diff --git a/docs/src/api/class-browsercontext.md b/docs/src/api/class-browsercontext.md
index 6d5558ae55..7b6c6aab1d 100644
--- a/docs/src/api/class-browsercontext.md
+++ b/docs/src/api/class-browsercontext.md
@@ -415,13 +415,42 @@ The order of evaluation of multiple scripts installed via [`method: BrowserConte
 [`method: Page.addInitScript`] is not defined.
 :::
 
+**Bundling**
+
+If you have a complex script split into several files, it needs to be bundled into a single file first. We recommend running [`esbuild`](https://esbuild.github.io/) or [`webpack`](https://webpack.js.org/) to produce a commonjs module and pass [`option: path`] and [`option: arg`].
+
+```js browser title="mocks/mockRandom.ts"
+// This script can import other files.
+import { defaultValue } from './defaultValue';
+
+export default function(value?: number) {
+  window.Math.random = () => value ?? defaultValue;
+}
+```
+
+```sh
+# bundle with esbuild
+esbuild mocks/mockRandom.ts --bundle --format=cjs --outfile=mocks/mockRandom.js
+```
+
+```js title="tests/example.spec.ts"
+const mockPath = { path: path.resolve(__dirname, '../mocks/mockRandom.js') };
+
+// Passing 42 as an argument to the default export function.
+await context.addInitScript({ path: mockPath }, 42);
+
+// Make sure to pass undefined even if you do not need to pass an argument.
+// This instructs Playwright to treat the file as a commonjs module.
+await context.addInitScript({ path: mockPath }, undefined);
+```
+
 ### param: BrowserContext.addInitScript.script
 * since: v1.8
 * langs: js
 - `script` <[function]|[string]|[Object]>
   - `path` ?<[path]> Path to the JavaScript file. If `path` is a relative path, then it is resolved relative to the
-    current working directory. Optional.
-  - `content` ?<[string]> Raw script content. Optional.
+    current working directory.
+  - `content` ?<[string]> Raw script content.
 
 Script to be evaluated in all pages in the browser context.
 
@@ -437,7 +466,9 @@ Script to be evaluated in all pages in the browser context.
 * langs: js
 - `arg` ?<[Serializable]>
 
-Optional argument to pass to [`param: script`] (only supported when passing a function).
+Optional JSON-serializable argument to pass to [`param: script`].
+* When `script` is a function, the argument is passed to it directly.
+* When `script` is a file path, the file is assumed to be a commonjs module. The default export, either `module.exports` or `module.exports.default`, should be a function that's going to be executed with this argument.
 
 ### param: BrowserContext.addInitScript.path
 * since: v1.8
diff --git a/docs/src/api/class-page.md b/docs/src/api/class-page.md
index ee9623c867..11d7cbdd74 100644
--- a/docs/src/api/class-page.md
+++ b/docs/src/api/class-page.md
@@ -619,13 +619,42 @@ The order of evaluation of multiple scripts installed via [`method: BrowserConte
 [`method: Page.addInitScript`] is not defined.
 :::
 
+**Bundling**
+
+If you have a complex script split into several files, it needs to be bundled into a single file first. We recommend running [`esbuild`](https://esbuild.github.io/) or [`webpack`](https://webpack.js.org/) to produce a commonjs module and pass [`option: path`] and [`option: arg`].
+
+```js browser title="mocks/mockRandom.ts"
+// This script can import other files.
+import { defaultValue } from './defaultValue';
+
+export default function(value?: number) {
+  window.Math.random = () => value ?? defaultValue;
+}
+```
+
+```sh
+# bundle with esbuild
+esbuild mocks/mockRandom.ts --bundle --format=cjs --outfile=mocks/mockRandom.js
+```
+
+```js title="tests/example.spec.ts"
+const mockPath = { path: path.resolve(__dirname, '../mocks/mockRandom.js') };
+
+// Passing 42 as an argument to the default export function.
+await page.addInitScript({ path: mockPath }, 42);
+
+// Make sure to pass undefined even if you do not need to pass an argument.
+// This instructs Playwright to treat the file as a commonjs module.
+await page.addInitScript({ path: mockPath }, undefined);
+```
+
 ### param: Page.addInitScript.script
 * since: v1.8
 * langs: js
 - `script` <[function]|[string]|[Object]>
   - `path` ?<[path]> Path to the JavaScript file. If `path` is a relative path, then it is resolved relative to the
-    current working directory. Optional.
-  - `content` ?<[string]> Raw script content. Optional.
+    current working directory.
+  - `content` ?<[string]> Raw script content.
 
