react-dom 19.0.0: prefetchDNS

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: verify react-dom.prefetchDNS in pkg:npm/react-dom@19.0.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/react-dom@19.0.0
Demonstrate these symbols/APIs:
  - react-dom.prefetchDNS

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.

{"case":{"caseId":"case:sha256:bab6a2627e8d99d273d23b424418117e38f2efb62ec96a89b6b8ef252e27ad88","contract":["prefetchDNS is exported as a function from the react-dom entrypoint","prefetchDNS returns undefined and accepts a target origin string","calling prefetchDNS during server rendering emits a link rel=\"dns-prefetch\" element","subsequent prefetchDNS calls with the same URL are deduplicated in rendered output","prefetchDNS calls with different URLs emit distinct link elements for each origin","imperative invocation of prefetchDNS outside of render pass executes without error"],"goal":"verify react-dom.prefetchDNS in pkg:npm/react-dom@19.0.0","kind":"HOW","packages":["pkg:npm/react-dom@19.0.0"],"schemaVersion":1,"symbols":["react-dom.prefetchDNS"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"npm","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/react-dom@19.0.0"],"schemaVersion":1,"subject":"pkg:npm/react-dom@19.0.0","symbols":["react-dom.prefetchDNS"],"verifierAdapter":"node-typescript@1"}

import { prefetchDNS } from 'react-dom';

/**
 * Prefetches DNS for a given origin URL using ReactDOM.prefetchDNS.
 *
 * @param {string} href - The domain or origin URL to prefetch DNS for.
 * @returns {void}
 */
export function triggerPrefetchDNS(href) {
  return prefetchDNS(href);
}

export { prefetchDNS };

{
  "name": "sample-react-dom-prefetchdns",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-react-dom-prefetchdns",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "react": "19.0.0",
        "react-dom": "19.0.0"
      }
    },
    "node_modules/react": {
      "version": "19.0.0",
      "resolved": "https://registry.npmjs.org/react/-/react-19.0.0.tgz",
      "integrity": "sha512-V8AVnmPIICiWpGfm6GLzCR/W5FXLchHop40W4nXBmdlEceh16rCN8O8LNWm5bh5XUX91fh7KpA+W0TgMKmgTpQ==",
      "license": "MIT",
      "engines": {
        "node": ">=0.10.0"
      }
    },
    "node_modules/react-dom": {
      "version": "19.0.0",
      "resolved": "https://registry.npmjs.org/react-dom/-/react-dom-19.0.0.tgz",
      "integrity": "sha512-4GV5sHFG0e/0AD4X+ySy6UJd3jVl1iNsNHdpad0qhABJ11twS3TTBnseqsKurKcsNqCEFeGL3uLpVChpIO3QfQ==",
      "license": "MIT",
      "dependencies": {
        "scheduler": "^0.25.0"
      },
      "peerDependencies": {
        "react": "^19.0.0"
      }
    },
    "node_modules/scheduler": {
      "version": "0.25.0",
      "resolved": "https://registry.npmjs.org/scheduler/-/scheduler-0.25.0.tgz",
      "integrity": "sha512-xFVuu11jh+xcO7JOAGJNOXld8/TcEHK/4CituBUeUb5hqxJLj9YuemAEuvm9gQ/+pgXYfbQuqAkiYu+u7YEsNA==",
      "license": "MIT"
    }
  }
}

{
  "name": "sample-react-dom-prefetchdns",
  "version": "1.0.0",
  "description": "Clean-room sample verifying react-dom prefetchDNS",
  "main": "index.js",
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "react": "19.0.0",
    "react-dom": "19.0.0"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify react-dom.prefetchDNS in pkg:npm/react-dom@19.0.0",
  "kind": "HOW",
  "packages": [
    "pkg:npm/react-dom@19.0.0"
  ],
  "symbols": [
    "react-dom.prefetchDNS"
  ]
}

import assert from 'node:assert/strict';
import React from 'react';
import * as ReactDOM from 'react-dom';
import { prefetchDNS } from 'react-dom';
import ReactDOMServer from 'react-dom/server';
import { triggerPrefetchDNS } from '../index.js';

// 1. prefetchDNS is exported as a function from the react-dom entrypoint
assert.equal(typeof ReactDOM.prefetchDNS, 'function', 'ReactDOM.prefetchDNS should be a function');
assert.equal(typeof prefetchDNS, 'function', 'prefetchDNS should be a function');
assert.equal(typeof triggerPrefetchDNS, 'function', 'triggerPrefetchDNS should be a function');

// 2. prefetchDNS returns undefined and accepts a target origin string
const ret = prefetchDNS('https://example.com');
assert.equal(ret, undefined, 'prefetchDNS must return undefined');
const helperRet = triggerPrefetchDNS('https://example.org');
assert.equal(helperRet, undefined, 'triggerPrefetchDNS helper must return undefined');

// 3. calling prefetchDNS during server rendering emits a link rel="dns-prefetch" element
function PageWithPrefetch() {
  prefetchDNS('https://api.example.com');
  return React.createElement('div', { id: 'app' }, 'hello world');
}

const html = ReactDOMServer.renderToString(React.createElement(PageWithPrefetch));
assert.match(
  html,
  /<link href="https:\/\/api\.example\.com" rel="dns-prefetch"\/>/,
  'renderToString should emit dns-prefetch link'
);
assert.ok(html.includes('<div id="app">hello world</div>'), 'rendered HTML should include component content');

// 4. subsequent prefetchDNS calls with the same URL are deduplicated in rendered output
function PageWithDuplicatePrefetch() {
  prefetchDNS('https://cdn.example.com');
  prefetchDNS('https://cdn.example.com');
  prefetchDNS('https://cdn.example.com');
  return React.createElement('div', null, 'content');
}

const htmlDup = ReactDOMServer.renderToString(React.createElement(PageWithDuplicatePrefetch));
const matches = htmlDup.match(/<link href="https:\/\/cdn\.example\.com" rel="dns-prefetch"\/>/g);
assert.equal(matches?.length, 1, 'identical prefetchDNS calls should be deduplicated to a single link element');

// 5. prefetchDNS calls with different URLs emit distinct link elements for each origin
function PageWithMultiplePrefetches() {
  prefetchDNS('https://cdn1.example.com');
  prefetchDNS('https://cdn2.example.com');
  return React.createElement('div', null, 'multi');
}

const htmlMulti = ReactDOMServer.renderToString(React.createElement(PageWithMultiplePrefetches));
assert.ok(htmlMulti.includes('<link href="https://cdn1.example.com" rel="dns-prefetch"/>'), 'should include first prefetch link');
assert.ok(htmlMulti.includes('<link href="https://cdn2.example.com" rel="dns-prefetch"/>'), 'should include second prefetch link');

// 6. imperative invocation of prefetchDNS outside of render pass executes without error
assert.doesNotThrow(() => {
  prefetchDNS('https://offline.example.com');
}, 'prefetchDNS outside of render pass executes without error');

console.log('CONTRACT PASS: react-dom.prefetchDNS verified');