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 3D Folding Animation

    Google Plus provides loads of inspiration for front-end developers, especially when it comes to the CSS and JavaScript wonders they create. Last year I duplicated their incredible PhotoStack effect with both MooTools and pure CSS; this time I'm going to duplicate...

  • By
    Create a CSS Cube

    CSS cubes really showcase what CSS has become over the years, evolving from simple color and dimension directives to a language capable of creating deep, creative visuals.  Add animation and you've got something really neat.  Unfortunately each CSS cube tutorial I've read is a bit...

Incredible Demos

  • By
    iPhone Click Effect Using MooTools or jQuery

    One thing I love about love about Safari on the iPhone is that Safari provides a darkened background effect when you click a link. It's the most subtle of details but just enforces than an action is taking place. So why not implement that...

  • By
    jQuery Link Nudging

    A few weeks back I wrote an article about MooTools Link Nudging, which is essentially a classy, subtle link animation achieved by adding left padding on mouseover and removing it on mouseout. Here's how to do it using jQuery: The jQuery JavaScript It's important to keep...

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!