diff --git a/src/common/Errors.ts b/src/common/Errors.ts
index 5b9a09a092c7c..2676caa893f6d 100644
--- a/src/common/Errors.ts
+++ b/src/common/Errors.ts
@@ -43,7 +43,7 @@ export class TimeoutError extends CustomError {}
  */
 export class ProtocolError extends CustomError {
   public code?: number;
-  public originalMessage: string;
+  public originalMessage = '';
 }
 /**
  * @public
diff --git a/src/common/EventEmitter.ts b/src/common/EventEmitter.ts
index 91cf7e0904be9..95eb0c10e0023 100644
--- a/src/common/EventEmitter.ts
+++ b/src/common/EventEmitter.ts
@@ -144,6 +144,6 @@ export class EventEmitter implements CommonEventEmitter {
   }
 
   private eventListenersCount(event: EventType): number {
-    return this.eventsMap.has(event) ? this.eventsMap.get(event).length : 0;
+    return this.eventsMap.get(event)?.length || 0;
   }
 }
diff --git a/src/common/helper.ts b/src/common/helper.ts
index ba20970d9253d..aa0dea6854a97 100644
--- a/src/common/helper.ts
+++ b/src/common/helper.ts
@@ -134,7 +134,9 @@ async function waitForEvent<T extends any>(
   timeout: number,
   abortPromise: Promise<Error>
 ): Promise<T> {
-  let eventTimeout, resolveCallback, rejectCallback;
+  let eventTimeout: NodeJS.Timeout;
+  let resolveCallback: (value: T | PromiseLike<T>) => void;
+  let rejectCallback: (value: Error) => void;
   const promise = new Promise<T>((resolve, reject) => {
     resolveCallback = resolve;
     rejectCallback = reject;
@@ -192,7 +194,7 @@ function pageBindingInitString(type: string, name: string): string {
     const binding = win[bindingName];
 
     win[bindingName] = (...args: unknown[]): Promise<unknown> => {
-      const me = window[bindingName];
+      const me = (window as any)[bindingName];
       let callbacks = me.callbacks;
       if (!callbacks) {
         callbacks = new Map();
@@ -216,8 +218,8 @@ function pageBindingDeliverResultString(
   result: unknown
 ): string {
   function deliverResult(name: string, seq: number, result: unknown): void {
-    window[name].callbacks.get(seq).resolve(result);
-    window[name].callbacks.delete(seq);
+    (window as any)[name].callbacks.get(seq).resolve(result);
+    (window as any)[name].callbacks.delete(seq);
   }
   return evaluationString(deliverResult, name, seq, result);
 }
@@ -236,8 +238,8 @@ function pageBindingDeliverErrorString(
   ): void {
     const error = new Error(message);
     error.stack = stack;
-    window[name].callbacks.get(seq).reject(error);
-    window[name].callbacks.delete(seq);
+    (window as any)[name].callbacks.get(seq).reject(error);
+    (window as any)[name].callbacks.delete(seq);
   }
   return evaluationString(deliverError, name, seq, message, stack);
 }
@@ -248,8 +250,8 @@ function pageBindingDeliverErrorValueString(
   value: unknown
 ): string {
   function deliverErrorValue(name: string, seq: number, value: unknown): void {
-    window[name].callbacks.get(seq).reject(value);
-    window[name].callbacks.delete(seq);
+    (window as any)[name].callbacks.get(seq).reject(value);
+    (window as any)[name].callbacks.delete(seq);
   }
   return evaluationString(deliverErrorValue, name, seq, value);
 }
@@ -266,7 +268,9 @@ function makePredicateString(
     if (!node) return waitForHidden;
     if (!waitForVisible && !waitForHidden) return node;
     const element =
-      node.nodeType === Node.TEXT_NODE ? node.parentElement : (node as Element);
+      node.nodeType === Node.TEXT_NODE
+        ? (node.parentElement as Element)
+        : (node as Element);
 
     const style = window.getComputedStyle(element);
     const isVisible =
@@ -296,7 +300,7 @@ async function waitWithTimeout<T extends any>(
   taskName: string,
   timeout: number
 ): Promise<T> {
-  let reject;
+  let reject: (reason?: Error) => void;
   const timeoutError = new TimeoutError(
     `waiting for ${taskName} failed: timeout ${timeout}ms exceeded`
   );
@@ -313,14 +317,14 @@ async function waitWithTimeout<T extends any>(
 async function getReadableAsBuffer(
   readable: Readable,
   path?: string
-): Promise<Buffer> {
+): Promise<Buffer | null> {
   if (!isNode && path) {
     throw new Error('Cannot write to a path outside of Node.js environment.');
   }
 
   const fs = isNode ? await importFSModule() : null;
 
-  let fileHandle: import('fs').promises.FileHandle;
+  let fileHandle: import('fs').promises.FileHandle | undefined;
 
   if (path && fs) {
     fileHandle = await fs.promises.open(path, 'w');
@@ -328,12 +332,12 @@ async function getReadableAsBuffer(
   const buffers = [];
   for await (const chunk of readable) {
     buffers.push(chunk);
-    if (fileHandle) {
+    if (fileHandle && fs) {
       await fs.promises.writeFile(fileHandle, chunk);
     }
   }
 
-  if (path) await fileHandle.close();
+  if (path && fileHandle) await fileHandle.close();
   let resultBuffer = null;
   try {
     resultBuffer = Buffer.concat(buffers);