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
    How I Stopped WordPress Comment Spam

    I love almost every part of being a tech blogger:  learning, preaching, bantering, researching.  The one part about blogging that I absolutely loathe:  dealing with SPAM comments.  For the past two years, my blog has registered 8,000+ SPAM comments per day.  PER DAY.  Bloating my database...

  • By
    Interview with a Pornhub Web Developer

    Regardless of your stance on pornography, it would be impossible to deny the massive impact the adult website industry has had on pushing the web forward. From pushing the browser's video limits to pushing ads through WebSocket so ad blockers don't detect them, you have...

Incredible Demos

  • By
    Highlight Table Rows, Columns, and Cells Using MooTools 1.2.3

    Row highlighting and individual cell highlighting in tables is pretty simple in every browser that supports :hover on all elements (basically everything except IE6). Column highlighting is a bit more difficult. Luckily MooTools 1.2.3 makes the process easy. The XHTML A normal table. The cells...

  • By
    “Top” Watermark Using MooTools

    Whenever you have a long page worth of content, you generally want to add a "top" anchor link at the bottom of the page so that your user doesn't have to scroll forever to get to the top. The only problem with this method is...

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!