polling / 2);
     });
@@ -190,15 +228,24 @@ describe('waittask specs', function () {
 
       let success = false;
       const watchdog = page
-        .waitForFunction(() => globalThis.__FOO === 'hit', {
-          polling: 'mutation',
-        })
-        .then(() => (success = true));
-      await page.evaluate(() => (globalThis.__FOO = 'hit'));
+        .waitForFunction(
+          () => {
+            return (globalThis as any).__FOO === 'hit';
+          },
+          {
+            polling: 'mutation',
+          }
+        )
+        .then(() => {
+          return (success = true);
+        });
+      await page.evaluate(() => {
+        return ((globalThis as any).__FOO = 'hit');
+      });
       expect(success).toBe(false);
-      await page.evaluate(() =>
-        document.body.appendChild(document.createElement('div'))
-      );
+      await page.evaluate(() => {
+        return document.body.appendChild(document.createElement('div'));
+      });
       await watchdog;
     });
     it('should poll on mutation async', async () => {
@@ -206,36 +253,56 @@ describe('waittask specs', function () {
 
       let success = false;
       const watchdog = page
-        .waitForFunction(async () => globalThis.__FOO === 'hit', {
-          polling: 'mutation',
-        })
-        .then(() => (success = true));
-      await page.evaluate(async () => (globalThis.__FOO = 'hit'));
+        .waitForFunction(
+          async () => {
+            return (globalThis as any).__FOO === 'hit';
+          },
+          {
+            polling: 'mutation',
+          }
+        )
+        .then(() => {
+          return (success = true);
+        });
+      await page.evaluate(async () => {
+        return ((globalThis as any).__FOO = 'hit');
+      });
       expect(success).toBe(false);
-      await page.evaluate(async () =>
-        document.body.appendChild(document.createElement('div'))
-      );
+      await page.evaluate(async () => {
+        return document.body.appendChild(document.createElement('div'));
+      });
       await watchdog;
     });
     it('should poll on raf', async () => {
       const { page } = getTestState();
 
-      const watchdog = page.waitForFunction(() => globalThis.__FOO === 'hit', {
-        polling: 'raf',
+      const watchdog = page.waitForFunction(
+        () => {
+          return (globalThis as any).__FOO === 'hit';
+        },
+        {
+          polling: 'raf',
+        }
+      );
+      await page.evaluate(() => {
+        return ((globalThis as any).__FOO = 'hit');
       });
-      await page.evaluate(() => (globalThis.__FOO = 'hit'));
       await watchdog;
     });
     it('should poll on raf async', async () => {
       const { page } = getTestState();
 
       const watchdog = page.waitForFunction(
-        async () => globalThis.__FOO === 'hit',
+        async () => {
+          return (globalThis as any).__FOO === 'hit';
+        },
         {
           polling: 'raf',
         }
       );
-      await page.evaluate(async () => (globalThis.__FOO = 'hit'));
+      await page.evaluate(async () => {
+        return ((globalThis as any).__FOO = 'hit');
+      });
       await watchdog;
     });
     itFailsFirefox('should work with strict CSP policy', async () => {
@@ -243,100 +310,148 @@ describe('waittask specs', function () {
 
       server.setCSP('/empty.html', 'script-src ' + server.PREFIX);
       await page.goto(server.EMPTY_PAGE);
-      let error = null;
+      let error!: Error;
       await Promise.all([
         page
-          .waitForFunction(() => globalThis.__FOO === 'hit', { polling: 'raf' })
-          .catch((error_) => (error = error_)),
-        page.evaluate(() => (globalThis.__FOO = 'hit')),
+          .waitForFunction(
+            () => {
+              return (globalThis as any).__FOO === 'hit';
+            },
+            {
+              polling: 'raf',
+            }
+          )
+          .catch((error_) => {
+            return (error = error_);
+          }),
+        page.evaluate(() => {
+          return ((globalThis as any).__FOO = 'hit');
+        }),
       ]);
-      expect(error).toBe(null);
+      expect(error).toBeUndefined();
     });
     it('should throw on bad polling value', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
-        await page.waitForFunction(() => !!document.body, {
-          polling: 'unknown',
-        });
+        await page.waitForFunction(
+          () => {
+            return !!document.body;
+          },
+          {
+            polling: 'unknown',
+          }
+        );
       } catch (error_) {
-        error = error_;
+        if (isErrorLike(error_)) {
+          error = error_ as Error;
+        }
       }
-      expect(error).toBeTruthy();
-      expect(error.message).toContain('polling');
+      expect(error?.message).toContain('polling');
     });
     it('should throw negative polling interval', async () => {
       const { page } = getTestState();
 
