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Zach Leatherman

Quick Performance Tip: jQuery and addClass

30 Jun 2009 Read in about 2 minutes

Abstractions are helpful and dangerous. But the more we know about a library’s internals, the less danger we’ll be in later. Here’s an issue I ran into where I had assumed that jQuery would be optimized for this case, but it wasn’t. I’ll go over my bad assumption and how to workaround it.

As of jQuery 1.3.2, adding multiple HTML classes to an element using jQuery’s addClass method will add them one at a time, modifying the className property of an element for each class.

$('#myElement').addClass('myFirstClass mySecondClass');

Here’s the original code inside of jQuery 1.3.2. Note how the classNames string is split, and elem.className is changed for each split entry.

add: function( elem, classNames ) {
jQuery.each((classNames || "").split(/\s+/), function(i, className){
    if ( elem.nodeType == 1 && !jQuery.className.has( elem.className, className ) )
        elem.className += (elem.className ? " " : "") + className;
// ...

This may cause longer than needed delays, as reflow may occur after every class is added individually. If absolutely necessary, you can always fall back to modifying the className yourself, like so:

$('#myElement').each(function() {
   this.className += ' myFirstClass mySecondClass';

Most likely, this isn’t a tip that will be needed, but it is useful to be aware of.


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Richard Neil Ilagan

12 Aug 2009 at 06:36AM

Great insight, and kudos to you for actually delving into the underlying source code.

I, myself, have been writing some code that dynamically modifies classes of some HTML elements as part of an event procedure (namely, hover in and out). As you can imagine, the performance load on this can get pretty hefty; it increases proportionally the more classes you add in and the more elements you modify in one go. This is most notable in Internet Explorer too.

While your aforementioned workaround for the .addClass() procedure would definitely work better in my opinion, I also wanted to ask if there was a better way to implement .removeClass() as well. Manually doing this meant string splitting and concatenation, which, I assume, is what the function already does.

Good work, and definitely bookmarking this.

Zach Leatherman

13 Aug 2009 at 10:24PM

I'm surprised you're seeing performance issues inside callbacks for hover events. How many nodes are in the DOM tree you're modifying the className for?

Actually, it looks like the removeClass method is optimized, and only sets the string once, instead of multiple times. It shouldn't need any changes.

elem.className = classNames !== undefined ?
jQuery.grep(elem.className.split(/\s+/), function(className){
return !jQuery.className.has( classNames, className );
}).join(" ") :