Searching Arrays with indexOf and lastIndexOf

In JavaScript, the indexOf() method returns the first index at which a specified element can be found in an array. If it does not exist in the array, -1 is returned instead. Similarly, lastIndexOf() returns the last index where an element appears or -1 if it doesn't exist within the array. These methods are incredibly useful when searching for elements and are commonly used in various coding instances.

Examples

Using lastIndexOf()

The following example uses lastIndexOf() to locate values in an array.

const numbers = [2, 5, 9, 2];
numbers.lastIndexOf(2); // 3
numbers.lastIndexOf(7); // -1
numbers.lastIndexOf(2, 3); // 3
numbers.lastIndexOf(2, 2); // 0
numbers.lastIndexOf(2, -2); // 0
numbers.lastIndexOf(2, -1); // 3

You cannot use lastIndexOf() to search for NaN.

const array = [NaN];
array.lastIndexOf(NaN); // -1

Finding all the occurrences of an element

The following example uses lastIndexOf to find all the indices of an element in a given array, using push to add them to another array as they are found.

const indices = [];
const array = ["a", "b", "a", "c", "a", "d"];
const element = "a";
let idx = array.lastIndexOf(element);
while (idx !== -1) {
  indices.push(idx);
  idx = idx > 0 ? array.lastIndexOf(element, idx - 1) : -1;
}

console.log(indices);
// [4, 2, 0]

Note that we have to handle the case idx === 0 separately here because the element will always be found regardless of the fromIndex parameter if it is the first element of the array. This is different from the indexOf method.

Using lastIndexOf() on sparse arrays

You cannot use lastIndexOf() to search for empty slots in sparse arrays.

console.log([1, , 3].lastIndexOf(undefined)); // -1

Calling lastIndexOf() on non-array objects

The lastIndexOf() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 2,
  3: 5, // ignored by lastIndexOf() since length is 3
};
console.log(Array.prototype.lastIndexOf.call(arrayLike, 2));
// 2
console.log(Array.prototype.lastIndexOf.call(arrayLike, 5));
// -1