String.prototype.lastIndexOf()

Summary: in this tutorial, you’ll learn how to use the JavaScript String lastIndexOf() method to locate the last occurrence of a substring in a string.

Introduction to the JavaScript String lastIndexOf() Method

The String.prototype.lastIndexOf() returns the last occurrence of a substring (substr) in a string (str).

Here’s the syntax of the lastIndexOf() method:

str.lastIndexOf(substr, [, fromIndex]);Code language: JavaScript (javascript)

The lastIndexOf() method returns -1 if the str does not contain the substr.

By default, the lastIndexOf() method searches for the substring backward from the end of a string. It searches for the substring using a case-sensitive match.

The fromIndex argument is optional and defaults to +Infinity. It means that if you omit the fromIndex, the search starts from the end of the string.

If the fromIndex is greater or equal to str.length, the lastIndexOf() will search for the substr in the whole string.

If the fromIndex is less than zero, the search behavior is the same as if the fromIndex were zero.

To find the index of the first occurrence of a substring within a string, you use the lastindexOf() method.

JavaScript String lastIndexOf() examples

Let’s take some examples of using the lastIndexOf() method.

1) Basic JavaScript lastIndexOf() method example

The following example uses the lastIndexOf() method to locate the last occurrence of the substring 'a' in the string 'JavaScript':

let str = 'JavaScript';
let index = str.lastIndexOf('a');

console.log(index);Code language: JavaScript (javascript)

Output:

{ index: 3 }Code language: JavaScript (javascript)

If you pass the fromIndex argument to the string, the lastIndexOf() method will start searching backward from the fromIndex as shown in the following example:

let str = "JavaScript";
let index = str.lastIndexOf("a", 2);

console.log({ index });
Code language: JavaScript (javascript)

Output:

{ index: 1 }Code language: JavaScript (javascript)

2) The lastIndexOf() method and case-sensitivity

The lastIndexOf() always perform a case-sensitive search. For example, the following uses the lastIndexOf() method to search for the substring "L" in "Hello, World!":

let str = "Hello, World!";
let substr = "L";

let index = str.lastIndexOf(substr);

console.log({ index });Code language: JavaScript (javascript)

Output:

{ index: -1 }Code language: CSS (css)

To perform a case-insensitive search for the index of the last occurrence of a substring within a string, you can convert both substring and string to lowercase before applying the lastIndexOf() method as follows:

let str = "Hello, World!";
let substr = "L";

let index = str.toLocaleLowerCase().lastIndexOf(substr.toLocaleLowerCase());

console.log({ index });Code language: JavaScript (javascript)

Output:

{ index: 10 }Code language: CSS (css)

Summary

• The lastIndexOf() returns the index of the last occurrence of a substring in a string, or -1 if the string does not contain the substring.
• The lastIndexOf() always performs a case-sensitive search.