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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Superceded by Stockfish with NNUE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

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.