 Script to be evaluated in the page.
 
@@ -641,7 +670,9 @@ Script to be evaluated in all pages in the browser context.
 * langs: js
 - `arg` ?<[Serializable]>
 
-Optional argument to pass to [`param: script`] (only supported when passing a function).
+Optional JSON-serializable argument to pass to [`param: script`].
+* When `script` is a function, the argument is passed to it directly.
+* When `script` is a file path, the file is assumed to be a commonjs module. The default export, either `module.exports` or `module.exports.default`, should be a function that's going to be executed with this argument.
 
 ### param: Page.addInitScript.path
 * since: v1.8
diff --git a/packages/playwright-core/src/client/browserContext.ts b/packages/playwright-core/src/client/browserContext.ts
index c4f7827840..b0b72917cf 100644
--- a/packages/playwright-core/src/client/browserContext.ts
+++ b/packages/playwright-core/src/client/browserContext.ts
@@ -308,7 +308,7 @@ export class BrowserContext extends ChannelOwner<channels.BrowserContextChannel>
   }
 
   async addInitScript(script: Function | string | { path?: string, content?: string }, arg?: any): Promise<void> {
-    const source = await evaluationScript(script, arg);
+    const source = await evaluationScript(script, arg, arguments.length > 1);
     await this._channel.addInitScript({ source });
   }
 
diff --git a/packages/playwright-core/src/client/clientHelper.ts b/packages/playwright-core/src/client/clientHelper.ts
index 540230a4fc..fcc785b71b 100644
--- a/packages/playwright-core/src/client/clientHelper.ts
+++ b/packages/playwright-core/src/client/clientHelper.ts
@@ -28,20 +28,37 @@ export function envObjectToArray(env: types.Env): { name: string, value: string
   return result;
 }
 
-export async function evaluationScript(fun: Function | string | { path?: string, content?: string }, arg?: any, addSourceUrl: boolean = true): Promise<string> {
+export async function evaluationScript(fun: Function | string | { path?: string, content?: string }, arg: any, hasArg: boolean, addSourceUrl: boolean = true): Promise<string> {
   if (typeof fun === 'function') {
     const source = fun.toString();
     const argString = Object.is(arg, undefined) ? 'undefined' : JSON.stringify(arg);
     return `(${source})(${argString})`;
   }
-  if (arg !== undefined)
-    throw new Error('Cannot evaluate a string with arguments');
-  if (isString(fun))
+  if (isString(fun)) {
+    if (arg !== undefined)
+      throw new Error('Cannot evaluate a string with arguments');
     return fun;
-  if (fun.content !== undefined)
+  }
+  if (fun.content !== undefined) {
+    if (arg !== undefined)
+      throw new Error('Cannot evaluate a string with arguments');
     return fun.content;
+  }
   if (fun.path !== undefined) {
     let source = await fs.promises.readFile(fun.path, 'utf8');
+    if (hasArg) {
+      // Assume a CJS module that has a function default export.
+      source = `(() => {
+        var exports = {}; var module = { exports };
+        ${source}
+        let __pw_result__ = module.exports;
+        if (__pw_result__ && typeof __pw_result__ === 'object' && ('default' in __pw_result__))
+          __pw_result__ = __pw_result__['default'];
+        if (typeof __pw_result__ !== 'function')
+          return __pw_result__;
+        return __pw_result__(${JSON.stringify(arg)});
+      })()`;
+    }
     if (addSourceUrl)
       source = addSourceUrlToScript(source, fun.path);
     return source;
diff --git a/packages/playwright-core/src/client/page.ts b/packages/playwright-core/src/client/page.ts
index a10286fa9a..5ff6d6178d 100644
--- a/packages/playwright-core/src/client/page.ts
+++ b/packages/playwright-core/src/client/page.ts
@@ -492,7 +492,7 @@ export class Page extends ChannelOwner<channels.PageChannel> implements api.Page
   }
 
