Here’s what the API docs say:
In a web browser, Stockfish.js can be run in a web-worker, which can be created like this:
var stockfish = new Worker("stockfish.js");
The output of the engine is again posted as a message. To receive it, you need to add a message handler:
stockfish.onmessage = function onmessage(event) { console.log(event.data); };
Input (standard UCI commands) to the engine is posted as a message to the worker:
stockfish.postMessage("go depth 15");