ExtendableMessageEvent.source

The source read-only property of the ExtendableMessageEvent interface returns a reference to the Client object from which the message was sent.

A Client, ServiceWorker or MessagePort object.

When the following code is used inside a service worker to respond to a push messages by sending the data received via PushMessageData to the main context via a channel message, the event object of onmessage will be a ExtendableMessageEvent.

var port;

self.addEventListener('push', function(e) {
  var obj = e.data.json();

  if(obj.action === 'subscribe' || obj.action === 'unsubscribe') {
    port.postMessage(obj);
  } else if(obj.action === 'init' || obj.action === 'chatMsg') {
    port.postMessage(obj);
  }
});

self.onmessage = function(e) {
  console.log(e.source);
  port = e.ports[0];
}

• Using Service Workers
• Service workers basic code example
• Is ServiceWorker ready?
• Channel Messaging