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WebAssembly port of the strong chess engine Stockfish. See it in action on

npm version CI Passively maintained

Superceded by Stockfish with NNUE

This port is branched from SF_classical. See for a WebAssembly port with NNUE support.


Uses the latest WebAssembly threading proposal. Requires these HTTP headers on the top level response:

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

And the following header on the included files:

Cross-Origin-Embedder-Policy: require-corp

If headers are not configured correctly on the top level response, wasmThreadsSupported() as defined below will return false. If headers are not configured correctly on the included files, something like pthread sent an error! undefined:undefined: undefined may be logged to the console.

Chromium based (desktop only)

  • Since Chromium 79: Full support.
  • Chromium 74: Supports treading, but cannot allocate additional memory. The default allocation suffices for up to 2 threads and 16 MB hash.
  • Chromium 70 to 73: Needs flag chrome://flags/#enable-webassembly-threads or Origin Trial.


  • Since Firefox 79: Full support.
  • Firefox 72 to 78: Structured cloning can no longer be enabled with flags, except on nightlies.
  • Firefox 71: Requires javascript.options.shared_memory and dom.postMessage.sharedArrayBuffer.withCOOP_COEP to be enabled in about:flags.
  • Firefox 68 to 70: Requires javascript.options.shared_memory to be enabled in about:flags

Other browsers

No support.

See stockfish.js for a more portable but single-threaded version.

Feature detection

function wasmThreadsSupported() {
  // WebAssembly 1.0
  const source = Uint8Array.of(0x0, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00);
  if (
    typeof WebAssembly !== "object" ||
    typeof WebAssembly.validate !== "function"
    return false;
  if (!WebAssembly.validate(source)) return false;

  // SharedArrayBuffer
  if (typeof SharedArrayBuffer !== "function") return false;

  // Atomics
  if (typeof Atomics !== "object") return false;

  // Shared memory
  const mem = new WebAssembly.Memory({ shared: true, initial: 8, maximum: 16 });
  if (!(mem.buffer instanceof SharedArrayBuffer)) return false;

  // Structured cloning
  try {
    // You have to make sure nobody cares about these messages!
    window.postMessage(mem, "*");
  } catch (e) {
    return false;

  // Growable shared memory (optional)
  try {
  } catch (e) {
    return false;

  return true;

Current limitations

  • Hashtable: 1024 MB. You may want to check navigator.deviceMemory before allocating.
  • Threads: 32. You may want to check navigator.hardwareConcurrency. May be capped lower (e.g., dom.workers.maxPerDomain in Firefox).
  • Can hang when UCI protocol is misused. (Do not send invalid commands or positions. While the engine is searching, do not change options or start additional searches).
  • No NNUE support.
  • No Syzygy tablebase support.


Assuming em++ (^2.0.13) is available:

npm run-script prepare


Requires stockfish.js, stockfish.wasm and stockfish.worker.js (total size ~400K, ~150K gzipped) to be served from the same directory.

<script src="stockfish.js"></script>
  Stockfish().then((sf) => {
    sf.addMessageListener((line) => {


Or from recent node (v14.4.0 tested) with flags --experimental-wasm-threads --experimental-wasm-bulk-memory:

const Stockfish = require("stockfish.wasm"); // the module, not the file

Stockfish().then((sf) => {
  // ...


Thanks to the Stockfish team for sharing the engine under the GPL3.