   async addInitScript(script: Function | string | { path?: string, content?: string }, arg?: any) {
-    const source = await evaluationScript(script, arg);
+    const source = await evaluationScript(script, arg, arguments.length > 1);
     await this._channel.addInitScript({ source });
   }
 
diff --git a/packages/playwright-core/src/client/selectors.ts b/packages/playwright-core/src/client/selectors.ts
index 2739be0e8d..c7a7967559 100644
--- a/packages/playwright-core/src/client/selectors.ts
+++ b/packages/playwright-core/src/client/selectors.ts
@@ -26,7 +26,7 @@ export class Selectors implements api.Selectors {
   private _registrations: channels.SelectorsRegisterParams[] = [];
 
   async register(name: string, script: string | (() => SelectorEngine) | { path?: string, content?: string }, options: { contentScript?: boolean } = {}): Promise<void> {
-    const source = await evaluationScript(script, undefined, false);
+    const source = await evaluationScript(script, undefined, false, false);
     const params = { ...options, name, source };
     for (const channel of this._channels)
       await channel._channel.register(params);
diff --git a/packages/playwright-core/types/types.d.ts b/packages/playwright-core/types/types.d.ts
index a4bf9fa812..0d018dcfd3 100644
--- a/packages/playwright-core/types/types.d.ts
+++ b/packages/playwright-core/types/types.d.ts
@@ -288,8 +288,41 @@ export interface Page {
    * [browserContext.addInitScript(script[, arg])](https://playwright.dev/docs/api/class-browsercontext#browser-context-add-init-script)
    * and [page.addInitScript(script[, arg])](https://playwright.dev/docs/api/class-page#page-add-init-script) is not
    * defined.
+   *
+   * **Bundling**
+   *
+   * If you have a complex script split into several files, it needs to be bundled into a single file first. We
+   * recommend running [`esbuild`](https://esbuild.github.io/) or [`webpack`](https://webpack.js.org/) to produce a
+   * commonjs module and pass `path` and `arg`.
+   *
+   * ```js
+   * // mocks/mockRandom.ts
+   * // This script can import other files.
+   * import { defaultValue } from './defaultValue';
+   *
+   * export default function(value?: number) {
+   *   window.Math.random = () => value ?? defaultValue;
+   * }
+   * ```
+   *
+   * ```js
+   * // tests/example.spec.ts
+   * const mockPath = { path: path.resolve(__dirname, '../mocks/mockRandom.js') };
+   *
+   * // Passing 42 as an argument to the default export function.
+   * await page.addInitScript({ path: mockPath }, 42);
+   *
+   * // Make sure to pass undefined even if you do not need to pass an argument.
+   * // This instructs Playwright to treat the file as a commonjs module.
+   * await page.addInitScript({ path: mockPath }, undefined);
+   * ```
+   *
    * @param script Script to be evaluated in the page.
-   * @param arg Optional argument to pass to `script` (only supported when passing a function).
+   * @param arg Optional JSON-serializable argument to pass to `script`.
+   * - When `script` is a function, the argument is passed to it directly.
+   * - When `script` is a file path, the file is assumed to be a commonjs module. The default export, either
+   * `module.exports` or `module.exports.default`, should be a function that's going to be executed with this
+   * argument.
    */
   addInitScript<Arg>(script: PageFunction<Arg, any> | { path?: string, content?: string }, arg?: Arg): Promise<void>;
 
