Selector optimization is less important than it used to be, as more browsers implement document.querySelectorAll()
and the burden of selection shifts from jQuery to the browser. However, there are still some tips to keep in mind when selector performance becomes a bottleneck.
link jQuery Extensions
When possible, avoid selectors that include jQuery extensions. These extensions cannot take advantage of the performance boost provided by the native querySelectorAll()
DOM method and, therefore, require the use of the Sizzle selector engine provided by jQuery.
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Keep in mind that many jQuery extensions, including :even
in the above example, do not have exact equivalents in the CSS specification. In some situations the convenience of these extensions could outweigh their performance cost.
link Avoid Excessive Specificity
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A "flatter" DOM also helps improve selector performance, as the selector engine has fewer layers to traverse when looking for an element.
link ID-Based Selectors
Beginning your selector with an ID is a safe bet.
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With the first approach, jQuery queries the DOM using document.querySelectorAll()
. With the second, jQuery uses document.getElementById()
, which is faster, although the speed improvement may be diminished by the subsequent call to .find()
.
link Tips for Older Browsers
When support for older browsers, such as Internet Explorer 8 and below, is necessary, consider the following tips:
link Specificity
Be specific on the right-hand side of your selector, and less specific on the left.
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Use tag.class
if possible on your right-most selector, and just tag or just .class
on the left.
link Avoid the Universal Selector
Selections that specify or imply that a match could be found anywhere can be very slow.
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