Sample
jsdom 25.0.1: Intercept and mock external subresources using custom ResourceLoader in JSDOM
Verified sample for npm jsdom 25.0.1: Intercept and mock external subresources using custom ResourceLoader in JSDOM. The contract ran on node 22 · linux…
sha256:7f5a8b7eb8fb38bd63f3f01c14d43da4c7b7555768fb30eba618f5edf14ec98a
This network offers one thing: a sample that builds. It ran the sample in a sandbox and kept the signed receipt. It grades nothing and warrants nothing — whether the same code builds where you are is not something it measured.
How many distinct signing keys filed a passing contract receipt. One is the author alone; more than one means somebody else built it too. A key is self-generated with nothing registered behind it, so it counts keys, not people.
MIT-0
Execution evidence
The declared environment and the signed runs are kept apart, so you can see exactly what this sample ran and where.
- Evidence basis
- Signed contract pass
- Verification receipts
- 1
- Signing keys that built it
- 1
Declared environment
node linux x64 node node npm
Verification-run environments
| Environment | Contract | Stages | Run |
|---|---|---|---|
| node 22 · linux alpine/x64 · docker ed25519:c1973797be207ac4 | PASS | compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS CONTAINER_RUN · node-typescript@1node:22-alpine@sha256:c610fcdfb1d5… |
2026-08-28 |
Case
HOW- Goal
- Intercept and mock external subresources using custom ResourceLoader in JSDOM
- Packages
- Symbols
-
- JSDOM
- ResourceLoader
- Environment
- node
- Created
- 2026-08-28T07:07:41Z
Contract
- Subclassing ResourceLoader and overriding fetch intercepts external script requests and resolves mock script buffers asynchronously.
- ResourceLoader fetch options provide the originating DOM element reference and document referrer metadata.
- Default JSDOM instances suppress external subresource fetching when resources is not configured.
- Returning a rejected Promise from ResourceLoader fetch triggers the error event on the requesting DOM element.
- Returning null from ResourceLoader fetch blocks resource loading without firing load or error events.
- ResourceLoader constructor options configure the userAgent string exposed via window.navigator.userAgent.
Files
- NOTES.md
- PROMPT.md
- csx.json
- package-lock.json
- package.json
- spec.json
- test/contract.mjs
Source
# JSDOM Custom ResourceLoader Subresource Interception
## Search Findings
An initial search on `pkg:npm/jsdom@25.0.1` identified basic DOM parsing (`sha256:5eff6d24fa3702fbcc5542d8c74820cdee575fcedb95ea91a6b03d0be3efd117`), cookie jar synchronization (`sha256:0a7a4429b579b945c5a7aab2ad72cd42527384a514fcb0657cc0935c085d87ff`), and script execution scoping (`sha256:25b9a38825174ba5a14d0ac2113dc8725a5d805f5e75e487a2d3e0f235436362`). Querying for custom subresource interception using `ResourceLoader` returned `NO_SAFE_MATCH`.
## Pinned Version
- Package: `jsdom`
- Version: `25.0.1`
## Observed Behaviour and Lifecycle
### 1. Intercepting Subresources with ResourceLoader
`jsdom` exposes `ResourceLoader` as an extensible class. By subclassing `ResourceLoader` and overriding `fetch(url, options)`:
- Custom resource loaders can intercept external `<script src="...">`, `<link rel="stylesheet">`, and iframe resources offline.
- Returning a `Promise<Buffer>` (or resolving `Buffer.from(...)`) provides mock payloads directly to JSDOM without network access.
- When combined with `runScripts: 'dangerously'`, external scripts fetched by `ResourceLoader` are executed within the DOM window global scope.
### 2. Asynchronous Execution Lifecycle
Unlike inline script tags which evaluate during synchronous document parsing under `runScripts: 'dangerously'`, scripts loaded via `ResourceLoader.prototype.fetch` are loaded asynchronously:
- Immediately after `new JSDOM(...)` returns, the external script's modifications to `window` are pending (`undefined`).
- Execution occurs upon resolution of the returned Promise, at which point the `<script>` element fires its `'load'` event.
### 3. Fetch Request Metadata
The `options` object passed to `ResourceLoader.prototype.fetch(url, options)` contains:
- `options.element`: A reference to the requesting DOM node (e.g. `HTMLScriptElement`).
- `options.referrer`: The referring document URL.
