The strong open source chess engine Stockfish compiled to JavaScript and WebAssembly using Emscripten. See it in action for local computer analysis on lichess.org.

About 1.4MB uncompressed, 250 KB gzipped.

Install Emscripten and uglifyjs, then:

./build.sh

Or using Docker:

docker run --user $(id -u):$(id -g) --volume $(pwd):/home/builder/stockfish.js:rw niklasf/emscripten-for-stockfish

var wasmSupported = typeof WebAssembly === 'object' && WebAssembly.validate(Uint8Array.of(0x0, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00));

var stockfish = new Worker(wasmSupported ? 'stockfish.wasm.js' : 'stockfish.js');

stockfish.addEventListener('message', function (e) {
  console.log(e.data);
});

stockfish.postMessage('uci');

• Expose as web worker.
• Web workers are inherently single threaded. Limit to one thread.
• Break down main iterative deepening loop to allow interrupting search.
• Limit total memory to 32 MB.
• Disable Syzygy tablebases.
• Disable benchmark.

Thanks to @nmrugg for doing the same thing with Stockfish 6, to @ddugovic for his multi-variant Stockfish fork and to the Stockfish team for ... Stockfish.