Posted in: Using jQuery Core

Using jQuery’s .index() Function

.index() is a method on jQuery objects that's generally used to search for a given element within the jQuery object that it's called on. This method has four different signatures with different semantics that can be confusing. This article covers details about how to understand the way .index() works with each signature.

link .index() with No Arguments

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<ul>
<div></div>
<li id="foo1">foo</li>
<li id="bar1">bar</li>
<li id="baz1">baz</li>
<div></div>
</ul>
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var foo = $( "#foo1" );
console.log( "Index: " + foo.index() ); // 1
var listItem = $( "li" );
// This implicitly calls .first()
console.log( "Index: " + listItem.index() ); // 1
console.log( "Index: " + listItem.first().index() ); // 1
var div = $( "div" );
// This implicitly calls .first()
console.log( "Index: " + div.index() ); // 0
console.log( "Index: " + div.first().index() ); // 0

In the first example, .index() gives the zero-based index of #foo1 within its parent. Since #foo1 is the second child of its parent, index() returns 1.

When .index() is called on a jQuery object that contains more than one element, it calculates the index of the first element.

link .index() with a String Argument

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<ul>
<div class="test"></div>
<li id="foo1">foo</li>
<li id="bar1" class="test">bar</li>
<li id="baz1">baz</li>
<div class="test"></div>
</ul>
<div id="last"></div>
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var foo = $( "li" );
// This implicitly calls .first()
console.log( "Index: " + foo.index( "li" ) ); // 0
console.log( "Index: " + foo.first().index( "li" ) ); // 0
var baz = $( "#baz1" );
console.log( "Index: " + baz.index( "li" )); // 2
var listItem = $( "#bar1" );
console.log( "Index: " + listItem.index( ".test" ) ); // 1
var div = $( "#last" );
console.log( "Index: " + div.index( "div" ) ); // 2

When .index() is called with a string argument, there are two things to consider. First, jQuery will implicitly call .first() on the original jQuery object. It will find the index of the first element, not the last element in this case. This is inconsistent, so be careful here.

The second point to consider is that jQuery is querying the entire DOM using the passed in string selector and checking the index within that newly queried jQuery object. For example, when using .index( "div" ) in the last example above, jQuery is selecting all of the <div> elements in the document, then searching for the index that contains the first element in the jQuery object .index() is called on.

link .index() with a jQuery Object Argument

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<ul>
<div class="test"></div>
<li id="foo1">foo</li>
<li id="bar1" class="test">bar</li>
<li id="baz1">baz</li>
<div class="test"></div>
</ul>
<div id="last"></div>
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var foo = $( "li" );
var baz = $( "#baz1" );
console.log( "Index: " + foo.index( baz ) ); // 2
var tests = $( ".test" );
var bar = $( "#bar1" );
// Implicitly calls .first() on the argument.
console.log( "Index: " + tests.index( bar ) ); // 1
console.log( "Index: " + tests.index( bar.first() ) ); // 1

In this case, the first element of the jQuery object that is passed into .index() is being checked against all of the elements in the original jQuery object. The original jQuery object, on the left side of .index(), is array-like and is searched from index 0 through length - 1 for the first element of the argument jQuery object.

link .index() with a DOM Element Argument

In this case, the DOM element that's passed into .index() is being checked against all of the elements in the original jQuery object. Once all other cases are understood, this should be the simplest case. It is very similar to the previous case, except since the DOM element is passed directly, it is not taken from a jQuery object container.