Incorrect information regarding access to the `document` of the `iframe`

[Alexandre887]

Cross-window communication

Code:

<script>
  iframe.onload = function() {
    // we can get the reference to the inner window
    let iframeWindow = iframe.contentWindow; // OK
    try {
      // ...but not to the document inside it
      let doc = iframe.contentDocument; // ERROR // 🟢
    } catch(e) {
      alert(e); // Security Error (another origin)
    }

    // also we can't READ the URL of the page in iframe
    try {
      // Can't read URL from the Location object
      let href = iframe.contentWindow.location.href; // ERROR
    } catch(e) {
      alert(e); // Security Error
    }

    // ...we can WRITE into location (and thus load something else into the iframe)!
    iframe.contentWindow.location = '/'; // OK

    iframe.onload = null; // clear the handler, not to run it after the location change
  };
</script>

In the line labeled 🟢, we will not get any error when trying to access iframe.contentDocument.

MDN: HTMLIFrameElement: contentDocument property:

If the iframe and the iframe's parent document are Same Origin, returns a Document (that is, the active document in the inline frame's nested browsing context), $\Huge\textcolor{red}{else}$ $\Huge\textcolor{red}{returns}$ $\Huge\textcolor{red}{null}$.