-      let error = null;
+      let error!: Error;
       try {
-        await page.waitForFunction(() => !!document.body, { polling: -10 });
+        await page.waitForFunction(
+          () => {
+            return !!document.body;
+          },
+          { polling: -10 }
+        );
       } catch (error_) {
-        error = error_;
+        if (isErrorLike(error_)) {
+          error = error_ as Error;
+        }
       }
-      expect(error).toBeTruthy();
-      expect(error.message).toContain('Cannot poll with non-positive interval');
+      expect(error?.message).toContain(
+        'Cannot poll with non-positive interval'
+      );
     });
     it('should return the success value as a JSHandle', async () => {
       const { page } = getTestState();
 
-      expect(await (await page.waitForFunction(() => 5)).jsonValue()).toBe(5);
+      expect(
+        await (
+          await page.waitForFunction(() => {
+            return 5;
+          })
+        ).jsonValue()
+      ).toBe(5);
     });
     it('should return the window as a success value', async () => {
       const { page } = getTestState();
 
-      expect(await page.waitForFunction(() => window)).toBeTruthy();
+      expect(
+        await page.waitForFunction(() => {
+          return window;
+        })
+      ).toBeTruthy();
     });
     it('should accept ElementHandle arguments', async () => {
       const { page } = getTestState();
 
       await page.setContent('<div></div>');
-      const div = await page.$('div');
+      const div = (await page.$('div'))!;
       let resolved = false;
       const waitForFunction = page
         .waitForFunction(
-          (element) => element.localName === 'div' && !element.parentElement,
+          (element: Element) => {
+            return element.localName === 'div' && !element.parentElement;
+          },
           {},
           div
         )
-        .then(() => (resolved = true));
+        .then(() => {
+          return (resolved = true);
+        });
       expect(resolved).toBe(false);
-      await page.evaluate((element: HTMLElement) => element.remove(), div);
+      await page.evaluate((element: HTMLElement) => {
+        return element.remove();
+      }, div);
       await waitForFunction;
     });
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
-      await page
-        .waitForFunction('false', { timeout: 10 })
-        .catch((error_) => (error = error_));
-      expect(error).toBeTruthy();
-      expect(error.message).toContain('waiting for function failed: timeout');
+      let error!: Error;
+      await page.waitForFunction('false', { timeout: 10 }).catch((error_) => {
+        return (error = error_);
+      });
+
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
+      expect(error?.message).toContain('waiting for function failed: timeout');
     });
     it('should respect default timeout', async () => {
       const { page, puppeteer } = getTestState();
 
       page.setDefaultTimeout(1);
-      let error = null;
-      await page.waitForFunction('false').catch((error_) => (error = error_));
+      let error!: Error;
+      await page.waitForFunction('false').catch((error_) => {
+        return (error = error_);
+      });
       expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
-      expect(error.message).toContain('waiting for function failed: timeout');
+      expect(error?.message).toContain('waiting for function failed: timeout');
     });
     it('should disable timeout when its set to 0', async () => {
       const { page } = getTestState();
 
       const watchdog = page.waitForFunction(
         () => {
-          globalThis.__counter = (globalThis.__counter || 0) + 1;
-          return globalThis.__injected;
+          (globalThis as any).__counter =
+            ((globalThis as any).__counter || 0) + 1;
+          return (globalThis as any).__injected;
         },
         { timeout: 0, polling: 10 }
       );
-      await page.waitForFunction(() => globalThis.__counter > 10);
-      await page.evaluate(() => (globalThis.__injected = true));
+      await page.waitForFunction(() => {
+        return (globalThis as any).__counter > 10;
+      });
+      await page.evaluate(() => {
+        return ((globalThis as any).__injected = true);
+      });
       await watchdog;
     });
     it('should survive cross-process navigation', async () => {
@@ -345,24 +460,32 @@ describe('waittask specs', function () {
       let fooFound = false;
       const waitForFunction = page
         .waitForFunction('globalThis.__FOO === 1')
-        .then(() => (fooFound = true));
+        .then(() => {
+          return (fooFound = true);
+        });
       await page.goto(server.EMPTY_PAGE);
       expect(fooFound).toBe(false);
       await page.reload();
       expect(fooFound).toBe(false);
       await page.goto(server.CROSS_PROCESS_PREFIX + '/grid.html');
       expect(fooFound).toBe(false);
-      await page.evaluate(() => (globalThis.__FOO = 1));
+      await page.evaluate(() => {
+        return ((globalThis as any).__FOO = 1);
+      });
       await waitForFunction;
       expect(fooFound).toBe(true);
     });
     it('should survive navigations', async () => {
       const { page, server } = getTestState();
 
