jquery: when does $("???") scan the whole DOM?

Question

When using $("#xxx") I guess under the hoods jQuery uses getElementById.

What about $(".xxx") does it scan the whole DOM every time?

view the source here (click the big button): http://code.google.com/p/jqueryjs/downloads/detail?name=jquery-1.3.2.js&downloadBtn= — geowa4, Sep 23 '09 at 18:34
oh, and jQuery uses the Sizzle selector engine nowadays. (both written by John Resig) — geowa4, Sep 23 '09 at 18:42

score 6 · Accepted Answer · edited May 23 '17 at 10:32

jQuery attempts to use the fastest selection method to get what you asked for. There are a number of good resources with performance optimization tips out there that relate directly to jQuery:

Good ways to improve jQuery selector performance?

http://www.artzstudio.com/2009/04/jquery-performance-rules/

http://www.componenthouse.com/article-19

http://www.learningjquery.com/2006/12/quick-tip-optimizing-dom-traversal

score 4 · Answer 2 · edited Sep 23 '09 at 19:06

4

See the context argument to the $ function. If not supplied, it defaults to the entire document.

So to answer your question:

$('whatever'); // scans the entire `document`

$('whatever', element); // scans only within element

edited Sep 23 '09 at 19:06

Crescent Fresh

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answered Sep 23 '09 at 18:41

Roatin Marth

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so this is basically like doing element.find('whatever') ? – Andreas Grech Sep 23 '09 at 19:07

score 1 · Answer 3 · answered Sep 23 '09 at 18:42

What about $(".xxx") does it scan the whole DOM every time?

If you don't do the caching: yes. Caching is simple enough:

var $myCachedElements = $('.myElements'); // DOM querying occurs

$myCachedElements.animate({left: '1000px'}, 'slow'); // no DOM Querying this time, as long as you use the variable.

score 0 · Answer 4 · answered Sep 23 '09 at 18:34

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Many browsers do not support getElementsByClassName as a native DOM function, so jQuery has to do the work itself by checking each element's classes.

answered Sep 23 '09 at 18:34

Rex M

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But it does try to use any Selector API method before gEBCN last I recall? Then the lowest common denominator is looping through each elements clsas. – meder omuraliev Sep 23 '09 at 18:40

score 0 · Answer 5 · answered Sep 23 '09 at 18:42

Here's a compatibility table for document.getElementsByClassName: http://www.quirksmode.org/dom/w3c_core.html#gettingelements

The browsers in green for getElementsByClassName will not require a full DOM scan for $(".className") selectors, and will use browser-native methods instead. The ones in red will be slower.

The difference isn't as pronounced as you'd think though, even for thousands of elements.

jquery: when does $("???") scan the whole DOM?

5 Answers5