Is the relation between <iframe> and it's document one way?

问题

You can get the nodes of the document object of an <iframe> with the contentDocument property of the HTMLIFrameElement

But I can't find a way getting the <iframe> back out of the node.
Is the relation between the <iframe> and it's document is only one way?
If it does, WHY?

A non working DEMO:

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

var div = doc.getElementsByTagName('div')[0];

console.log('Did we find the iframe? ' + ($(div).closest('iframe').length > 0));​
// Output: "Did we find the iframe? false"

回答1:

The iframe element is in different document (it's an element in the main page) than the div element (it's an element in the iframe page).

You can get the iframe element like this:

iframe.contentWindow.frameElement === iframe

Or when inside the iframe script environment:

window.frameElement

Or using the div

div.ownerDocument.defaultView.frameElement === iframe

Updated demo

回答2:

thing is, for (div) in the console.log line, they are part of the frame, they don't know they are in a frame so .closest('iframe') will be empty.

来源：https://stackoverflow.com/questions/11249702/is-the-relation-between-iframe-and-its-document-one-way