-      const watchdog = page.waitForFunction(() => globalThis.__done);
+      const watchdog = page.waitForFunction(() => {
+        return (globalThis as any).__done;
+      });
       await page.goto(server.EMPTY_PAGE);
       await page.goto(server.PREFIX + '/consolelog.html');
-      await page.evaluate(() => (globalThis.__done = true));
+      await page.evaluate(() => {
+        return ((globalThis as any).__done = true);
+      });
       await watchdog;
     });
   });
@@ -401,8 +524,9 @@ describe('waittask specs', function () {
   });
 
   describe('Frame.waitForSelector', function () {
-    const addElement = (tag) =>
-      document.body.appendChild(document.createElement(tag));
+    const addElement = (tag: string) => {
+      return document.body.appendChild(document.createElement(tag));
+    };
 
     it('should immediately resolve promise if node exists', async () => {
       const { page, server } = getTestState();
@@ -417,13 +541,18 @@ describe('waittask specs', function () {
     itFailsFirefox('should work with removed MutationObserver', async () => {
       const { page } = getTestState();
 
-      await page.evaluate(() => delete window.MutationObserver);
+      await page.evaluate(() => {
+        // @ts-expect-error We want to remove it for the test.
+        return delete window.MutationObserver;
+      });
       const [handle] = await Promise.all([
         page.waitForSelector('.zombo'),
         page.setContent(`<div class='zombo'>anything</div>`),
       ]);
       expect(
-        await page.evaluate((x: HTMLElement) => x.textContent, handle)
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, handle)
       ).toBe('anything');
     });
 
@@ -435,10 +564,8 @@ describe('waittask specs', function () {
       const watchdog = frame.waitForSelector('div');
       await frame.evaluate(addElement, 'br');
       await frame.evaluate(addElement, 'div');
-      const eHandle = await watchdog;
-      const tagName = await eHandle
-        .getProperty('tagName')
-        .then((e) => e.jsonValue());
+      const eHandle = (await watchdog)!;
+      const tagName = await (await eHandle.getProperty('tagName')).jsonValue();
       expect(tagName).toBe('DIV');
     });
 
@@ -448,10 +575,10 @@ describe('waittask specs', function () {
       await page.goto(server.EMPTY_PAGE);
       const watchdog = page.waitForSelector('h3 div');
       await page.evaluate(addElement, 'span');
-      await page.evaluate(
-        () =>
-          (document.querySelector('span').innerHTML = '<h3><div></div></h3>')
-      );
+      await page.evaluate(() => {
+        return (document.querySelector('span')!.innerHTML =
+          '<h3><div></div></h3>');
+      });
       await watchdog;
     });
 
@@ -461,43 +588,43 @@ describe('waittask specs', function () {
         const { page, server } = getTestState();
 
         await page.goto(server.EMPTY_PAGE);
-        await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-        const otherFrame = page.frames()[1];
+        await attachFrame(page, 'frame1', server.EMPTY_PAGE);
+        const otherFrame = page.frames()[1]!;
         const watchdog = page.waitForSelector('div');
         await otherFrame.evaluate(addElement, 'div');
         await page.evaluate(addElement, 'div');
         const eHandle = await watchdog;
-        expect(eHandle.executionContext().frame()).toBe(page.mainFrame());
+        expect(eHandle?.executionContext().frame()).toBe(page.mainFrame());
       }
     );
 
     itFailsFirefox('should run in specified frame', async () => {
       const { page, server } = getTestState();
 
