Array

`Array.prototype.findLastIndex`

Iterates the array in reverse order and returns the index of the first element that satisfies the provided testing function

Syntax

JavaScript

array.findLastIndex(callbackFn, thisArg?)

Parameters

Parameter	Type	Description
callbackFn	(element, index, array) => boolean	Function to test each element
thisArg	any	Value to use as this when executing callbackFn

Return Value

The index of the last matching element, or -1

Examples

Basic Usage

const numbers = [5, 12, 50, 130, 44];
const idx = numbers.findLastIndex(n => n > 45);
console.log(idx); // 3

Practical Example

const arr = [1, 2, 3, 4, 3, 2, 1];
console.log(arr.findLastIndex(n => n === 3)); // 4

Advanced Usage

const items = ['a', 'b', 'c', 'b'];
console.log(items.findLastIndex(x => x === 'b')); // 3

Understanding Array.prototype.findLastIndex

The Array.prototype.findLastIndex method in JavaScript iterates the array in reverse order and returns the index of the first element that satisfies the provided testing function. It belongs to the Array object and is one of the most widely used methods for working with array values in modern JavaScript and TypeScript applications.

The method signature is array.findLastIndex(callbackFn, thisArg?). It accepts 2 parameters: callbackFn, thisArg. When called, it returns the index of the last matching element, or -1. Understanding when and how to use findLastIndex() helps you write more expressive, readable code.

Common use cases for Array.prototype.findLastIndex include data transformation, input validation, API response processing, and building reusable utility functions. It works well alongside related methods like array-findindex, array-findlast, array-lastindexof, enabling you to chain operations together for complex data manipulation pipelines.

Supported in Chrome 97+, Firefox 104+, Safari 15.4+, Edge 97+, and Node.js 18+.