Optimize Selectors

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.

// Slower (the zero-based :even selector is a jQuery extension)
$( "#my-table tr:even" );
 
// Better, though not exactly equivalent
$( "#my-table tr:nth-child(odd)" );

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

$( ".data table.attendees td.gonzalez" );
 
// Better: Drop the middle if possible.
$( ".data td.gonzalez" );

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.

// Fast:
$( "#container div.robotarm" );
 
// Super-fast:
$( "#container" ).find( "div.robotarm" );

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.

// Unoptimized:
$( "div.data .gonzalez" );
 
// Optimized:
$( ".data td.gonzalez" );

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.

$( ".buttons > *" ); // Extremely expensive.
$( ".buttons" ).children(); // Much better.
 
$( ":radio" ); // Implied universal selection.
$( "*:radio" ); // Same thing, explicit now.
$( "input:radio" ); // Much better.