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
    JavaScript Promise API

    While synchronous code is easier to follow and debug, async is generally better for performance and flexibility. Why "hold up the show" when you can trigger numerous requests at once and then handle them when each is ready?  Promises are becoming a big part of the JavaScript world...

  • By
    Facebook Open Graph META Tags

    It's no secret that Facebook has become a major traffic driver for all types of websites.  Nowadays even large corporations steer consumers toward their Facebook pages instead of the corporate websites directly.  And of course there are Facebook "Like" and "Recommend" widgets on every website.  One...

Incredible Demos

  • By
    Select Dropdowns, MooTools, and CSS Print

    I know I've harped on this over and over again but it's important to enhance pages for print. You can do some things using simple CSS but today's post features MooTools and jQuery. We'll be taking the options of a SELECT element and generating...

  • By
    MooTools 1.2 Image Protector: dwProtector

    Image protection is a hot topic on the net these days, and why shouldn't it be? If you spent two hours designing an awesome graphic, would you want it ripped of in matter of seconds? Hell no! That's why I've created an image...

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!