-      await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      await utils.attachFrame(page, 'frame2', server.EMPTY_PAGE);
-      const frame1 = page.frames()[1];
-      const frame2 = page.frames()[2];
+      await attachFrame(page, 'frame1', server.EMPTY_PAGE);
+      await attachFrame(page, 'frame2', server.EMPTY_PAGE);
+      const frame1 = page.frames()[1]!;
+      const frame2 = page.frames()[2]!;
       const waitForSelectorPromise = frame2.waitForSelector('div');
       await frame1.evaluate(addElement, 'div');
       await frame2.evaluate(addElement, 'div');
       const eHandle = await waitForSelectorPromise;
-      expect(eHandle.executionContext().frame()).toBe(frame2);
+      expect(eHandle?.executionContext().frame()).toBe(frame2);
     });
 
     itFailsFirefox('should throw when frame is detached', async () => {
       const { page, server } = getTestState();
 
-      await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      const frame = page.frames()[1];
-      let waitError = null;
-      const waitPromise = frame
-        .waitForSelector('.box')
-        .catch((error) => (waitError = error));
-      await utils.detachFrame(page, 'frame1');
+      await attachFrame(page, 'frame1', server.EMPTY_PAGE);
+      const frame = page.frames()[1]!;
+      let waitError: Error | undefined;
+      const waitPromise = frame.waitForSelector('.box').catch((error) => {
+        return (waitError = error);
+      });
+      await detachFrame(page, 'frame1');
       await waitPromise;
       expect(waitError).toBeTruthy();
-      expect(waitError.message).toContain(
+      expect(waitError?.message).toContain(
         'waitForFunction failed: frame got detached.'
       );
     });
@@ -505,9 +632,9 @@ describe('waittask specs', function () {
       const { page, server } = getTestState();
 
       let boxFound = false;
-      const waitForSelector = page
-        .waitForSelector('.box')
-        .then(() => (boxFound = true));
+      const waitForSelector = page.waitForSelector('.box').then(() => {
+        return (boxFound = true);
+      });
       await page.goto(server.EMPTY_PAGE);
       expect(boxFound).toBe(false);
       await page.reload();
@@ -522,18 +649,22 @@ describe('waittask specs', function () {
       let divFound = false;
       const waitForSelector = page
         .waitForSelector('div', { visible: true })
-        .then(() => (divFound = true));
+        .then(() => {
+          return (divFound = true);
+        });
       await page.setContent(
         `<div style='display: none; visibility: hidden;'>1</div>`
       );
       expect(divFound).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('display')
-      );
+      await page.evaluate(() => {
+        return document.querySelector('div')?.style.removeProperty('display');
+      });
       expect(divFound).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('visibility')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')
+          ?.style.removeProperty('visibility');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divFound).toBe(true);
     });
@@ -543,18 +674,22 @@ describe('waittask specs', function () {
       let divVisible = false;
       const waitForSelector = page
         .waitForSelector('div#inner', { visible: true })
-        .then(() => (divVisible = true));
+        .then(() => {
+          return (divVisible = true);
+        });
       await page.setContent(
         `<div style='display: none; visibility: hidden;'><div id="inner">hi</div></div>`
       );
       expect(divVisible).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('display')
-      );
+      await page.evaluate(() => {
+        return document.querySelector('div')?.style.removeProperty('display');
+      });
       expect(divVisible).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.removeProperty('visibility')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')
+          ?.style.removeProperty('visibility');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divVisible).toBe(true);
     });
@@ -565,12 +700,16 @@ describe('waittask specs', function () {
       await page.setContent(`<div style='display: block;'></div>`);
       const waitForSelector = page
         .waitForSelector('div', { hidden: true })
-        .then(() => (divHidden = true));
+        .then(() => {
+          return (divHidden = true);
+        });
       await page.waitForSelector('div'); // do a round trip
       expect(divHidden).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.setProperty('visibility', 'hidden')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')
+          ?.style.setProperty('visibility', 'hidden');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divHidden).toBe(true);
     });
@@ -581,12 +720,16 @@ describe('waittask specs', function () {
       await page.setContent(`<div style='display: block;'></div>`);
       const waitForSelector = page
         .waitForSelector('div', { hidden: true })
-        .then(() => (divHidden = true));
+        .then(() => {
+          return (divHidden = true);
+        });
       await page.waitForSelector('div'); // do a round trip
       expect(divHidden).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.setProperty('display', 'none')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')
+          ?.style.setProperty('display', 'none');
+      });
       expect(await waitForSelector).toBe(true);
       expect(divHidden).toBe(true);
     });
@@ -597,10 +740,14 @@ describe('waittask specs', function () {
       let divRemoved = false;
       const waitForSelector = page
         .waitForSelector('div', { hidden: true })
-        .then(() => (divRemoved = true));
+        .then(() => {
+          return (divRemoved = true);
+        });
       await page.waitForSelector('div'); // do a round trip
       expect(divRemoved).toBe(false);
-      await page.evaluate(() => document.querySelector('div').remove());
+      await page.evaluate(() => {
+        return document.querySelector('div')?.remove();
+      });
       expect(await waitForSelector).toBe(true);
       expect(divRemoved).toBe(true);
     });
@@ -615,26 +762,27 @@ describe('waittask specs', function () {
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
-      await page
-        .waitForSelector('div', { timeout: 10 })
-        .catch((error_) => (error = error_));
-      expect(error).toBeTruthy();
-      expect(error.message).toContain(
+      let error!: Error;
+      await page.waitForSelector('div', { timeout: 10 }).catch((error_) => {
+        return (error = error_);
+      });
+      expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
+      expect(error?.message).toContain(
         'waiting for selector `div` failed: timeout'
       );
-      expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     it('should have an error message specifically for awaiting an element to be hidden', async () => {
       const { page } = getTestState();
 
