Array.prototype.lastIndexOf()

lastIndexOf(searchElement, fromIndex)

Returns: number · Added in vES5 · Updated March 13, 2026 · Array Methods

array index search

The lastIndexOf() method returns the last index at which a given element can be found in an array. If the element is not found, it returns -1.

How It Differs from indexOf()

const colors = ['red', 'blue', 'green', 'blue', 'yellow'];

colors.indexOf('blue');     // 1 (first occurrence)
colors.lastIndexOf('blue'); // 3 (last occurrence)

Strict Equality Comparison

lastIndexOf() uses strict equality (===) to compare elements. This means type coercion does not occur:

const mixed = [1, '1', true, 'true'];

mixed.lastIndexOf(1);    // 0 (number)
mixed.lastIndexOf('1'); // 1 (string)
mixed.lastIndexOf(true); // 2 (boolean)

The fromIndex Parameter

The optional second parameter fromIndex specifies the starting position for the backward search. By default, it searches from the end of the array.

Important behavior:

When fromIndex is omitted, search starts from arr.length - 1
A positive fromIndex counts forward from the start
A negative fromIndex counts backward from the end

const letters = ['a', 'b', 'c', 'b', 'a', 'b'];

// Search from the end (default)
letters.lastIndexOf('b'); // 5

// Start searching backwards from index 3
letters.lastIndexOf('b', 3); // 3

// Negative fromIndex: start from 2 positions before the end
letters.lastIndexOf('b', -2); // 3

Examples

Finding the Last Occurrence

const sentence = 'the quick brown fox jumps over the lazy dog';
const chars = sentence.split('');

// Find last 'o'
const lastO = chars.lastIndexOf('o');
console.log(lastO); // 43
console.log(chars[lastO]); // 'o'

Limiting Search Range with fromIndex

const items = ['apple', 'banana', 'cherry', 'banana', 'date'];

// Find 'banana' before index 3 (exclusive)
const index = items.lastIndexOf('banana', 2);
console.log(index); // 1

Detecting Duplicate Elements

function hasDuplicate(arr) {
  const unique = new Set(arr);
  return unique.size !== arr.length;
}

function findDuplicates(arr) {
  const duplicates = [Global_Objects::eval];
  
  for (let i = 0; i < arr.length; i++) {
    // If current index equals lastIndexOf, it's the last occurrence
    // Check if we've already found this element earlier
    if (arr.indexOf(arr[i]) !== i) {
      if (!duplicates.includes(arr[i])) {
        duplicates.push(arr[i]);
      }
    }
  }
  
  return duplicates;
}

console.log(hasDuplicate([1, 2, 3, 4]));      // false
console.log(hasDuplicate([1, 2, 2, 3]));      // true
console.log(findDuplicates([1, 2, 3, 2, 4])); // [2]

Notes

Returns -1 when the element is not found
Empty arrays always return -1
fromIndex does not change which index is returned—it only limits where the search begins