Array.prototype.lastIndexOf()

Overview.

The Array.prototype.lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, from the end to the beginning.

Syntax

array.lastIndexOf(searchElement[, fromIndex])

Parameters

searchElement (required): The element to search for in the array.
fromIndex (optional): The index at which to start searching backwards. By default, the search starts at the last index in the array.

return value

The index of the last occurrence of the specified element in the array, or -1 if it is not found.

Examples

const arr = [1, 2, 3, 4, 5, 4, 3, 2, 1];
const lastIndex = arr.lastIndexOf(4);
console.log(lastIndex); // Output: 5

Use Cases

Searching for the last occurrence of a value in an array

const arr = [1, 2, 3, 4, 5, 4, 3, 2, 1];
const lastIndex = arr.lastIndexOf(4);
console.log(lastIndex); // Output: 5

Checking if an element exists in an array

const arr = [1, 2, 3, 4, 5, 4, 3, 2, 1];
const hasElement = arr.lastIndexOf(6) !== -1;
console.log(hasElement); // Output: false

Finding the last index of a specific value in an array

const arr = [1, 2, 3, 4, 5, 4, 3, 2, 1];
const lastIndex = arr.lastIndexOf(3);
if (lastIndex !== -1) {
  arr.splice(lastIndex, 1);
}
console.log(arr); // Output: [1, 2, 3, 4, 5, 4, 2, 1]