Node.js 21.x: What's New, Release History & EOL Status

What Is New in Node.js 21?

Stable WebSocket Client -- No More Undici Import

Node.js 21 ships a built-in WebSocket global that works without any flags or third-party packages. It follows the WhatWG WebSocket standard and uses Undici under the hood.

const ws = new WebSocket('wss://echo.example.com');
ws.addEventListener('open', () => ws.send('hello'));
ws.addEventListener('message', (e) => console.log(e.data));

This is significant for server-to-server communication, live data feeds, and any tooling that previously needed ws or undici's WebSocket implementation as a dependency.

New JavaScript Features via V8 11.8

Array grouping (Object.groupBy and Map.groupBy) ships natively -- a long-requested feature for grouping collections by a key without a reducer. Promise.withResolvers() simplifies creating manually-resolved promises.

// Object.groupBy
const items = [{type: 'a', v: 1}, {type: 'b', v: 2}, {type: 'a', v: 3}];
const grouped = Object.groupBy(items, (i) => i.type);
// { a: [...], b: [...] }

// Promise.withResolvers
const { promise, resolve, reject } = Promise.withResolvers();
setTimeout(() => resolve('done'), 1000);

Test Runner Improvements

The built-in test runner now supports --test-reporter and --test-reporter-destination flags. You can pipe test output to spec, tap, junit, or dot formatters and redirect each to a file or stdout independently.

This makes the built-in runner a more complete replacement for Jest/Mocha in CI pipelines that require structured test output (e.g., JUnit XML for GitHub Actions or Jenkins).

Web API Globals Expanded

navigator.hardwareConcurrency is now available in the global scope, matching browser behavior. Scripts that dynamically size worker pools based on CPU count no longer need to os.cpus().length workarounds when targeting both browser and Node environments.