AG-20096 Improve 'prevent-addEventListener' — save reference to 'toString()' method. #292

Squashed commit of the following:

commit 9cc48ae
Author: Adam Wróblewski <adam@adguard.com>
Date:   Tue Jun 9 15:09:24 2026 +0200

    Fix changelog
    Lint error

commit 3c3bd27
Author: Adam Wróblewski <adam@adguard.com>
Date:   Tue Jun 9 14:53:26 2026 +0200

    Improve 'prevent-addEventListener' — save reference to 'toString()' method

Author: AdamWr

CHANGELOG.md

@@ -20,6 +20,8 @@ The format is based on [Keep a Changelog], and this project adheres to [Semantic
 
 ### Changed
 
+- `listenerToString` helper now uses native `Function.prototype.toString`
+  to prevent issues where websites redefine it [#292].
 - `prevent-fetch` now supports a structured `responseConfig` argument for overriding synthetic response fields such as
   `ok`, `redirected`, `status`, `statusText`, and extended `type` values [#529].
 - Redirect resources with a `.ts` extension are now converted to `.js`.
@@ -34,6 +36,7 @@ The format is based on [Keep a Changelog], and this project adheres to [Semantic
 [#560]: https://github.com/AdguardTeam/Scriptlets/issues/560
 [#529]: https://github.com/AdguardTeam/Scriptlets/issues/529
 [#467]: https://github.com/AdguardTeam/Scriptlets/issues/467
+[#292]: https://github.com/AdguardTeam/Scriptlets/issues/292
 
 ## [2.4.2] - 2026-04-24
 

src/helpers/add-event-listener-utils.ts

@@ -33,10 +33,17 @@ export const validateListener = (listener: unknown): boolean => {
  * https://developer.mozilla.org/en-US/docs/Web/API/EventListener
  *
  * @param listener valid listener
+ * @param nativeToString native Function.prototype.toString method
  * @returns listener string
  */
-export const listenerToString = (listener: EventListener | EventListenerObject): string => {
+export const listenerToString = (
+    listener: EventListener | EventListenerObject,
+    nativeToString: typeof Function.prototype.toString,
+): string => {
     return typeof listener === 'function'
-        ? listener.toString()
-        : listener.handleEvent.toString();
+        // Using native toString() is required to fix issues
+        // where websites redefine Function.prototype.toString
+        // https://github.com/AdguardTeam/Scriptlets/issues/292
+        ? nativeToString.call(listener)
+        : nativeToString.call(listener.handleEvent);
 };

src/scriptlets/log-addEventListener.ts

@@ -36,6 +36,7 @@ import { type Source } from './scriptlets';
  */
 export function logAddEventListener(source: Source, noProtect?: string) {
     const nativeAddEventListener = window.EventTarget.prototype.addEventListener;
+    const nativeToString = Function.prototype.toString;
 
     function addEventListenerWrapper(
         this: EventTarget | null | undefined,
@@ -46,7 +47,7 @@ export function logAddEventListener(source: Source, noProtect?: string) {
         if (validateType(type) && validateListener(listener)) {
             let targetElement: Element | undefined;
             let targetElementInfo: string | undefined;
-            const listenerInfo = listenerToString(listener as EventListener | EventListenerObject);
+            const listenerInfo = listenerToString(listener as EventListener | EventListenerObject, nativeToString);
 
             if (this) {
                 if (this instanceof Window) {

src/scriptlets/prevent-addEventListener.js

@@ -152,12 +152,13 @@ export function preventAddEventListener(
     };
 
     const nativeAddEventListener = window.EventTarget.prototype.addEventListener;
+    const nativeToString = Function.prototype.toString;
 
     function addEventListenerWrapper(type, listener, ...args) {
         let shouldPrevent = false;
         if (validateType(type) && validateListener(listener)) {
             shouldPrevent = typeSearchRegexp.test(type.toString())
-                && listenerSearchRegexp.test(listenerToString(listener))
+                && listenerSearchRegexp.test(listenerToString(listener, nativeToString))
                 && elementMatches(this);
         }
 

tests/scriptlets/prevent-addEventListener.test.js

@@ -5,6 +5,7 @@ const { test, module } = QUnit;
 const name = 'prevent-addEventListener';
 
 const nativeDescriptor = Object.getOwnPropertyDescriptor(window.EventTarget.prototype, 'addEventListener');
+const nativeToString = Function.prototype.toString;
 
 const beforeEach = () => {
     window.__debug = () => {
@@ -17,6 +18,8 @@ const afterEach = () => {
     Object.defineProperty(window.EventTarget.prototype, 'addEventListener', nativeDescriptor);
     Object.defineProperty(window, 'addEventListener', nativeDescriptor);
     Object.defineProperty(document, 'addEventListener', nativeDescriptor);
+    // eslint-disable-next-line no-extend-native
+    Function.prototype.toString = nativeToString;
 };
 
 module(name, { beforeEach, afterEach });
@@ -92,7 +95,7 @@ test('should not throw error when event type is null', (assert) => {
     // This should not throw an error
     assert.expect(2);
     try {
-        document.addEventListener(null, () => {});
+        document.addEventListener(null, () => { });
         window[testProp] = 'end';
     } catch (e) {
         assert.ok(false, `Should not throw error: ${e.message}`);
@@ -119,6 +122,30 @@ test('does not allow to add event listener', (assert) => {
     clearGlobalProps(testProp);
 });
 
+test('does not allow to add event listener - native toString mocked', (assert) => {
+    const scriptletArgs = ['click', 'clicked'];
+    runScriptlet(name, scriptletArgs);
+
+    const nativeToString = Function.prototype.toString;
+    // eslint-disable-next-line no-extend-native
+    Function.prototype.toString = function mockToString() {
+        return nativeToString.call(this).includes('clicked')
+            ? ''
+            : nativeToString.call(this);
+    };
+
+    const testProp = 'testProp';
+    const element = document.createElement('div');
+    element.addEventListener('click', () => {
+        window[testProp] = 'clicked';
+    });
+    element.click();
+
+    assert.strictEqual(window.hit, 'FIRED', 'hit function fired');
+    assert.strictEqual(window[testProp], undefined, 'property should be undefined');
+    clearGlobalProps(testProp);
+});
+
 test('event listeners not corresponding to scriptlet arguments should be added correctly', (assert) => {
     const scriptletArgs = ['click', undefined];
     runScriptlet(name, scriptletArgs);
@@ -384,7 +411,7 @@ test('noProtect parameter allows subsequent override of addEventListener', (asse
     };
 
     const element2 = document.createElement('div');
-    element2.addEventListener('click', () => {});
+    element2.addEventListener('click', () => { });
 
     assert.strictEqual(overrideWorked, true, 'addEventListener should be overridable with noProtect');
     clearGlobalProps(testProp);