JavaScript Program to Find the Index of the Last Occurrence of a Substring in a String

Finding the index of the last occurrence of a substring in a string is a common task in JavaScript. The last occurrence of a substring in a string in JavaScript refers to finding the final position where a specified substring appears within a given string. We may want to know the position of the last occurrence of a specific substring within a given string.

There are several methods that can be used to find the index of the last occurrence of a substring in a string in JavaScript, which are listed below:

We will explore all the above methods along with their basic implementation with the help of examples.

Using lastIndexOf() Method

The lastIndexOf() method in JavaScript is used to find the index of the last occurrence of a specified substring in a string. It searches the string from the end to the beginning and returns the index of the last occurrence of the substring, or -1 if the substring is not found.

Syntax:

str.lastIndexOf(searchValue , index)

Example: In this example, we find the last occurrence index of the substring “Computer” in the text. It displays the index if found; otherwise, indicates absence.

const text = "GeeksforGeeks, A Computer science Portal.";

// Substring to search for
const substring = "Computer";

// Using lastIndexOf() method
const lastIndex = text.lastIndexOf(substring);

if (lastIndex !== -1) {
    console.log(`Last occurrence of '
    ${substring}
    ' found at index ${lastIndex}`);
} else {
    console.log(
        `'${substring}' not found in the text.`);
}

Last occurrence of '
    Computer
    ' found at index 17

Using Regular Expression

Regular expressions provide a powerful way to search for patterns in strings. You can use the RegExp constructor along with the exec() method to find the last occurrence of a substring in a string.

Syntax:

const regex = new RegExp(substring, 'g');

Example: In this example,we are using a RegExp and exec(), to finds the last occurrence index of the substring “sample” in the text and displays it.

const text = "Hello, this is a sample text. This text is a sample.";

// Substring to search for
const substring = "sample";

// Create a regular expression
const regex = new RegExp(substring, 'g');
let match;
let lastIndex = -1;

while ((match = regex.exec(text)) !== null) {
    lastIndex = match.index;
}

if (lastIndex !== -1) {
    console.log(`Last occurrence of '
    ${substring}
    ' found at index 
    ${lastIndex}`);
} else {
    console.log(`'${substring}
    ' not found in the text.`);
};

Last occurrence of '
    sample
    ' found at index 
    45

Using split() and pop()

Using split() and pop(), this approach splits the text at the substring, extracts the last segment, and calculates the index of its start within the original string.

Syntax:

const segments = str.split(substring);
const lastIndex = str.length - segments.pop().length - substring.length;

Example: In this example, the index of the last occurrence of “Science” is found by splitting the string and calculating the index based on the lengths of segments and substring.

function lastIndexOfSubstring(str, substr) {
    let lastIndex = -1;
    for (let i = 0; i <= str.length - substr.length; i++) {
        if (str.substr(i, substr.length) === substr) {
            lastIndex = i;
        }
    }
    return lastIndex;
}


console.log(lastIndexOfSubstring("hello world hello", "hello")); // Output: 12

Last occurrence of Science found at index :26

Using a Loop

Using a loop, iterate through the string, comparing substrings of the same length as the target substring with it. Track the index of the last occurrence. Return the index found after iterating through the entire string.

Example: In this example we defines a function lastIndexOfSubstring that finds the last occurrence of a substring within a string by iterating through the string and updating the last index each time the substring is found.

function lastIndexOfSubstring(str, substr) {
    let lastIndex = -1;
    for (let i = 0; i <= str.length - substr.length; i++) {
        if (str.substr(i, substr.length) === substr) {
            lastIndex = i;
        }
    }
    return lastIndex;
}


console.log(lastIndexOfSubstring("hello world hello", "hello")); 

12

Using Array.prototype.reduceRight() Method

The Array.prototype.reduceRight() method applies a function against an accumulator and each value of the array (from right to left) to reduce it to a single value. We can use this method to find the last occurrence of a substring by iterating the string array in reverse.

Example: In this example, we use reduceRight() to find the last occurrence index of the substring “example” in the given text. The method accumulates the index if the substring is found, otherwise returns -1.

const text = "This is an example text with example as a repeated example.";

// Substring to search for
const substring = "example";

// Using reduceRight() method
const lastIndex = text.split('').reduceRight((acc, _, i, arr) => {
    if (arr.slice(i, i + substring.length).join('') === substring && acc === -1) {
        return i;
    }
    return acc;
}, -1);

if (lastIndex !== -1) {
    console.log(`Last occurrence of '${substring}' found at index ${lastIndex}`);
} else {
    console.log(`'${substring}' not found in the text.`);
}

Last occurrence of 'example' found at index 51

Using indexOf() with a Loop

In this approach, we repeatedly use the indexOf method to find occurrences of the substring, starting the search just after the last found occurrence until no more occurrences are found. This way, we keep track of the last found index.

Example: This example demonstrates how to find the index of the last occurrence of a substring using indexOf in a loop.

function lastIndexOfSubstring(str, substring) {
    let lastIndex = -1;
    let currentIndex = -1;

    while ((currentIndex = str.indexOf(substring, currentIndex + 1)) !== -1) {
        lastIndex = currentIndex;
    }

    return lastIndex;
}

// Example usage:
let text = "This is a sample example of a sample example text example.";
let substring = "example";
console.log(lastIndexOfSubstring(text, substring));  // Output: 49

50