Will jQuery search for the ID before filtering other parameters in the selector?

Question

This question is related to performance.

If I use a selector like the following

$('#myID a') // Does this find #myID and filter by a?

Or should I write the statement like this?

$('#myID').find('a')

I'm not sure if jQuery is smart enough to execute this statement using the ID first or if it operates exactly like CSS and reads right to left. It's not such a big deal using tags but when you run something like

$('#myID .myClass')

It makes a HUGE difference in performance.

Answer 1

From a NetTuts article: http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-think-right-to-left-with-jquery/

As an example, if Sizzle comes across a selector like $('#box p'), it’s true that it works right-to-left, but there’s also a quick regex optimization that will first determine whether the first section of the selector is an id. If so, it’ll use that as the context, when searching for the paragraph tags.

Relevant comment from SizzleJS:

// Take a shortcut and set the context if the root selector is an ID
// (but not if it'll be faster if the inner selector is an ID)

Answer 2

When an Id is in the selector. jQuery will first execute document.getElementById then begin filtering for child elements.

basically this is why it is never a great idea to use just attribute or class selectors $('.someclass') or $('[name=myname]') without being more specific. Because it causes the code to traverse the DOM and look at every element to find that class.

By just adding a tagname to the same selector $('div.someclass') or $('div.[name=myname]') you improve efficiency becuase it will first run. document.getElementsByTagName narrowing the number of elements to search.