files/en-us/web/javascript/reference/global_objects/array/findlastindex/index.md

---
title: Array.prototype.findLastIndex()
short-title: findLastIndex()
slug: Web/JavaScript/Reference/Global_Objects/Array/findLastIndex
page-type: javascript-instance-method
browser-compat: javascript.builtins.Array.findLastIndex
sidebar: jsref
---

The **`findLastIndex()`** method of {{jsxref("Array")}} instances iterates the array in reverse order and returns the index of the first element that satisfies the provided testing function.
If no elements satisfy the testing function, -1 is returned.

See also the {{jsxref("Array/findLast", "findLast()")}} method, which returns the value of last element that satisfies the testing function (rather than its index).

{{InteractiveExample("JavaScript Demo: Array.prototype.findLastIndex()", "shorter")}}

```js interactive-example
const array = [5, 12, 50, 130, 44];

const isLargeNumber = (element) => element > 45;

console.log(array.findLastIndex(isLargeNumber));
// Expected output: 3
// Index of element with value: 130
```

## Syntax

```js-nolint
findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)
```

### Parameters

- `callbackFn`
  - : A function to execute for each element in the array. It should return a [truthy](/en-US/docs/Glossary/Truthy) value to indicate a matching element has been found, and a [falsy](/en-US/docs/Glossary/Falsy) value otherwise. The function is called with the following arguments:
    - `element`
      - : The current element being processed in the array.
    - `index`
      - : The index of the current element being processed in the array.
    - `array`
      - : The array `findLastIndex()` was called upon.
- `thisArg` {{optional_inline}}
  - : A value to use as `this` when executing `callbackFn`. See [iterative methods](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#iterative_methods).

### Return value

The index of the last (highest-index) element in the array that passes the test.
Otherwise `-1` if no matching element is found.

## Description

The `findLastIndex()` method is an [iterative method](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#iterative_methods). It calls a provided `callbackFn` function once for each element in an array in descending-index order, until `callbackFn` returns a [truthy](/en-US/docs/Glossary/Truthy) value. `findLastIndex()` then returns the index of that element and stops iterating through the array. If `callbackFn` never returns a truthy value, `findLastIndex()` returns `-1`. Read the [iterative methods](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#iterative_methods) section for more information about how these methods work in general.

`callbackFn` is invoked for _every_ index of the array, not just those with assigned values. Empty slots in [sparse arrays](/en-US/docs/Web/JavaScript/Guide/Indexed_collections#sparse_arrays) behave the same as `undefined`.

The `findLastIndex()` method is [generic](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#generic_array_methods). It only expects the `this` value to have a `length` property and integer-keyed properties.

## Examples

### Find the index of the last prime number in an array

The following example returns the index of the last element in the array that is a prime number, or `-1` if there is no prime number.

```js
function isPrime(n) {
  if (n < 2) {
    return false;
  }
  if (n % 2 === 0) {
    return n === 2;
  }
  for (let factor = 3; factor * factor <= n; factor += 2) {
    if (n % factor === 0) {
      return false;
    }
  }
  return true;
}

console.log([4, 6, 8, 12].findLastIndex(isPrime)); // -1, not found
console.log([4, 5, 7, 8, 9, 11, 12].findLastIndex(isPrime)); // 5
```

> [!NOTE]
> The `isPrime()` implementation is for demonstration only. For a real-world application, you would want to use a heavily memoized algorithm such as the [Sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes) to avoid repeated calculations.

### Using the third argument of callbackFn

The `array` argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first uses `filter()` to extract the positive values and then uses `findLastIndex()` to find the last element that is less than its neighbors.

```js
const numbers = [3, -1, 1, 4, 1, 5, 9, 2, 6];
const lastTrough = numbers
  .filter((num) => num > 0)
  .findLastIndex((num, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    if (idx > 0 && num >= arr[idx - 1]) return false;
    if (idx < arr.length - 1 && num >= arr[idx + 1]) return false;
    return true;
  });
console.log(lastTrough); // 6
```

### Using findLastIndex() on sparse arrays

You can search for `undefined` in a sparse array and get the index of an empty slot.

```js
console.log([1, , 3].findLastIndex((x) => x === undefined)); // 1
```

### Calling findLastIndex() on non-array objects

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

```js
const arrayLike = {
  length: 3,
  0: 2,
  1: 7.3,
  2: 4,
  3: 3, // ignored by findLastIndex() since length is 3
};
console.log(
  Array.prototype.findLastIndex.call(arrayLike, (x) => Number.isInteger(x)),
); // 2
```

## Specifications

{{Specifications}}

## Browser compatibility

{{Compat}}

## See also

- [Polyfill of `Array.prototype.findLastIndex` in `core-js`](https://github.com/zloirock/core-js#array-find-from-last)
- [es-shims polyfill of `Array.prototype.findLastIndex`](https://www.npmjs.com/package/array.prototype.findlastindex)
- [Indexed collections](/en-US/docs/Web/JavaScript/Guide/Indexed_collections) guide
- {{jsxref("Array")}}
- {{jsxref("Array.prototype.find()")}}
- {{jsxref("Array.prototype.findIndex()")}}
- {{jsxref("Array.prototype.findLast()")}}
- {{jsxref("Array.prototype.indexOf()")}}
- {{jsxref("Array.prototype.lastIndexOf()")}}
- {{jsxref("TypedArray.prototype.findLastIndex()")}}