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)