unithread

True multithreading for JavaScript with one API across Node and the browser. Wraps node:worker_threads and Web Workers behind a single surface: function shipping, worker pools, RPC services, and shared memory via SharedArrayBuffer + Atomics.

import { runInThread, Task, WorkerPool, SharedCounter, Mutex, Signal } from "@jasonkneen/unithread";

// One-shot: run a function on a real OS thread
const n = await runInThread((x) => {
  const fib = (i) => (i < 2 ? i : fib(i - 1) + fib(i - 2));
  return fib(x);
}, 35);

// Persistent thread, call repeatedly
const task = await Task.spawn((a, b) => a + b);
await task.run([2, 3]); // 5
await task.terminate();

// RPC service: object of named methods on one thread
const svc = await Task.spawnService({
  square: (x) => x * x,
  hash: (s) => { /* self-contained work */ },
});
await svc.call("square", [12]); // 144

// Pool sized to hardware cores, order-preserving parallel map
const pool = await WorkerPool.create((n) => heavyWork(n));
const results = await pool.map(inputs, (n) => [n]);
await pool.close();

// True shared memory across threads
const counter = new SharedCounter(0);       // Atomics-backed
const mutex   = new Mutex();                // lock() / lockAsync() / withLock()
const signal  = new Signal();               // wait() / waitAsync() / fire()
await runInThread((buf) => {
  new Int32Array(buf); Atomics.add(new Int32Array(buf), 0, 1);
}, counter.buffer);

• Node: new Worker(source, { eval: true }) from node:worker_threads.
• Browser: new Worker(URL.createObjectURL(new Blob([source]))).
• One env-detecting bootstrap script runs in both worker types and speaks a small id-correlated message protocol (call / method / result / error), so Task, WorkerPool, and RPC behave identically in either runtime.

• Shipped functions must be self-contained. They cross the thread boundary via Function.prototype.toString() — no closure captures, no imports from outer scope. Pass data as arguments; use SharedArrayBuffer for shared state.
• Browser SharedArrayBuffer requires cross-origin isolation (COOP/COEP headers). Atomics.wait is forbidden on the browser main thread — use lockAsync() / waitAsync() there. Node has no such restriction.
• Everything non-shared is copied (structured clone). Pass ArrayBuffers in the transfer list to move instead of copy.
• Every function shipped via runInThread, Task.spawn, or WorkerPool.create receives one extra trailing argument: a WorkerEnv ({ isMainThread, threadId, runtime, emit, transfer }). Task.spawnService methods are the one exception — they're invoked as plain methods (task.call(name, args)) with no env appended, so reach for globalThis.__unithread inside a service method instead.

Node / bundlers — npm install @jasonkneen/unithread, then import { ... } from "@jasonkneen/unithread".

Browser, zero build step — copy dist/unithread.bundle.min.js (8.5 kB) next to your page:

<script type="module">
  import { runInThread, WorkerPool, spawnRemote } from "./unithread.bundle.min.js";
  const svc = await spawnRemote({ square: (x) => x * x });
  console.log(await svc.square(12)); // 144 — computed on a real thread
  await svc.terminate();
</script>

For SharedArrayBuffer in the browser, serve with COOP/COEP (see demo/serve.mjs for a 30-line reference server):

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Everything except shared memory (spawn, pools, RPC proxies, transfers) works without those headers.

Node 24 — 21/21 (npm test) and real Chromium via Playwright — 11/11 (npm run test:browser), including the proofs that are impossible without independent OS threads:

• main thread timers kept firing (~42 ticks) during a 400 ms synchronous worker spin — both runtimes
• Node main thread blocked on Atomics.wait, released by a worker's notify; browser main thread same via waitAsync
• 4 threads × 50 k atomic increments = exactly 200 000 on one SharedArrayBuffer — both runtimes
• mutex-serialized non-atomic read-modify-write exact across 4 threads — both runtimes
• zero-copy ArrayBuffer transfer worker→main — both runtimes
• browser suite ran cross-origin-isolated (crossOriginIsolated === true)

1. npm tarball: npm i ./unithread-0.2.0 — full package with types.
2. Single file: copy dist/unithread.bundle.js (13 kB, zero deps) or .min.js (8.5 kB) into your project; import { ... } from "./unithread.bundle.js" works in Node and browsers unchanged.
3. Browser check: npm run demo → http://localhost:8080. Serves with COOP/COEP already set, so the SharedArrayBuffer proofs run rather than skip.

• spawnRemote(methods) / wrap(task) — Comlink-style proxy: await svc.pow(2, 10).
• Transferables both directions: pass a transfer list to run/call; return via env.transfer(value, [buffers]). Moved, not copied — source detaches.
• Streaming: worker calls env.emit(event, data) mid-task; main thread subscribes with task.onEvent(cb). Also on globalThis.__unithread inside service methods.

skill/ packages unithread as a Claude Code skill: an agent can audit a codebase for main-thread hogs, offload them to threads, and prove the main thread was freed.

ln -s "$PWD/skill" ~/.claude/skills/unithread

Then ask an agent to "make this app stop janking". It will measure candidates, refuse to offload anything under ~16 ms, verify the shipped function captures nothing, and report a before/after — reverting if the number did not move.

npm install && npm run build   # tsc -> dist/, then esbuild -> dist/unithread.bundle[.min].js
npm test                       # node suite      — 21/21
npm run test:browser           # chromium suite  — 11/11 (needs: npx playwright install chromium)
npm run test:skill             # skill suite     — 28/28 (portability + measurement gates)
npm run demo                   # http://localhost:8080, served with COOP/COEP

npm run demo opens a live page with one switch on it. The same workload (fib(34) on a loop, ~110 ms a job) runs non-stop; the switch moves it between the main thread and a pool of worker threads, and a frame-time trace shows the result immediately:

Measured on a 16-core M-series in Chromium. The frame trace goes from a solid wall of red (every frame missing the 16.7 ms budget) to solid green the instant you flip it.