CodeSampleX

Sample

react-dom 19.0.0: flushSync

Verified sample for npm react-dom 19.0.0: flushSync. The contract ran on node 22 · linux debian/x64 · docker and passed: flushSync is exported as a function…

sha256:ee9daa6e0f56b0b611eb07a2dff2d65ebe8bfb24350f1b5d19057bdc82b47230

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 linux 24 · ubuntu · glibc 2.39 x64 npm

Verification-run environments

Environment Contract Stages Run
node 22 · linux debian/x64 · docker ed25519:c1973797be207ac4 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1node:22@sha256:8a34c4ab3ea2…		 2026-09-06

Case

HOW
Goal
verify react-dom.flushSync in pkg:npm/react-dom@19.0.0
Packages
Symbols
  • react-dom.flushSync
Created
2026-09-06T22:10:44Z

Contract

  1. flushSync is exported as a function from the react-dom entrypoint
  2. flushSync executes the provided callback synchronously and returns its return value
  3. flushSync around root.render commits the root DOM synchronously before flushSync returns
  4. flushSync around state updates commits component state changes synchronously to the DOM
  5. flushSync re-throws exceptions thrown by its callback
  6. flushSync called without arguments returns undefined

Files

  • PROMPT.md
  • csx.json
  • index.js
  • package-lock.json
  • package.json
  • spec.json
  • test/contract.mjs

Download the source artifact (tar.gz)

Source

PROMPT.md
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.flushSync 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.flushSync

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.
csx.json
{"case":{"caseId":"case:sha256:a73a027a1c2b41fbb974c8546c38018612a501e6bb38b35d22b5df8ad7ef5b49","contract":["flushSync is exported as a function from the react-dom entrypoint","flushSync executes the provided callback synchronously and returns its return value","flushSync around root.render commits the root DOM synchronously before flushSync returns","flushSync around state updates commits component state changes synchronously to the DOM","flushSync re-throws exceptions thrown by its callback","flushSync called without arguments returns undefined"],"goal":"verify react-dom.flushSync in pkg:npm/react-dom@19.0.0","kind":"HOW","packages":["pkg:npm/react-dom@19.0.0"],"schemaVersion":1,"symbols":["react-dom.flushSync"]},"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.flushSync"],"verifierAdapter":"node-typescript@1"}
index.js
import { flushSync } from 'react-dom';

/**
 * Executes a callback synchronously within React's flushSync boundary,
 * forcing any pending updates to be committed to the DOM before returning.
 *
 * @template T
 * @param {() => T} [fn] - The function to execute synchronously.
 * @returns {T | undefined} The return value of the callback, or undefined if no callback is passed.
 */
export function runSync(fn) {
  return flushSync(fn);
}

export { flushSync };
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package.json
{
  "name": "sample-react-dom-flushsync",
  "version": "1.0.0",
  "description": "Clean-room sample verifying react-dom flushSync",
  "main": "index.js",
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "react": "19.0.0",
    "react-dom": "19.0.0"
  },
  "devDependencies": {
    "jsdom": "29.1.1"
  }
}
spec.json
{
  "schemaVersion": 1,
  "goal": "verify react-dom.flushSync in pkg:npm/react-dom@19.0.0",
  "kind": "HOW",
  "packages": [
    "pkg:npm/react-dom@19.0.0"
  ],
  "symbols": [
    "react-dom.flushSync"
  ]
}
test/contract.mjs
import assert from 'node:assert/strict';
import { JSDOM } from 'jsdom';
import React, { useState } from 'react';
import * as ReactDOM from 'react-dom';
import { flushSync } from 'react-dom';
import { createRoot } from 'react-dom/client';
import { runSync } from '../index.js';

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

// 2. flushSync executes the provided callback synchronously and returns its return value
const result = flushSync(() => 42);
assert.equal(result, 42, 'flushSync should return the value returned by its callback');

const helperResult = runSync(() => 'synced');
assert.equal(helperResult, 'synced', 'runSync should return callback return value');

// 3. flushSync around root.render commits the root DOM synchronously before flushSync returns
const dom = new JSDOM('<!doctype html><html><body><div id="app"></div></body></html>', {
  url: 'https://example.com/',
});
const priorGlobals = new Map();
for (const name of ['window', 'document']) {
  priorGlobals.set(name, Object.getOwnPropertyDescriptor(globalThis, name));
}
Object.defineProperty(globalThis, 'window', {
  configurable: true,
  writable: true,
  value: dom.window,
});
Object.defineProperty(globalThis, 'document', {
  configurable: true,
  writable: true,
  value: dom.window.document,
});

let root;
try {
  const container = document.getElementById('app');
  root = createRoot(container);

  // Without flushSync, render is deferred in concurrent React
  root.render(React.createElement('div', { id: 'status' }, 'deferred'));
  assert.equal(container.innerHTML, '', 'render without flushSync should not commit synchronously');

  // With flushSync, render is committed immediately
  flushSync(() => {
    root.render(React.createElement('div', { id: 'status' }, 'synchronous'));
  });
  assert.equal(container.innerHTML, '<div id="status">synchronous</div>', 'flushSync should commit DOM synchronously');

  // 4. flushSync around state updates commits component state changes synchronously to the DOM
  let setCounterState;
  function Counter() {
    const [count, setCount] = useState(0);
    setCounterState = setCount;
    return React.createElement('span', { id: 'count-val' }, `count:${count}`);
  }

  flushSync(() => {
    root.render(React.createElement(Counter));
  });
  assert.equal(container.innerHTML, '<span id="count-val">count:0</span>');

  // State update without flushSync does not commit synchronously
  setCounterState(1);
  assert.equal(container.innerHTML, '<span id="count-val">count:0</span>', 'state update without flushSync should be deferred');

  // State update wrapped in flushSync commits synchronously
  flushSync(() => {
    setCounterState(2);
  });
  assert.equal(container.innerHTML, '<span id="count-val">count:2</span>', 'state update with flushSync should commit synchronously');

  // 5. flushSync re-throws exceptions thrown by its callback
  assert.throws(
    () => {
      flushSync(() => {
        throw new Error('sync callback error');
      });
    },
    /sync callback error/
  );

  // 6. flushSync called without arguments returns undefined
  const noArgRet = flushSync();
  assert.equal(noArgRet, undefined, 'flushSync() without callback returns undefined');
} finally {
  if (root) {
    try {
      root.unmount();
    } catch {
      // ignore
    }
  }
  await new Promise((resolve) => setImmediate(resolve));
  dom.window.close();
  for (const [name, descriptor] of priorGlobals) {
    if (descriptor) {
      Object.defineProperty(globalThis, name, descriptor);
    } else {
      delete globalThis[name];
    }
  }
}

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

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