emsdk/upstream/emscripten/src/build_as_worker.js

var workerResponded = false, workerCallbackId = -1;

(() => {
  var messageBuffer = null, buffer = 0;

  function flushMessages() {
    if (!messageBuffer) return;
    if (runtimeInitialized) {
      var temp = messageBuffer;
      messageBuffer = null;
      temp.forEach((message) => onmessage(message));
    }
  }

  function messageResender() {
    flushMessages();
    if (messageBuffer) {
      setTimeout(messageResender, 100); // still more to do
    }
  }

  onmessage = (msg) => {
    // if main has not yet been called (mem init file, other async things), buffer messages
    if (!runtimeInitialized) {
      if (!messageBuffer) {
        messageBuffer = [];
        setTimeout(messageResender, 100);
      }
      messageBuffer.push(msg);
      return;
    }
    flushMessages();

    var func = Module['_' + msg.data['funcName']];
    if (!func) abort('invalid worker function to call: ' + msg.data['funcName']);
    var data = msg.data['data'];
    if (data) {
      if (!data.byteLength) data = new Uint8Array(data);
      buffer = _realloc(buffer, data.length);
      HEAPU8.set(data, buffer);
    }

    workerResponded = false;
    workerCallbackId = msg.data['callbackId'];
    if (data) {
      func(buffer, data.length);
    } else {
      func(0, 0);
    }
  }
})();