What Does jQuery .index() Return If Not Found?

When working with DOM elements in jQuery, the .index() method is commonly used to determine the zero-based position of an element relative to its siblings or a specific collection. If the target element cannot be found, jQuery's .index() method returns -1. This article explains how this return value works across different use cases of the method and how to properly handle it in your code.

The Return Value: -1

In JavaScript and jQuery, an index of -1 is the standard convention to represent that an item does not exist within a collection or list. Because .index() uses zero-based indexing—meaning the first element is at position 0, the second at 1, and so on—a positive integer or zero indicates a successful match. The return value -1 explicitly signals a failure to find the element.

Scenarios Where .index() Returns -1

There are three common ways to use .index(), and each will return -1 under specific conditions:

1. Passing a Selector That Matches Nothing: When you pass a selector string into .index('selector'), jQuery looks for the calling element within the set matched by that selector. If the calling element is not part of that matched set, it returns -1.

   // Returns -1 if $('#myElement') is not an <li> with class .active
   var idx = $('#myElement').index('li.active');

2. Passing an Element That Does Not Exist in the Collection: When calling .index(target) on a jQuery collection and passing in a DOM element or jQuery object as an argument, jQuery searches for that specific item within the calling set. If the item is not present, it returns -1.

   var items = $('ul li');
   var externalElement = $('#notInList');
   var idx = items.index(externalElement); // Returns -1

3. Calling .index() on an Empty Selection: If you call .index() without arguments on a jQuery object that matched zero elements, it cannot determine sibling position and returns -1.

   var idx = $('.non-existent-class').index(); // Returns -1

Handling the Return Value in Conditional Statements

Because 0 is a valid index (representing the first element) and evaluates to false in loose JavaScript boolean checks, you must explicitly check for -1 rather than relying on truthy/falsy evaluation.

var position = $('li').index($('#targetItem'));

if (position !== -1) {
    console.log("Element found at index: " + position);
} else {
    console.log("Element was not found.");
}

Checking strictly against -1 ensures that elements at index 0 are not mistakenly treated as missing.