IFRAME contentWindow is null

By  on  

I like clean code so I do what I can to avoid unwanted JavaScript global variables.  I initially thought that keys(window) would give me window property leaks but that didn't work because browsers returned different results, so I moved on to using an IFRAME to compare default window property keys.

When I first tried this method, I got a lame error about an IFRAME element's contentWindow property being null.  Ugh.  It didn't take long to figure out why:  you need to wait until the IFRAME has loaded to get the contentWindow:

var iframe = document.createElement('iframe');
iframe.onload = function() {
	// contentWindow is set!	
};
iframe.src = 'about:blank';
document.body.appendChild(iframe);

Of course you'll want to add the onload event before setting the src.  If you use the load event to check for the contentWindow property, you'll be in business!

Recent Features

  • By
    Introducing MooTools Templated

    One major problem with creating UI components with the MooTools JavaScript framework is that there isn't a great way of allowing customization of template and ease of node creation. As of today, there are two ways of creating: new Element Madness The first way to create UI-driven...

  • By
    How to Create a RetroPie on Raspberry Pi – Graphical Guide

    Today we get to play amazing games on our super powered game consoles, PCs, VR headsets, and even mobile devices.  While I enjoy playing new games these days, I do long for the retro gaming systems I had when I was a kid: the original Nintendo...

Incredible Demos

  • By
    Create Digg URLs Using PHP

    Digg recently came out with a sweet new feature that allows users to create Tiny Digg URLs which show a Digg banner at the top allowing easy access to vote for the article from the page. While I love visiting Digg every once in a...

  • By
    Scrolling “Go To Top” Link Using Dojo

    One of the most popular code snippets of posted on my blog has been the scrolling "Go To Top" link snippet. The premise of the snippet is simple: once the user scrolls an element (usually the BODY element) past a given threshold, a "Go...

Discussion

  1. Sisyphe

    Don’t you need to append your iframe element to a DOM tree so that the browser fetches its target content ? I mean, I know that old IE will load scripts as you parse an “HTML string” but in modern browsers, I thought that the asset does not get loaded until you append the element to a document (and in my opinion this it what makes constructors such as Image() so useful).

  2. Yes, good catch! Updated!

  3. James

    Unfortunately this does not appear to be 100% reliable in chrome (i’m currently using version 62.0.3202.94, but this appears to have been an issue for a while), as sometimes contentWindow can still be null when onload is triggered.

  4. bell.jackit

    This solution worked for me! Thanks a lot!

  5. Sharry Stowell

    Thank you, saved me hours!

    Sharry

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!