AG-31638 Improve 'prevent-fetch' — add ability to set random response content. #416

Squashed commit of the following:

commit afcdf8d
Merge: b61d76f bd11dba
Author: Adam Wróblewski <[email protected]>
Date:   Tue Apr 22 08:07:44 2025 +0200

    Merge branch 'master' into feature/AG-31638

commit b61d76f
Merge: 23139ca 61825cc
Author: Slava Leleka <[email protected]>
Date:   Thu Apr 17 13:57:52 2025 -0400

    merge the parent branch into the current branch, resolve conflicts

commit 23139ca
Author: Adam Wróblewski <[email protected]>
Date:   Wed Apr 16 12:47:33 2025 +0200

    Improve 'prevent-fetch' — add ability to set random response content

Author: AdamWr

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog], and this project adheres to [Semantic
 
 ### Added
 
+- Ability to set random response content in `prevent-fetch` scriptlet [#416].
 - Ability to choose CSS injection method in `inject-css-in-shadow-dom` scriptlet [#477].
 - TypeScript types for CoreLibs provided [`ContentScriptApi`](./README.md#scriptlets-api--content-script-api).
 - Trusted Types API utility — [`PolicyApi`](./README.md#scriptlets-api--content-script-api--policy-api).
@@ -24,6 +25,7 @@ The format is based on [Keep a Changelog], and this project adheres to [Semantic
 
 [Unreleased]: https://github.com/AdguardTeam/Scriptlets/compare/v2.1.7...HEAD
 [#392]: https://github.com/AdguardTeam/Scriptlets/issues/392
+[#416]: https://github.com/AdguardTeam/Scriptlets/issues/416
 [#477]: https://github.com/AdguardTeam/Scriptlets/issues/477
 
 ## [v2.1.7] - 2025-04-03

src/helpers/noop-utils.ts

@@ -113,6 +113,9 @@ export const noopPromiseResolve = (
     }
 
     const response = new Response(responseBody, {
+        headers: {
+            'Content-Length': `${responseBody.length}`,
+        },
         status: 200,
         statusText: 'OK',
     });

src/scriptlets/prevent-fetch.js

@@ -15,6 +15,12 @@ import {
     parseMatchProps,
     isValidParsedData,
     getMatchPropsData,
+    generateRandomResponse,
+    nativeIsFinite,
+    nativeIsNaN,
+    getNumberFromString,
+    getRandomIntInclusive,
+    getRandomStrByLength,
 } from '../helpers';
 
 /* eslint-disable max-len */
@@ -46,6 +52,10 @@ import {
  *     - `emptyObj` — empty object
  *     - `emptyArr` — empty array
  *     - `emptyStr` — empty string
+ *     - `true` — random alphanumeric string of 10 symbols
+ *     - colon-separated pair `name:value` string value to customize `responseBody` where
+ *         - `name` — only `length` supported for now
+ *         - `value` — range on numbers, for example `100-300`, limited to 500000 characters
  * - `responseType` — optional, string for defining response type,
  *   original response type is used if not specified. Possible values:
  *     - `basic`
@@ -97,6 +107,12 @@ import {
  *
  *     ! Specify response body for all fetch calls
  *     example.org#%#//scriptlet('prevent-fetch', '', 'emptyArr')
+ *
+ *     ! Specify response body to random alphanumeric string of 10 symbols for all fetch calls
+ *     example.org#%#//scriptlet('prevent-fetch', '', 'true')
+ *
+ *     ! Specify response body to random alphanumeric string with specific range for all fetch calls
+ *     example.org#%#//scriptlet('prevent-fetch', '', 'length:100-300')
  *     ```
  *
  * 1. Prevent all fetch calls and specify response type value
@@ -128,6 +144,8 @@ export function preventFetch(source, propsToMatch, responseBody = 'emptyObj', re
         strResponseBody = '[]';
     } else if (responseBody === 'emptyStr') {
         strResponseBody = '';
+    } else if (responseBody === 'true' || responseBody.match(/^length:\d+-\d+$/)) {
+        strResponseBody = generateRandomResponse(responseBody);
     } else {
         logMessage(source, `Invalid responseBody parameter: '${responseBody}'`);
         return;
@@ -249,4 +267,10 @@ preventFetch.injections = [
     parseMatchProps,
     isValidParsedData,
     getMatchPropsData,
+    generateRandomResponse,
+    nativeIsFinite,
+    nativeIsNaN,
+    getNumberFromString,
+    getRandomIntInclusive,
+    getRandomStrByLength,
 ];

tests/scriptlets/prevent-fetch.test.js

@@ -294,6 +294,39 @@ if (!isSupported) {
         done();
     });
 
+    test('simple fetch - returns random string of 10 symbols', async (assert) => {
+        const INPUT_JSON_PATH_1 = `${FETCH_OBJECTS_PATH}/test01.json`;
+        const inputRequest1 = new Request(INPUT_JSON_PATH_1);
+
+        runScriptlet(name, ['test01', 'true']);
+        const done = assert.async();
+
+        const response = await fetch(inputRequest1);
+        const parsedData = await response.text();
+
+        assert.strictEqual(parsedData.length, 10, 'Response length is correct');
+        assert.strictEqual(window.hit, 'FIRED', 'hit function fired');
+        done();
+    });
+
+    test('simple fetch - returns random string with specific range', async (assert) => {
+        // blocked_request.json doesn't exist,
+        // it's required for test for blocked requests
+        const BLOCKED_REQUEST = `${FETCH_OBJECTS_PATH}/blocked_request.json`;
+
+        runScriptlet(name, ['blocked_request', 'length:50-100']);
+        const done = assert.async();
+
+        const response = await fetch(BLOCKED_REQUEST);
+        const parsedData = await response.text();
+        const headerLength = Number(response.headers.get('Content-Length'));
+
+        assert.ok(parsedData.length >= 50 && parsedData.length <= 100, 'Response length is correct');
+        assert.strictEqual(headerLength, parsedData.length, 'Header length matches response length');
+        assert.strictEqual(window.hit, 'FIRED', 'hit function fired');
+        done();
+    });
+
     test('simple fetch - valid response type', async (assert) => {
         const OPAQUE_RESPONSE_TYPE = 'opaque';
         const INPUT_JSON_PATH = `${FETCH_OBJECTS_PATH}/test01.json`;