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?

Answer 1

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

Answer 2

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