       await page.setContent(`<div></div>`);
-      let error = null;
+      let error!: Error;
       await page
         .waitForSelector('div', { hidden: true, timeout: 10 })
-        .catch((error_) => (error = error_));
+        .catch((error_) => {
+          return (error = error_);
+        });
       expect(error).toBeTruthy();
-      expect(error.message).toContain(
+      expect(error?.message).toContain(
         'waiting for selector `div` to be hidden failed: timeout'
       );
     });
@@ -643,14 +791,14 @@ describe('waittask specs', function () {
       const { page } = getTestState();
 
       let divFound = false;
-      const waitForSelector = page
-        .waitForSelector('.zombo')
-        .then(() => (divFound = true));
+      const waitForSelector = page.waitForSelector('.zombo').then(() => {
+        return (divFound = true);
+      });
       await page.setContent(`<div class='notZombo'></div>`);
       expect(divFound).toBe(false);
-      await page.evaluate(
-        () => (document.querySelector('div').className = 'zombo')
-      );
+      await page.evaluate(() => {
+        return (document.querySelector('div')!.className = 'zombo');
+      });
       expect(await waitForSelector).toBe(true);
     });
     it('should return the element handle', async () => {
@@ -659,28 +807,28 @@ describe('waittask specs', function () {
       const waitForSelector = page.waitForSelector('.zombo');
       await page.setContent(`<div class='zombo'>anything</div>`);
       expect(
-        await page.evaluate(
-          (x: HTMLElement) => x.textContent,
-          await waitForSelector
-        )
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, await waitForSelector)
       ).toBe('anything');
     });
     it('should have correct stack trace for timeout', async () => {
       const { page } = getTestState();
 
-      let error;
-      await page
-        .waitForSelector('.zombo', { timeout: 10 })
-        .catch((error_) => (error = error_));
-      expect(error.stack).toContain('waiting for selector `.zombo` failed');
+      let error!: Error;
+      await page.waitForSelector('.zombo', { timeout: 10 }).catch((error_) => {
+        return (error = error_);
+      });
+      expect(error?.stack).toContain('waiting for selector `.zombo` failed');
       // The extension is ts here as Mocha maps back via sourcemaps.
-      expect(error.stack).toContain('waittask.spec.ts');
+      expect(error?.stack).toContain('waittask.spec.ts');
     });
   });
 
   describe('Frame.waitForXPath', function () {
-    const addElement = (tag) =>
-      document.body.appendChild(document.createElement(tag));
+    const addElement = (tag: string) => {
+      return document.body.appendChild(document.createElement(tag));
+    };
 
     it('should support some fancy xpath', async () => {
       const { page } = getTestState();
@@ -690,51 +838,51 @@ describe('waittask specs', function () {
         '//p[normalize-space(.)="hello world"]'
       );
       expect(
-        await page.evaluate(
-          (x: HTMLElement) => x.textContent,
-          await waitForXPath
-        )
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, await waitForXPath)
       ).toBe('hello  world  ');
     });
     it('should respect timeout', async () => {
       const { page, puppeteer } = getTestState();
 
