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
    CSS @supports

    Feature detection via JavaScript is a client side best practice and for all the right reasons, but unfortunately that same functionality hasn't been available within CSS.  What we end up doing is repeating the same properties multiple times with each browser prefix.  Yuck.  Another thing we...

  • By
    Camera and Video Control with HTML5

    Client-side APIs on mobile and desktop devices are quickly providing the same APIs.  Of course our mobile devices got access to some of these APIs first, but those APIs are slowly making their way to the desktop.  One of those APIs is the getUserMedia API...

Incredible Demos

  • By
    WebKit-Specific Style:  -webkit-appearance

    I was recently scoping out the horrid source code of the Google homepage when I noticed the "Google Search" and "I'm Feeling Lucky" buttons had a style definition I hadn't seen before:  -webkit-appearance.  The value assigned to the style was "push-button."  They are buttons so that...

  • By
    Fx.Rotate:  Animated Element Rotation with MooTools

    I was recently perusing the MooTools Forge and I saw a neat little plugin that allows for static element rotation: Fx.Rotate. Fx.Rotate is an extension of MooTools' native Fx class and rotates the element via CSS within each A-grade browser it...

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!