diff --git a/src/Worker.js b/src/Worker.ts
similarity index 52%
rename from src/Worker.js
rename to src/Worker.ts
index 2a750b2712812..b075370bcefa5 100644
--- a/src/Worker.js
+++ b/src/Worker.ts
@@ -13,71 +13,55 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-const EventEmitter = require('events');
-const {debugError} = require('./helper');
-const {ExecutionContext} = require('./ExecutionContext');
-const {JSHandle} = require('./JSHandle');
-// Used as a TypeDef
-// eslint-disable-next-line no-unused-vars
-const {CDPSession} = require('./Connection');
+import {EventEmitter} from 'events';
+import {debugError} from './helper';
+import {ExecutionContext} from './ExecutionContext';
+import {JSHandle} from './JSHandle';
+import {CDPSession} from './Connection';
 
-class Worker extends EventEmitter {
-  /**
-   * @param {CDPSession} client
-   * @param {string} url
-   * @param {function(string, !Array<!JSHandle>, Protocol.Runtime.StackTrace=):void} consoleAPICalled
-   * @param {function(!Protocol.Runtime.ExceptionDetails):void} exceptionThrown
-   */
-  constructor(client, url, consoleAPICalled, exceptionThrown) {
+type ConsoleAPICalledCallback = (eventType: string, handles: JSHandle[], trace: Protocol.Runtime.StackTrace) => void;
+type ExceptionThrownCallback = (details: Protocol.Runtime.ExceptionDetails) => void;
+type JSHandleFactory = (obj: Protocol.Runtime.RemoteObject) => JSHandle;
+
+export class Worker extends EventEmitter {
+  _client: CDPSession;
+  _url: string;
+  _executionContextPromise: Promise<ExecutionContext>;
+  _executionContextCallback: (value: ExecutionContext) => void;
+
+  constructor(client: CDPSession, url: string, consoleAPICalled: ConsoleAPICalledCallback, exceptionThrown: ExceptionThrownCallback) {
     super();
     this._client = client;
     this._url = url;
-    this._executionContextPromise = new Promise(x => this._executionContextCallback = x);
-    /** @type {function(!Protocol.Runtime.RemoteObject):!JSHandle} */
-    let jsHandleFactory;
+    this._executionContextPromise = new Promise<ExecutionContext>(x => this._executionContextCallback = x);
+
+    let jsHandleFactory: JSHandleFactory;
     this._client.once('Runtime.executionContextCreated', async event => {
+      // eslint-disable-next-line @typescript-eslint/explicit-function-return-type
       jsHandleFactory = remoteObject => new JSHandle(executionContext, client, remoteObject);
       const executionContext = new ExecutionContext(client, event.context, null);
       this._executionContextCallback(executionContext);
     });
+
     // This might fail if the target is closed before we recieve all execution contexts.
     this._client.send('Runtime.enable', {}).catch(debugError);
-
     this._client.on('Runtime.consoleAPICalled', event => consoleAPICalled(event.type, event.args.map(jsHandleFactory), event.stackTrace));
     this._client.on('Runtime.exceptionThrown', exception => exceptionThrown(exception.exceptionDetails));
   }
 
-  /**
-   * @return {string}
-   */
-  url() {
+  url(): string {
     return this._url;
   }
 
-  /**
-   * @return {!Promise<ExecutionContext>}
-   */
-  async executionContext() {
+  async executionContext(): Promise<ExecutionContext> {
     return this._executionContextPromise;
   }
 
-  /**
-   * @param {Function|string} pageFunction
-   * @param {!Array<*>} args
-   * @return {!Promise<*>}
-   */
-  async evaluate(pageFunction, ...args) {
-    return (await this._executionContextPromise).evaluate(pageFunction, ...args);
+  async evaluate<ReturnType extends any>(pageFunction: Function|string, ...args: any[]): Promise<ReturnType> {
+    return (await this._executionContextPromise).evaluate<ReturnType>(pageFunction, ...args);
   }
 
-  /**
-   * @param {Function|string} pageFunction
-   * @param {!Array<*>} args
-   * @return {!Promise<!JSHandle>}
-   */
-  async evaluateHandle(pageFunction, ...args) {
+  async evaluateHandle(pageFunction: Function|string, ...args: any[]): Promise<JSHandle> {
     return (await this._executionContextPromise).evaluateHandle(pageFunction, ...args);
   }
 }
-
-module.exports = {Worker};