Quick Performance Tip: jQuery and addClass

By Zach Leatherman | Published: June 30, 2009

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.

This entry was posted in JavaScript and tagged , . Bookmark the permalink. Both comments and trackbacks are currently closed.
  • If you found this article useful, you should subscribe to my feed (or get an e-mail). I'm also on Twitter and GitHub.

  • About the Author

    Zach Leatherman is a Professional Front End Engineer. He loves building for the web, and has been contributing to the community through his blog since February 2007. Despite his propensity for software, he has a Bachelors degree in Computer Engineering and is currently on the User Experience Team at Union Pacific Railroad. The views expressed on this website do not represent the views of his employer.

    He enjoys spending time with his beautiful wife Traci and their two Great Danes, Roxie and Ella. They also have a cat, a rabbit, goldfish, and one or more tarantulas. Read more »

2 Comments

  1. Richard Neil Ilagan
    Posted August 12, 2009 at 1:36 am | Permalink

    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.

  2. Zach Leatherman
    Posted August 13, 2009 at 5:24 pm | Permalink

    Richard,
    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(" ") :
	"";