code: function _messageWorker(message, callback) {
      const worker = _getWorker();
  
      function _listen(e) {
          if (e.data.id === message.id) {
              callback(e.data);
              worker.removeEventListener('message', _listen);
          }
      }
  
      worker.addEventListener('message', _listen);
      worker.postMessage(message);
  }
docs: Helper for matching up callbacks directly with the
  post message requests to a web worker.
  
  @param {object} message      data to send to web worker
  @param {Function} callback   callback function for worker to reply to
  @return {void}