Create a Web Worker on the fly

useWorkerToDo.coffee

useWorkerToDo = (fn, data) ->
  window.URL or = window.webkitURL
  workerSupported = window.Worker and window.URL and window.Blob
  callbacks = []
  done = false
  returnVal = undefined
  resolve = ->
    while callbacks.length
      returnVal = callbacks.shift()(returnVal)
    return
  onmessage = (e) ->
    returnVal = e.data
    resolve()
    done = true
    return
  promise = {
    then: (cb) ->
      callbacks.push cb
      resolve() if done
      return promise
  }

  if workerSupported
    blob = new Blob(["
      self.onmessage = function(e) {
        self.postMessage( (#{fn.toString()})(e.data) );
      };
    "], { type: 'application/javascript' })
    blobURL = window.URL.createObjectURL blob
    worker = new Worker blobURL
    worker.onmessage = onmessage
    worker.postMessage data
    window.URL.revokeObjectURL blobURL
  else
    setTimeout ->
      onmessage data: fn(data)
      return
    , 1

  return promise