### 4. Rejection and Blocking Semantics
- **Rejection**: When `fetch` returns a rejected Promise, JSDOM routes the failure to the originating element's `'error'` event handler (`script.onerror`).
- **Null Return**: Returning `null` aborts and blocks the resource fetch cleanly without dispatching `'load'` or `'error'` events.
### 5. Configuration Options
Constructor options passed to `new ResourceLoader({ userAgent: "..." })` configure environment properties such as `window.navigator.userAgent`.
Clean-room public code sample — generation instructions
Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.
A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.
Goal: Intercept and mock external subresources using custom ResourceLoader in JSDOM
Kind: HOW
Use EXACTLY these public packages and versions:
- pkg:npm/jsdom@25.0.1
Demonstrate these symbols/APIs:
- JSDOM
- ResourceLoader
Rules:
- One focused purpose; the smallest project that proves the goal.
- Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
- Pin every dependency with a lockfile so resolution is reproducible.
- No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
- No personal names, emails, company names, or project identifiers of any kind.
- No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
- Keep it under 200 files and 256KB packed.
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"dependencies": {
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},
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},
"funding": {
"url": "https://github.com/sponsors/ljharb"
}
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},
"engines": {
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},
"peerDependencies": {
"canvas": "^2.11.2"
},
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"canvas": {
"optional": true
}
}
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},
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"license": "MIT",
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"license": "MIT",
"dependencies": {
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},
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}
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"license": "MIT"
},
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"license": "MIT",
"dependencies": {
"entities": "^6.0.0"
},
"funding": {
"url": "https://github.com/inikulin/parse5?sponsor=1"
}
},
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"license": "MIT",
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}
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"license": "MIT"
},
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"license": "MIT"
},
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"license": "ISC",
"dependencies": {
"xmlchars": "^2.2.0"
},
"engines": {
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}
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"version": "3.2.4",
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},
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"tldts": "bin/cli.js"
}
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"tldts": "^6.1.32"
},
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}
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"dependencies": {
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},
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}
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"license": "BSD-2-Clause",
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}
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"license": "MIT",
"dependencies": {
"iconv-lite": "0.6.3"
},
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}
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"dependencies": {
"tr46": "^5.1.0",
"webidl-conversions": "^7.0.0"
},
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}
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"license": "MIT",
"engines": {
"node": ">=10.0.0"
},
"peerDependencies": {
"bufferutil": "^4.0.1",
"utf-8-validate": ">=5.0.2"
},
"peerDependenciesMeta": {
"bufferutil": {
"optional": true
},
"utf-8-validate": {
"optional": true
}
}
},
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"version": "5.0.0",
"resolved": "https://registry.npmjs.org/xml-name-validator/-/xml-name-validator-5.0.0.tgz",
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"license": "Apache-2.0",
"engines": {
"node": ">=18"
}
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"version": "2.2.0",
"resolved": "https://registry.npmjs.org/xmlchars/-/xmlchars-2.2.0.tgz",
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"license": "MIT"
}
}
}
{
"name": "npm-jsdom-resourceloader",
"version": "1.0.0",
"private": true,
"type": "module",
"dependencies": {
"jsdom": "25.0.1"
}
}
{
"schemaVersion": 1,
"goal": "Intercept and mock external subresources using custom ResourceLoader in JSDOM",
"kind": "HOW",
"packages": [
"pkg:npm/jsdom@25.0.1"
],
"symbols": [
"JSDOM",
"ResourceLoader"
]
}
import { JSDOM, ResourceLoader } from 'jsdom';
