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<script>{ "title": "Don't Act on Absent Elements", "level": "intermediate", "source": "", "attribution": [ "jQuery Fundamentals" ] }</script>

jQuery won't tell you if you're trying to run a whole lot of code on an empty selection – it will proceed as though nothing's wrong. It's up to you to verify that your selection contains some elements.

// Bad: This runs three functions before it
// realizes there's nothing in the selection
$( "#nosuchthing" ).slideUp();

// Better:
var elem = $( "#nosuchthing" );

if ( elem.length ) {



// Best: Add a doOnce plugin.
jQuery.fn.doOnce = function( func ) {

	this.length && func.apply( this );

	return this;


$( "li.cartitems" ).doOnce(function() {

	// make it ajax! \o/


This guidance is especially applicable for jQuery UI widgets, which have a lot of overhead even when the selection doesn't contain elements.

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