-      let error = null;
-      await page
-        .waitForXPath('//div', { timeout: 10 })
-        .catch((error_) => (error = error_));
-      expect(error).toBeTruthy();
-      expect(error.message).toContain(
+      let error!: Error;
+      await page.waitForXPath('//div', { timeout: 10 }).catch((error_) => {
+        return (error = error_);
+      });
+      expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
+      expect(error?.message).toContain(
         'waiting for XPath `//div` failed: timeout'
       );
-      expect(error).toBeInstanceOf(puppeteer.errors.TimeoutError);
     });
     itFailsFirefox('should run in specified frame', async () => {
       const { page, server } = getTestState();
 
-      await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      await utils.attachFrame(page, 'frame2', server.EMPTY_PAGE);
-      const frame1 = page.frames()[1];
-      const frame2 = page.frames()[2];
+      await attachFrame(page, 'frame1', server.EMPTY_PAGE);
+      await attachFrame(page, 'frame2', server.EMPTY_PAGE);
+      const frame1 = page.frames()[1]!;
+      const frame2 = page.frames()[2]!;
       const waitForXPathPromise = frame2.waitForXPath('//div');
       await frame1.evaluate(addElement, 'div');
       await frame2.evaluate(addElement, 'div');
       const eHandle = await waitForXPathPromise;
-      expect(eHandle.executionContext().frame()).toBe(frame2);
+      expect(eHandle?.executionContext().frame()).toBe(frame2);
     });
     itFailsFirefox('should throw when frame is detached', async () => {
       const { page, server } = getTestState();
 
-      await utils.attachFrame(page, 'frame1', server.EMPTY_PAGE);
-      const frame = page.frames()[1];
-      let waitError = null;
+      await attachFrame(page, 'frame1', server.EMPTY_PAGE);
+      const frame = page.frames()[1]!;
+      let waitError: Error | undefined;
       const waitPromise = frame
         .waitForXPath('//*[@class="box"]')
-        .catch((error) => (waitError = error));
-      await utils.detachFrame(page, 'frame1');
+        .catch((error) => {
+          return (waitError = error);
+        });
+      await detachFrame(page, 'frame1');
       await waitPromise;
       expect(waitError).toBeTruthy();
-      expect(waitError.message).toContain(
+      expect(waitError?.message).toContain(
         'waitForFunction failed: frame got detached.'
       );
     });
@@ -745,12 +893,16 @@ describe('waittask specs', function () {
       await page.setContent(`<div style='display: block;'></div>`);
       const waitForXPath = page
         .waitForXPath('//div', { hidden: true })
-        .then(() => (divHidden = true));
+        .then(() => {
+          return (divHidden = true);
+        });
       await page.waitForXPath('//div'); // do a round trip
       expect(divHidden).toBe(false);
-      await page.evaluate(() =>
-        document.querySelector('div').style.setProperty('display', 'none')
-      );
+      await page.evaluate(() => {
+        return document
+          .querySelector('div')
+          ?.style.setProperty('display', 'none');
+      });
       expect(await waitForXPath).toBe(true);
       expect(divHidden).toBe(true);
     });
@@ -760,10 +912,9 @@ describe('waittask specs', function () {
       const waitForXPath = page.waitForXPath('//*[@class="zombo"]');
       await page.setContent(`<div class='zombo'>anything</div>`);
       expect(
-        await page.evaluate(
-          (x: HTMLElement) => x.textContent,
-          await waitForXPath
-        )
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, await waitForXPath)
       ).toBe('anything');
     });
     it('should allow you to select a text node', async () => {
@@ -771,7 +922,7 @@ describe('waittask specs', function () {
 
       await page.setContent(`<div>some text</div>`);
       const text = await page.waitForXPath('//div/text()');
-      expect(await (await text.getProperty('nodeType')).jsonValue()).toBe(
+      expect(await (await text!.getProperty('nodeType')!).jsonValue()).toBe(
         3 /* Node.TEXT_NODE */
       );
     });
@@ -781,10 +932,9 @@ describe('waittask specs', function () {
       await page.setContent(`<div>some text</div>`);
       const waitForXPath = page.waitForXPath('/html/body/div');
       expect(
-        await page.evaluate(
-          (x: HTMLElement) => x.textContent,
-          await waitForXPath
-        )
+        await page.evaluate((x: HTMLElement) => {
+          return x.textContent;
+        }, await waitForXPath)
       ).toBe('some text');
     });
   });
diff --git a/test/src/worker.spec.ts b/test/src/worker.spec.ts
index fde54500e5b2d..f2869ccc64871 100644
--- a/test/src/worker.spec.ts
+++ b/test/src/worker.spec.ts
@@ -15,16 +15,15 @@
  */
 
