Summary: in this tutorial, you will learn how to use the JavaScript Array lastIndexOf()
method to return the last index of a matching element in an array or -1 if no matching is found.
Introduction to JavaScript Array lastIndexOf() method
The Array lastIndexOf()
method returns the index of the last matching element in an array or -1
if there is no matching element.
Here’s the syntax of the lastIndexOf()
method:
const index = array.lastIndexOf(searchElement, fromIndex)
Code language: PHP (php)
The lastIndexOf()
method accepts two arguments:
searchElement
is the element to locate in the array.fromIndex
is a zero-based index at which the method starts searching. ThefromIndex
parameter is optional.
Note that the lastIndexOf()
method searches for the elements from the end of the array to the beginning array, starting from the fromIndex
.
If you omit the fromIndex
, the indexOf()
method starts searching from the end of the array.
The fromIndex
argument can be a positive or negative integer.
If fromIndex
is positive, the method starts searching from the fromIndex
backward to the beginning of the array.
If fromIndex
is negative (-array.length <= fromIndex < 0
), a negative fromIndex
counts back from the end of the array. The method also searches backward from the end of the array.
If fromIndex < - array.length
, the method does not search and return -1
.
The lastIndexOf()
method uses the strict equality comparison algorithm for comparing the searchElement
with the elements in the array when searching.
JavaScript Array lastIndexOf() method examples
Let’s take some examples of using the lastIndexOf()
method.
Basic JavaScript Array lastIndexOf() method example
The following example uses the lastIndexOf()
method to locate the number 20
in the scores
array:
const scores = [10, 20, 30, 10, 40, 20];
const index = scores.lastIndexOf(20);
console.log({ index });
Code language: JavaScript (javascript)
Output:
{ index: 5 }
Code language: CSS (css)
Using the fromIndex argument
The following example uses the lastIndexOf()
method to locate the number 20
in the scores
array starting from the index 2:
const scores = [10, 20, 30, 10, 40, 20];
const index = scores.lastIndexOf(20, 2);
console.log({ index });
Code language: JavaScript (javascript)
Output:
{ index: 1 }
Code language: CSS (css)
Using a negative fromIndex argument
The following example uses the lastIndexOf()
method to locate the number 20
in the scores
array starting from a negative index -3
:
const scores = [10, 20, 30, 10, 40, 20];
const index = scores.lastIndexOf(20, -3);
console.log({ index });
Code language: JavaScript (javascript)
Output:
{ index: 1 }
Code language: CSS (css)
Output:
Summary
- Use the JavaScript array
lastIndexOf()
method to return the index of the last matching element in the array.