Array.prototype.findLastIndex(callback) returns the index of the last element that passes your test. It scans from the end toward the start (ES2023). No match → -1. It does not mutate the array.
array.findLastIndex(callback)
scans: last → first
match → that index
no match → -1
findIndex() → first match index
findLast() → last match value (not index)Call array.findLastIndex. Start with this line.
array.findLastIndex(callback, thisArg?)| Call | What it does |
|---|---|
array.findLastIndex(callback, thisArg?) | Primary signature. |
array.findLastIndex(...) | Call with the arguments your case needs. |
Built into JavaScript. Call findLastIndex(...) on an array instance — no import required.
array.findLastIndex(callback, thisArg?)| Parameter | Description |
|---|---|
| Array | Required. Source to search |
| callback | Required. Return truthy for a match |
| thisArg | Optional. Optional this for the callback |
| Return Value | Description |
|---|---|
| * | Index number, or -1 |
Find the index of the last even number.
const numbers = [1, 2, 3, 4, 5, 2];
const lastEven = numbers.findLastIndex((n) => n % 2 === 0);
console.log(lastEven); // 51 and 5.n % 2 === 0.5 is even → return immediately.numbers stays unchanged; you only get an index.-1Always check for -1 before using the result as an index.
const numbers = [1, 3, 5];
const idx = numbers.findLastIndex((n) => n % 2 === 0);
console.log(idx); // -1
if (idx === -1) {
console.log("No even number");
}-1.-1 before reading or writing arr[idx].-1, not undefined.findLastIndex() vs findIndex()Same test, different direction: first match vs last match.
const numbers = [1, 2, 3, 4, 5, 2];
const isTwo = (n) => n === 2;
console.log(numbers.findIndex(isTwo)); // 1
console.log(numbers.findLastIndex(isTwo)); // 5n === 2.2 at index 1.2 at index 5.findIndex; last hit → findLastIndex.Array.prototype.findLastIndex() is part of ES2023. Evergreen browsers shipped it around 2022; Node.js includes it from version 18.
Supported in Chrome 97+, Firefox 104+, Safari 15.4+, Edge 97+, and Node 18+. Not available in Internet Explorer.
Bottom line: Safe for modern web apps and current Node LTS releases. For older targets, feature-detect with if ("findLastIndex" in Array.prototype) and fall back to a reverse loop or a polyfill.
Learn how to flatten nested arrays into a single level.
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