 import expect from 'expect';
+import { ConsoleMessage } from '../../lib/cjs/puppeteer/common/ConsoleMessage.js';
+import { WebWorker } from '../../lib/cjs/puppeteer/common/WebWorker.js';
 import {
+  describeFailsFirefox,
   getTestState,
   setupTestBrowserHooks,
   setupTestPageAndContextHooks,
-  describeFailsFirefox,
-} from './mocha-utils'; // eslint-disable-line import/extensions
-import utils from './utils.js';
-import { WebWorker } from '../../lib/cjs/puppeteer/common/WebWorker.js';
-import { ConsoleMessage } from '../../lib/cjs/puppeteer/common/ConsoleMessage.js';
-const { waitEvent } = utils;
+} from './mocha-utils.js';
+import { waitEvent } from './utils.js';
 
 describeFailsFirefox('Workers', function () {
   setupTestBrowserHooks();
@@ -33,15 +32,19 @@ describeFailsFirefox('Workers', function () {
     const { page, server } = getTestState();
 
     await Promise.all([
-      new Promise((x) => page.once('workercreated', x)),
+      new Promise((x) => {
+        return page.once('workercreated', x);
+      }),
       page.goto(server.PREFIX + '/worker/worker.html'),
     ]);
-    const worker = page.workers()[0];
-    expect(worker.url()).toContain('worker.js');
+    const worker = page.workers()[0]!;
+    expect(worker?.url()).toContain('worker.js');
 
-    expect(await worker.evaluate(() => globalThis.workerFunction())).toBe(
-      'worker function result'
-    );
+    expect(
+      await worker?.evaluate(() => {
+        return (globalThis as any).workerFunction();
+      })
+    ).toBe('worker function result');
 
     await page.goto(server.EMPTY_PAGE);
     expect(page.workers().length).toBe(0);
@@ -49,25 +52,26 @@ describeFailsFirefox('Workers', function () {
   it('should emit created and destroyed events', async () => {
     const { page } = getTestState();
 
-    const workerCreatedPromise = new Promise<WebWorker>((x) =>
-      page.once('workercreated', x)
-    );
-    const workerObj = await page.evaluateHandle(
-      () => new Worker('data:text/javascript,1')
-    );
+    const workerCreatedPromise = new Promise<WebWorker>((x) => {
+      return page.once('workercreated', x);
+    });
+    const workerObj = await page.evaluateHandle(() => {
+      return new Worker('data:text/javascript,1');
+    });
     const worker = await workerCreatedPromise;
-    const workerThisObj = await worker.evaluateHandle(() => this);
-    const workerDestroyedPromise = new Promise((x) =>
-      page.once('workerdestroyed', x)
-    );
-    await page.evaluate(
-      (workerObj: Worker) => workerObj.terminate(),
-      workerObj
-    );
+    const workerThisObj = await worker.evaluateHandle(() => {
+      return this;
+    });
+    const workerDestroyedPromise = new Promise((x) => {
+      return page.once('workerdestroyed', x);
+    });
+    await page.evaluate((workerObj: Worker) => {
+      return workerObj.terminate();
+    }, workerObj);
     expect(await workerDestroyedPromise).toBe(worker);
-    const error = await workerThisObj
-      .getProperty('self')
-      .catch((error) => error);
+    const error = await workerThisObj.getProperty('self').catch((error) => {
+      return error;
+    });
     expect(error.message).toContain('Most likely the worker has been closed.');
   });
   it('should report console logs', async () => {
@@ -75,7 +79,9 @@ describeFailsFirefox('Workers', function () {
 
     const [message] = await Promise.all([
       waitEvent(page, 'console'),
-      page.evaluate(() => new Worker(`data:text/javascript,console.log(1)`)),
+      page.evaluate(() => {
+        return new Worker(`data:text/javascript,console.log(1)`);
+      }),
     ]);
     expect(message.text()).toBe('1');
     expect(message.location()).toEqual({
@@ -87,39 +93,42 @@ describeFailsFirefox('Workers', function () {
   it('should have JSHandles for console logs', async () => {
     const { page } = getTestState();
 
