Example
Each time you use a selector in jQuery the DOM is searched for elements that match your query. Doing this too often or repeatedly will decrease performance. If you refer to a specific selector more than once you should add it to the cache by assigning it to a variable:
var nav = $('#navigation');
nav.show();
This would replace:
$('#navigation').show();
Caching this selector could prove helpful if your website needs to show/hide this element often. If there are multiple elements with the same selector the variable will become an array of these elements:
<div class="parent">
<div class="child">Child 1</div>
<div class="child">Child 2</div>
</div>
<script>
var children = $('.child');
var firstChildText = children[0].text();
console.log(firstChildText);
// output: "Child 1"
</script>
NOTE: The element has to exist in the DOM at the time of its assignment to a variable. If there is no element in the DOM with a class called child you will be storing an empty array in that variable.
<div class="parent"></div>
<script>
var parent = $('.parent');
var children = $('.child');
console.log(children);
// output: []
parent.append('<div class="child">Child 1</div>');
children = $('.child');
console.log(children[0].text());
// output: "Child 1"
</script>
Remember to reassign the selector to the variable after adding/removing elements in the DOM with that selector.
Note: When caching selectors, many developers will start the variable name with a $ to denote that the variable is a jQuery object like so:
var $nav = $('#navigation');
$nav.show();
