Question

I was hoping $('#childDiv2 .txtClass') or $('#childDiv2 input.txtClass') perform better when selecting <input type="text" id="txtID" class="txtClass"/> element. But according to this performance analysis $('.txtClass'); is the best selector among this. I’m using JQuery 1.7.2
Does anybody have explanation for this?

HTML

<div class="childDiv2">
    <input type="text" id="txtID" class="txtClass"/>
    <p class="child">Blah Blah Blah</p>
</div>​

JS

$('.txtClass');
$('#childDiv2 .txtClass')
$('#childDiv2 > .txtClass')
$('input.txtClass')
$('#childDiv2 input.txtClass')

Answer 1

Modern browsers expose a very efficient getElementsByClassName() method that returns the elements having a given class. That’s why a single class selector is faster in your case.

To elaborate on your examples:

$(".txtClass")                  =>  getElementsByClassName()

$("#childDiv2 .txtClass")       =>  getElementById(),
                                    then getElementsByClassName()

$("#childDiv2 > .txtClass")     =>  getElementById(),
                                    then iterate over children and check class

$("input.txtClass")             =>  getElementsByTagName(),
                                    then iterate over results and check class

$("#childDiv2 input.txtClass")  =>  getElementById(),
                                    then getElementsByTagName(),
                                    then iterate over results and check class

As you can see, it’s quite logical for the first form to be the fastest on modern browsers.