JavaScript Array lastIndexOf() Method

JavaScript Array lastIndexOf() method is used to search for the last occurrence of a specified element in an array. It returns the index of the element if found, or -1 if the element does not exist in the array. The search is performed in reverse order, starting from the end of the array.

Pre-requisites to Learn

JavaScript Array

Syntax

arr.lastIndexOf(searchElement, fromIndex);

Parameters

Parameter	Description
searchElement (Required)	The element to search for in the array.
fromIndex (Optional)	The index to start searching backward from. Defaults to the last index of the array.

Return Value

The lastIndexOf() method returns the index of the last occurrence of the searchElement in the array.-1 if the searchElement is not found.

Note:

Searches Backward: Looks for the element starting from the end of the array.
Handles Duplicates: Returns the last index of the specified element, even if it appears multiple times.
Uses Strict Equality (===): Compares elements using the strict equality operator, which means 5 is not equal to '5'.

Examples of JavaScript Array lastIndexOf() Method

Example 1: Finding the last occurrence of 2, which is at index 3.

let arr = [1, 2, 3, 2, 4];

let lastIndex = arr.lastIndexOf(2);
console.log(lastIndex); // Output: 3

Example 2: Start searching backward from index 2 and finds the first occurrence of 2 at index 1.

let arr = [1, 2, 3, 2, 4];

let lastIndex = arr.lastIndexOf(2, 2);
console.log(lastIndex); // Output: 1

Example 3: Element Not Found – If the element is not found, the method returns -1.

let numbers = [1, 2, 3, 4];

let lastIndex = numbers.lastIndexOf(5);
console.log(lastIndex); // Output: -1

Example 4: Identifies the last occurrence of 'banana' at index 3.

let arr = ['apple', 'banana', 'cherry', 'banana'];

let lastIndex = arr.lastIndexOf('banana');
console.log(lastIndex); // Output: 3

Example 5: A negative fromIndex counts backward from the end of the array. In this case, -3 corresponds to index 2.

let arr = [10, 20, 30, 40, 50];

let lastIndex = arr.lastIndexOf(30, -3);
console.log(lastIndex); // Output: 2

Example 6: The lastIndexOf() method is case-sensitive, so 'apple' and 'Apple' are treated as different elements.

let arr = ['Apple', 'Banana', 'apple'];

let lastIndex = arr.lastIndexOf('apple');
console.log(lastIndex); // Output: 2

let caseMismatch = arr.lastIndexOf('Apple');
console.log(caseMismatch); // Output: 0

Example 7: The lastIndexOf() method treats empty slots in sparse arrays as undefined.

let sparseArray = [1, , 3, , 5];

let lastIndex = sparseArray.lastIndexOf(undefined);
console.log(lastIndex); // Output: 3

Supported Browsers

Browser	Support
Chrome	1+
Firefox	1+
Safari	3+
Edge	12+
Opera	4+
Internet Explorer	9+

Comparison with Other Methods

Method	Purpose
Array.lastIndexOf() Method	Searches for the last occurrence of an element, starting from the end of the array.
Array.indexOf() Method	Searches for the first occurrence of an element, starting from the beginning of the array.
Array.includes() Method	Checks if an array contains a value, but does not return the index.