helpMePeter.js

function sendMsgToSW(){
    sendMessage({
        command: 'helloWorld',
        url: 'foo'
        }).then(function() {
        // If the promise resolves, just display a success message.
        writeLog('postMessage promise returned');
      }).then(function(val){
        writeLog(val);
      }).catch(console.error);
}

function sendMessage(message) {
  // This wraps the message posting/response in a promise, which will resolve if the response doesn't
  // contain an error, and reject with the error if it does. If you'd prefer, it's possible to call
  // controller.postMessage() and set up the onmessage handler independently of a promise, but this is
  // a convenient wrapper.
  return new Promise(function(resolve, reject) {
    var messageChannel = new MessageChannel();
    messageChannel.port1.onmessage = function(event) {
      if (event.data.error) {
        reject(event.data.error);
      } else {
        resolve(event.data);
        // writeLog('port1.onmessage got: ' + event.data);

      }
    };

    // This sends the message data as well as transferring messageChannel.port2 to the service worker.
    // The service worker can then use the transferred port to reply via postMessage(), which
    // will in turn trigger the onmessage handler on messageChannel.port1.
    // See https://html.spec.whatwg.org/multipage/workers.html#dom-worker-postmessage
    navigator.serviceWorker.controller.postMessage(message, [messageChannel.port2]);
  });
}