@@ -7666,8 +7699,41 @@ export interface BrowserContext {
    * [browserContext.addInitScript(script[, arg])](https://playwright.dev/docs/api/class-browsercontext#browser-context-add-init-script)
    * and [page.addInitScript(script[, arg])](https://playwright.dev/docs/api/class-page#page-add-init-script) is not
    * defined.
+   *
+   * **Bundling**
+   *
+   * If you have a complex script split into several files, it needs to be bundled into a single file first. We
+   * recommend running [`esbuild`](https://esbuild.github.io/) or [`webpack`](https://webpack.js.org/) to produce a
+   * commonjs module and pass `path` and `arg`.
+   *
+   * ```js
+   * // mocks/mockRandom.ts
+   * // This script can import other files.
+   * import { defaultValue } from './defaultValue';
+   *
+   * export default function(value?: number) {
+   *   window.Math.random = () => value ?? defaultValue;
+   * }
+   * ```
+   *
+   * ```js
+   * // tests/example.spec.ts
+   * const mockPath = { path: path.resolve(__dirname, '../mocks/mockRandom.js') };
+   *
+   * // Passing 42 as an argument to the default export function.
+   * await context.addInitScript({ path: mockPath }, 42);
+   *
+   * // Make sure to pass undefined even if you do not need to pass an argument.
+   * // This instructs Playwright to treat the file as a commonjs module.
+   * await context.addInitScript({ path: mockPath }, undefined);
+   * ```
+   *
    * @param script Script to be evaluated in all pages in the browser context.
-   * @param arg Optional argument to pass to `script` (only supported when passing a function).
+   * @param arg Optional JSON-serializable argument to pass to `script`.
+   * - When `script` is a function, the argument is passed to it directly.
+   * - When `script` is a file path, the file is assumed to be a commonjs module. The default export, either
+   * `module.exports` or `module.exports.default`, should be a function that's going to be executed with this
+   * argument.
    */
   addInitScript<Arg>(script: PageFunction<Arg, any> | { path?: string, content?: string }, arg?: Arg): Promise<void>;
 
diff --git a/tests/assets/injectedmodule.js b/tests/assets/injectedmodule.js
new file mode 100644
index 0000000000..bc099f243f
--- /dev/null
+++ b/tests/assets/injectedmodule.js
@@ -0,0 +1,33 @@
+"use strict";
+var __defProp = Object.defineProperty;
+var __getOwnPropDesc = Object.getOwnPropertyDescriptor;
+var __getOwnPropNames = Object.getOwnPropertyNames;
+var __hasOwnProp = Object.prototype.hasOwnProperty;
+var __export = (target, all) => {
+  for (var name in all)
+    __defProp(target, name, { get: all[name], enumerable: true });
+};
+var __copyProps = (to, from, except, desc) => {
+  if (from && typeof from === "object" || typeof from === "function") {
+    for (let key of __getOwnPropNames(from))
+      if (!__hasOwnProp.call(to, key) && key !== except)
+        __defProp(to, key, { get: () => from[key], enumerable: !(desc = __getOwnPropDesc(from, key)) || desc.enumerable });
+  }
+  return to;
+};
+var __toCommonJS = (mod) => __copyProps(__defProp({}, "__esModule", { value: true }), mod);
+
+// one.ts
+var one_exports = {};
+__export(one_exports, {
+  default: () => one_default
+});
+module.exports = __toCommonJS(one_exports);
+
+// two.ts
+var value = 42;
+
+// one.ts
+function one_default(arg) {
+  window.injected = arg ?? value;
+}
diff --git a/tests/page/page-add-init-script.spec.ts b/tests/page/page-add-init-script.spec.ts
index b2b7782eba..2b12f00251 100644
--- a/tests/page/page-add-init-script.spec.ts
+++ b/tests/page/page-add-init-script.spec.ts
@@ -31,6 +31,18 @@ it('should work with a path', async ({ page, server, asset }) => {
   expect(await page.evaluate(() => window['result'])).toBe(123);
 });
 
+it('should assume CJS module with a path and arg', async ({ page, server, asset }) => {
+  await page.addInitScript({ path: asset('injectedmodule.js') }, 17);
+  await page.goto(server.EMPTY_PAGE);
+  expect(await page.evaluate(() => window['injected'])).toBe(17);
+});
+
+it('should assume CJS module with a path and undefined arg', async ({ page, server, asset }) => {
+  await page.addInitScript({ path: asset('injectedmodule.js') }, undefined);
+  await page.goto(server.EMPTY_PAGE);
+  expect(await page.evaluate(() => window['injected'])).toBe(42);
+});
+
 it('should work with content @smoke', async ({ page, server }) => {
   await page.addInitScript({ content: 'window["injected"] = 123' });
   await page.goto(server.PREFIX + '/tamperable.html');