Access parent iframe with postMessage

Rarely someone would need to use postMessage across nested iframes in cross domain scenario, but I came across this the other day:

<iframe src="//www.example.com/" name="www">
  <!-- nested body -->
  <iframe src="//foo.example.com/" name="foo">
    <!-- another nested body -->
    <iframe src="//nested.example.com/" name="nested">
      <script>
        // I want to postMessage to foo.example.com
      </script>
    </iframe>
  </iframe>
</iframe>

You can access the parent iframe just as if it wasn't on a different subdomain. But the caveat is that the parent's parent frame cannot be accessed.

// within iframe[name=nested]
parent.postMessage('some message', 'https://foo.example.com');

If access to iframe[name=www] is necessary, and is the topmost frame, send messages with:

// within iframe[name=nested]
top.postMessage('some message', 'https://www.example.com');