MessageEvent.source
  The source read-only property of the
  MessageEvent interface is a MessageEventSource (which can be
  a WindowProxy, MessagePort, or
  ServiceWorker object) representing the message emitter.
Value
  a MessageEventSource (which can be a WindowProxy,
  MessagePort, or ServiceWorker object) representing the
  message emitter.
Examples
myWorker.onmessage = function(e) {
  result.textContent = e.data;
  console.log('Message received from worker');
  console.log(e.source);
};
Specifications
| Specification | 
|---|
| HTML Standard # dom-messageevent-source-dev | 
Browser compatibility
BCD tables only load in the browser
See also
- 
    ExtendableMessageEvent— similar to this interface but used in interfaces that needs to give more flexibility to authors.