Making a PromiseWorker that can shutdown

gistfile1.js

let myWorker = {
  //
  // Worker initialization
  //
  get worker() {
    if (this._worker) {
      return this._worker;
    }
    // Otherwise, initialize PromiseWorker.
    // TODO
  },
  _worker: null,

  //
  // Sending a message to the worker
  //
  post: function(...msg) {
    if (this.promiseShutdownComplete) {
      return Promise.reject(new Error(`Too late to send message ${msg}`));
    }
    return this.worker.post(...msg);
  },

  function shutdown() {
    if (this._promiseShutDownComplete) {
      // Shutdown has already started
      return this._promiseShutdownComplete;
    }
    if (this._worker) {
      // The worker has never been initialized
      return this._promiseShutdownComplete = Promise.resolve();
    }
    return this._promiseShutdownComplete = this._worker.post(/*some message that will trigger shutdown*/);
  },
  // Initially, `null`. Once we have started the shutdown sequence,
  // a promise that will resolve only one shutdown is complete.
  _promiseShutdownComplete: null,
};