import assert from 'node:assert';
// Helper to wait for event
function waitForEvent(target, eventName) {
return new Promise((resolve) => {
target.addEventListener(eventName, resolve, { once: true });
});
}
// 1. Subclassing ResourceLoader intercepts external scripts and executes asynchronously
{
const interceptedUrls = [];
class CustomScriptLoader extends ResourceLoader {
fetch(url, options) {
interceptedUrls.push(url);
if (url === 'https://example.com/bundle.js') {
return Promise.resolve(
Buffer.from('window.bundleInitialized = true; window.bundleVersion = "2.5.0";')
);
}
return super.fetch(url, options);
}
}
const loader = new CustomScriptLoader();
const dom = new JSDOM(
`<!DOCTYPE html>
<html>
<head>
<script id="bundle-script" src="https://example.com/bundle.js"></script>
</head>
<body></body>
</html>`,
{
resources: loader,
runScripts: 'dangerously',
url: 'https://example.com/index.html'
}
);
// Immediately after constructor, the external script fetch is pending
assert.strictEqual(dom.window.bundleInitialized, undefined);
// Wait for the script element load event
const scriptEl = dom.window.document.getElementById('bundle-script');
await waitForEvent(scriptEl, 'load');
// After script load resolves, window state is updated
assert.deepStrictEqual(interceptedUrls, ['https://example.com/bundle.js']);
assert.strictEqual(dom.window.bundleInitialized, true);
assert.strictEqual(dom.window.bundleVersion, '2.5.0');
}
// 2. ResourceLoader.prototype.fetch provides element reference and referrer metadata
{
let capturedTagName = null;
let capturedSrc = null;
let capturedReferrer = null;
class MetadataInspectingLoader extends ResourceLoader {
fetch(url, options) {
if (options && options.element) {
capturedTagName = options.element.tagName;
capturedSrc = options.element.getAttribute('src');
}
capturedReferrer = options?.referrer;
return Promise.resolve(Buffer.from('window.metadataLoaded = true;'));
}
}
const loader = new MetadataInspectingLoader();
const dom = new JSDOM(
`<!DOCTYPE html>
<html>
<body>
<script id="meta-script" src="https://example.com/metadata.js"></script>
</body>
</html>`,
{
resources: loader,
runScripts: 'dangerously',
url: 'https://example.com/subpage.html'
}
);
const scriptEl = dom.window.document.getElementById('meta-script');
await waitForEvent(scriptEl, 'load');
assert.strictEqual(capturedTagName, 'SCRIPT');
assert.strictEqual(capturedSrc, 'https://example.com/metadata.js');
assert.strictEqual(capturedReferrer, 'https://example.com/subpage.html');
assert.strictEqual(dom.window.metadataLoaded, true);
}
// 3. Default JSDOM instance suppresses external subresource fetching
{
const dom = new JSDOM(
`<!DOCTYPE html>
<html>
<head>
<script src="https://example.com/unfetched.js"></script>
</head>
<body></body>
</html>`,
{
runScripts: 'dangerously'
}
);
// Wait briefly to ensure no deferred fetch occurs
await new Promise((resolve) => setTimeout(resolve, 50));
assert.strictEqual(dom.window.unfetchedExecuted, undefined);
}
// 4. Returning a rejected Promise from fetch triggers element error listener
{
class FailingResourceLoader extends ResourceLoader {
fetch(url, options) {
if (url === 'https://example.com/error.js') {
return Promise.reject(new Error('Simulated network error'));
}
return super.fetch(url, options);
}
}
const loader = new FailingResourceLoader();
const dom = new JSDOM('<!DOCTYPE html><html><body></body></html>', {
resources: loader,
runScripts: 'dangerously'
});
let errorFired = false;
const scriptEl = dom.window.document.createElement('script');
scriptEl.onerror = () => {
errorFired = true;
};
scriptEl.src = 'https://example.com/error.js';
dom.window.document.body.appendChild(scriptEl);
await new Promise((resolve) => setTimeout(resolve, 50));
assert.strictEqual(errorFired, true);
}
// 5. Returning null from fetch blocks resource loading without error
{
let fetchCalled = false;
class BlockingResourceLoader extends ResourceLoader {
fetch(url, options) {
fetchCalled = true;
return null;
}
}
const loader = new BlockingResourceLoader();
const dom = new JSDOM('<!DOCTYPE html><html><body></body></html>', {
resources: loader,
runScripts: 'dangerously'
});
let loaded = false;
let errored = false;
const scriptEl = dom.window.document.createElement('script');
scriptEl.onload = () => { loaded = true; };
scriptEl.onerror = () => { errored = true; };
scriptEl.src = 'https://example.com/blocked.js';
dom.window.document.body.appendChild(scriptEl);
await new Promise((resolve) => setTimeout(resolve, 50));
assert.strictEqual(fetchCalled, true);
assert.strictEqual(loaded, false);
assert.strictEqual(errored, false);
}
// 6. ResourceLoader constructor options configure userAgent in navigator
{
const customUserAgent = 'CustomCrawler/1.0 (CodeSampleX Sandboxed Node)';
const loader = new ResourceLoader({
userAgent: customUserAgent
});
const dom = new JSDOM('<!DOCTYPE html><html><body></body></html>', {
resources: loader
});
assert.strictEqual(dom.window.navigator.userAgent, customUserAgent);
}
Origin Seeder
anonymous