Jquery element+class selector performance

问题

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')

回答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.

回答2:

var obj = document.getElementById("childDiv");
xxx = obj.getElementsByClassName("txtClass");

is 30x faster.

http://jsperf.com/selectors-perf/6

回答3:

CSS Selectors are parsed from right to left. So your example

$('#childDiv2 .txtClass')

will take two actions to complete. First find all elements with class txtClass. Then check each element for being a child of the element with the id childDiv2.

$('.txtClass')

This selector will just take one action. Find all elements with class txtClass

Have a look at this article on css-tricks.com

回答4:

Looks like it also depends on the density of the elements with the class among the elements of the type.

I ran the tests with Google Chrome Version 30.0.1599.69 using JQuery 1.10.1. Feel free to try it on another browser and/or using another JQuery version.

I tried to run the following tests:

1. Sparse (10% of the div's have the class) link to the test on jsbin

2. Dense (90% of the div's have the class) link to the test on jsbin

Looks like in the Dense case div.class wins, but in the Sparse case .class wins.

来源：https://stackoverflow.com/questions/11698935/jquery-elementclass-selector-performance