javascript - Fastest selector method in jquery and CSS - ID or not?

Question

What is fastest in jquery/javascript?

$('#myID .myClass')

or

$('.myClass')

What is best to use in CSS?

#myID .myClass{}

or

.myClass{}

I see now that I should have explained better. Sorry!

Ofceauce ID is a faster selector in both CSS and JavaScript. But some times you need to use class since there are multiple selectors.

Say forexample that I have i BIG html document. In the middle of the page I have:

<div id="myID">

<a class="myClass">link1</a>

<a class="myClass">link1</a>

<a class="myClass">link1</a>

</div>

If I want to target all "myClass". Would it then be better to target the ID before targeting the classes? (then I wouldn't have to do domtravel of the entire HTML document) Eg.:

Would this:

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

Be faster than:

$('.myClass')

Answer 1

My testing on modern browsers suggests that you should go with either,

$('#id').find('.class') // or
$('.class')

but not,

$('#id .class')

Reason being that all modern browsers implement getElementsByClassName resulting in almost-constant time lookups by class name (assuming a hash implementation). Which browsers are modern is another subjective question.