String.prototype.lastIndexOf()

lastIndexOf(searchString, fromIndex?)

Returns: number · Added in ves6 · Updated March 13, 2026 · String Methods

javascript es6 string search

The lastIndexOf() method searches for the last occurrence of a substring within a string, starting from the end. It returns the index position where the substring starts, or -1 if the substring is not found. This method is the inverse of indexOf(), which searches from the beginning.

Syntax

string.lastIndexOf(searchString)
string.lastIndexOf(searchString, fromIndex)

searchString: The substring to search for. If omitted, lastIndexOf() searches for the string "undefined".
fromIndex (optional): The index to start searching backward from. Defaults to string.length - 1. If greater than or equal to the string length, the entire string is searched.
Returns: A number representing the index of the last occurrence, or -1 if not found.

TypeScript Signature

interface String {
  lastIndexOf(searchString: string, fromIndex?: number): number;
}

Basic Examples

const text = "hello world hello";

console.log(text.lastIndexOf("hello"));  // 12 (last occurrence)
console.log(text.lastIndexOf("world"));  // 6
console.log(text.lastIndexOf("!"));      // -1 (not found)

Using the fromIndex Parameter

The second parameter controls where the search starts from the end:

const text = "hello world hello";

console.log(text.lastIndexOf("hello"));        // 12 (searches entire string)
console.log(text.lastIndexOf("hello", 10));   // 0 (stops before index 10)

// Finding all occurrences manually
const str = "to be or not to be";
const firstBe = str.lastIndexOf("be");        // 16
const secondBe = str.lastIndexOf("be", 15);   // 3
console.log(firstBe, secondBe);               // 16, 3

Practical Use Case: File Extension Extraction

lastIndexOf() is ideal for extracting file extensions from paths:

function getExtension(filename) {
  const dotIndex = filename.lastIndexOf(".");
  if (dotIndex === -1 || dotIndex === 0) {
    return ""; // No extension or hidden file
  }
  return filename.slice(dotIndex + 1);
}

console.log(getExtension("document.pdf"));      // "pdf"
console.log(getExtension("image.png"));         // "png"
console.log(getExtension("archive.tar.gz"));    // "gz" (last dot)
console.log(getExtension("README"));            // "" (no extension)

Finding the Last Separator in a Path

function getBasename(filepath) {
  const lastSlash = Math.max(
    filepath.lastIndexOf("/"),
    filepath.lastIndexOf("\\")
  );
  return lastSlash === -1 ? filepath : filepath.slice(lastSlash + 1);
}

console.log(getBasename("/home/user/documents/file.txt")); // "file.txt"
console.log(getBasename("C:\\Users\\Admin\\data.csv"));    // "data.csv"

Case Sensitivity

Like indexOf(), lastIndexOf() is case sensitive:

const text = "JavaScript JavaScript";

console.log(text.lastIndexOf("Java"));    // 10
console.log(text.lastIndexOf("java"));    // -1 (lowercase doesn't match)
console.log(text.lastIndexOf("SCRIPT")); // -1 (uppercase doesn't match)

Edge Cases

const text = "hello";

// Empty string returns fromIndex or string length
console.log(text.lastIndexOf(""));        // 5 (string length)
console.log(text.lastIndexOf("", 3));    // 3
console.log(text.lastIndexOf("", -1));   // 5 (treated as 0)

// fromIndex greater than string length
console.log(text.lastIndexOf("h", 100)); // 0 (searches entire string)

// Negative fromIndex
console.log(text.lastIndexOf("h", -5));  // -1 (searches nothing)

// Search string longer than target
console.log(text.lastIndexOf("hello world")); // -1

Key Differences from indexOf()

Aspect	`indexOf()`	`lastIndexOf()`
Search direction	Forward (start → end)	Backward (end → start)
Default fromIndex	0	string.length - 1
Use case	First occurrence	Last occurrence

Browser Compatibility

Browser	Version
Chrome	1+
Firefox	1+
Safari	1+
Edge	12+
Node.js	0.1.0+

Universal support across all modern and legacy JavaScript environments.