Get element from within an iFrame

var iframe = document.getElementById('iframeId');
var innerDoc = (iframe.contentDocument) ? iframe.contentDocument : iframe.contentWindow.document;

You could more simply write:

var iframe = document.getElementById('iframeId');
var innerDoc = iframe.contentDocument || iframe.contentWindow.document;

and the first valid inner doc will be returned.

Once you get the inner doc, you can just access its internals the same way as you would access any element on your current page. (innerDoc.getElementById...etc.)

IMPORTANT: Make sure that the iframe is on the same domain, otherwise you can't get access to its internals. That would be cross-site scripting.

Do not forget to access iframe after it is loaded. Old but reliable way without jQuery:

<iframe src="samedomain.com/page.htm" id="iframe" onload="access()"></iframe>

<script>
function access() {
   var iframe = document.getElementById("iframe");
   var innerDoc = iframe.contentDocument || iframe.contentWindow.document;
   console.log(innerDoc.body);
}
</script>