Finding the last occurrence of a substring

String lastIndexOf: String.lastIndexOf()

The lastIndexOf() method in JavaScript returns the index of the last occurrence of a specified value in a string. It searches the string from the end to the beginning and returns the position where the specified value is found. If the value is not found, it returns -1.

Syntax:

string.lastIndexOf(searchValue, fromIndex)

Parameters:

1. searchValue: The string to search for.
2. fromIndex (optional): The position in the string at which to begin searching backward. If omitted, the default is the length of the string, which means the entire string is searched.

Return Value:

The lastIndexOf() method returns the index of the last occurrence of the specified value in the string, searching backward from the specified position. If the value is not found, it returns -1.

Examples:

Basic Usage:

const text = 'JavaScript is amazing. JavaScript is fun.';
const lastIndex = text.lastIndexOf('JavaScript');
console.log(lastIndex); // Output: 25

In this example, the lastIndexOf() method finds the last occurrence of ‘JavaScript’ in the string, which starts at index 25.

Using fromIndex Parameter:

const text = 'JavaScript is amazing. JavaScript is fun.';
const lastIndex = text.lastIndexOf('JavaScript', 20);
console.log(lastIndex); // Output: 0

By specifying a fromIndex of 20, the search starts from position 20 and goes backward. The first occurrence of ‘JavaScript’ (starting from index 0) is found.

Searching for a Character:

const text = 'Hello, world!';
const lastIndex = text.lastIndexOf('o');
console.log(lastIndex); // Output: 8

The last occurrence of the character ‘o’ is at index 8.

Not Found Case:

const text = 'Hello, world!';
const lastIndex = text.lastIndexOf('z');
console.log(lastIndex); // Output: -1

Since the character ‘z’ is not in the string, the method returns -1.

Common Use Cases:

1. File Extension Extraction:

function getFileExtension(filename) {
  const dotIndex = filename.lastIndexOf('.');
  return dotIndex === -1 ? '' : filename.substring(dotIndex + 1);
}

console.log(getFileExtension('document.pdf')); // Output: "pdf"
console.log(getFileExtension('image.large.jpg')); // Output: "jpg"
console.log(getFileExtension('noextension')); // Output: ""

2. Finding the Last Word in a Sentence:

function getLastWord(sentence) {
  const lastSpaceIndex = sentence.lastIndexOf(' ');
  return lastSpaceIndex === -1 ? sentence : sentence.substring(lastSpaceIndex + 1);
}

console.log(getLastWord('Hello, world!')); // Output: "world!"
console.log(getLastWord('JavaScript')); // Output: "JavaScript"

3. Checking if a String Ends with a Specific Substring:

function endsWith(str, suffix) {
  const position = str.lastIndexOf(suffix);
  return position !== -1 && position === str.length - suffix.length;
}

console.log(endsWith('Hello, world!', 'world!')); // Output: true
console.log(endsWith('Hello, world!', 'Hello')); // Output: false

Note: Modern JavaScript has the built-in endsWith() method which is preferred for this use case.

By using the String.lastIndexOf() method, you can efficiently locate the last occurrence of a substring within a string, which is particularly useful for parsing file paths, extracting extensions, and performing other string manipulation tasks in JavaScript applications.