-    const logPromise = new Promise<ConsoleMessage>((x) =>
-      page.on('console', x)
-    );
-    await page.evaluate(
-      () => new Worker(`data:text/javascript,console.log(1,2,3,this)`)
-    );
+    const logPromise = new Promise<ConsoleMessage>((x) => {
+      return page.on('console', x);
+    });
+    await page.evaluate(() => {
+      return new Worker(`data:text/javascript,console.log(1,2,3,this)`);
+    });
     const log = await logPromise;
     expect(log.text()).toBe('1 2 3 JSHandle@object');
     expect(log.args().length).toBe(4);
-    expect(await (await log.args()[3].getProperty('origin')).jsonValue()).toBe(
+    expect(await (await log.args()[3]!.getProperty('origin')).jsonValue()).toBe(
       'null'
     );
   });
   it('should have an execution context', async () => {
     const { page } = getTestState();
 
-    const workerCreatedPromise = new Promise<WebWorker>((x) =>
-      page.once('workercreated', x)
-    );
-    await page.evaluate(
-      () => new Worker(`data:text/javascript,console.log(1)`)
-    );
+    const workerCreatedPromise = new Promise<WebWorker>((x) => {
+      return page.once('workercreated', x);
+    });
+    await page.evaluate(() => {
+      return new Worker(`data:text/javascript,console.log(1)`);
+    });
     const worker = await workerCreatedPromise;
     expect(await (await worker.executionContext()).evaluate('1+1')).toBe(2);
   });
   it('should report errors', async () => {
     const { page } = getTestState();
 
-    const errorPromise = new Promise<Error>((x) => page.on('pageerror', x));
-    await page.evaluate(
-      () =>
-        new Worker(`data:text/javascript, throw new Error('this is my error');`)
-    );
+    const errorPromise = new Promise<Error>((x) => {
+      return page.on('pageerror', x);
+    });
+    await page.evaluate(() => {
+      return new Worker(
+        `data:text/javascript, throw new Error('this is my error');`
+      );
+    });
     const errorLog = await errorPromise;
     expect(errorLog.message).toContain('this is my error');
   });
diff --git a/test/tsconfig.json b/test/tsconfig.json
index abe9d11796b53..f7bdf270f7d0c 100644
--- a/test/tsconfig.json
+++ b/test/tsconfig.json
@@ -1,17 +1,11 @@
 {
+  "extends": "../tsconfig.base.json",
   "compilerOptions": {
-    "esModuleInterop": true,
-    "allowJs": true,
-    "checkJs": false,
-    "target": "esnext",
-    "module": "CommonJS",
-    "moduleResolution": "node",
     "declaration": false,
     "declarationMap": false,
-    "resolveJsonModule": true,
-    "sourceMap": true,
-    "rootDir": "src",
-    "outDir": "build"
+    "module": "CommonJS",
+    "outDir": "build",
+    "rootDir": "src"
   },
   "include": ["src"],
   "references": [
diff --git a/utils/testserver/src/index.ts b/utils/testserver/src/index.ts
index 203b113a6f380..cdcff72d93b88 100644
--- a/utils/testserver/src/index.ts
+++ b/utils/testserver/src/index.ts
@@ -43,10 +43,10 @@ interface Subscriber {
 type TestIncomingMessage = IncomingMessage & { postBody?: Promise<string> };
 
 export class TestServer {
-  PORT?: number;
-  PREFIX?: string;
-  CROSS_PROCESS_PREFIX?: string;
-  EMPTY_PAGE?: string;
+  PORT!: number;
+  PREFIX!: string;
+  CROSS_PROCESS_PREFIX!: string;
+  EMPTY_PAGE!: string;
 
   #dirPath: string;
   #server: HttpsServer | HttpServer;
diff --git a/utils/testserver/tsconfig.json b/utils/testserver/tsconfig.json
index 16efc53207aca..c2576c2564f7f 100644
--- a/utils/testserver/tsconfig.json
+++ b/utils/testserver/tsconfig.json
@@ -1,11 +1,11 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "rootDir": "src",
-    "outDir": "lib",
-    "composite": true,
     "allowJs": true,
-    "module": "CommonJS"
+    "composite": true,
+    "module": "CommonJS",
+    "outDir": "lib",
+    "rootDir": "src"
   },
   "include": ["src"]
 }