Originally Published: 2019-04-30

If you’re looking to implement Web Workers with a Worker written in CoffeeScript, you may run into the snag that CoffeeScript will compile your worker with a wrapper/closure that looks like (function() {}).call(this);, which prevents your Worker’s onmessage function from properly registering. Have no fear, you don’t need to pass --bare to your CoffeeScript compiler to fix this! If you just add the line self.addEventListener('message', onmessage, false), your onmessage function will be registered and correctly handle postMessage() invocations from the parent. Here’s a small example with the compiled JS output:


  console.log('worker loaded')

  onmessage = (e) ->
    console.log('worker received message')

  self.addEventListener('message', onmessage, false)

Compiled output worker.js:

  (function() {
    var onmessage;

    console.log('worker loaded');

    onmessage = function(e) {
      console.log('worker received message');
      return console.log(e.data);

    self.addEventListener('message', onmessage, false);