detect which iframe sent post message

Question

Assume I have several iframes on the page and one of them sent a post message. Is there a simple and crossbrowser way to detect which one did it and be able to reply?

I see the source property of the message event but I am unable respond by using event.source.contentWindow.postMessage on it:

Error: Permission denied to access property 'contentWindow'

Answer 1

You can detect IFrame without names, just use comparison iframe.contentWindow === event.source.

window.addEventListener('message', function (event) {
  var iframes = document.getElementsByTagName('IFRAME');

  for (var i = 0, iframe, win; i < iframes.length; i++) {
    iframe = iframes[i];

    // Cross-browser way to get iframe's window object
    win = iframe.contentWindow || iframe.contentDocument.defaultView;

    // Comparison
    text.innerText += iframe.src +
      (win === event.source ? ' MATCHES.\n' : ' is not our IFrame.\n');
  }
});

// Creates iframe and sends postMessage from it
function createIFrameWithMessage(id) {
  var iframe = document.createElement('IFRAME');
  iframe.src = 'javascript:parent.postMessage("IFrame #' + id + '", "*");';
  document.body.appendChild(iframe);
}

createIFrameWithMessage(1);
createIFrameWithMessage(2);
createIFrameWithMessage(3);

<p id="text"></p>

Answer 2

iframe

<script>
    document.addEventListener('mousedown',event =>
        parent.postMessage({ name: window.name }, window.location.origin)
    );
</script>

parent window

<script>
   window.addEventListener("message", event => {
       if (event.origin != window.location.origin) return;
       console.warn(event.source.frameElement.name);
       // here you can find your iframe by name
   });
</script>
<iframe name='myname1' src='/url1'/>
<iframe name='myname2' src='/url2'/>
<iframe name='myname3' src='/url3'/>