JavaScript Array findLastIndex() Method

What You’ll Learn

The findLastIndex() method returns the index of the last element that passes your test, searching from the end. If nothing matches, it returns -1. This ES2023 tutorial covers syntax, five examples, comparisons with findIndex() and findLast(), and practical update/remove patterns.

Introduction

findLast() tells you what matched at the tail. findLastIndex() tells you where it matched—so you can remove it, replace it, or scroll a list to that row.

It is the tail-first partner of findIndex(), added in ES2023 alongside findLast().

Understanding the findLastIndex() Method

array.findLastIndex(callback) invokes your callback starting at the last index and moving toward 0. The first truthy result returns that index immediately.

If every callback is falsy, the result is -1.

📝 Syntax

General form of Array.prototype.findLastIndex:

array.findLastIndex(callback(element, index, array), thisArg)

Parameters

• callback — predicate tested on each element from end to start.
• element — current array item.
• index — optional; position of the element.
• array — optional; the array being searched.
• thisArg — optional; value to use as this inside the callback.

Return value

• Index of the last matching element (0-based).
• -1 if no element passes the test.
• Original array unchanged by the search itself.

Common patterns

• arr.findLastIndex(n => n % 2 === 0) — last even index.
• arr.findLastIndex(x => x.id === id) — last object with id.
• if (i !== -1) arr.splice(i, 1) — remove last match.

Examples Gallery

Open DevTools Console (F12) or use Try-it labs. Example N maps to ?tryit=N.

const numbers = [1, 2, 3, 4, 5, 2];

const lastIndex = numbers.findLastIndex((n) => n % 2 === 0);

console.log(lastIndex);
// 5

5

const numbers = [1, 2, 3, 4, 5, 2];

const lastIndex = numbers.findLastIndex((n) => n > 10);

if (lastIndex === -1) {
  console.log("No element greater than 10 found.");
} else {
  console.log("Index:", lastIndex);
}
// No element greater than 10 found.

No element greater than 10 found.

const numbers = [1, 2, 3, 4, 5, 2];

const firstTwo = numbers.findIndex((n) => n === 2);
const lastTwo = numbers.findLastIndex((n) => n === 2);

console.log(firstTwo);
// 1
console.log(lastTwo);
// 5

1
5

const numbers = [1, 2, 3, 4, 5, 2];
const target = 2;

const lastOccurrence = numbers.findLastIndex((n) => n === target);

console.log(lastOccurrence);
// 5

5

const tags = ["js", "html", "css", "js", "node"];

const i = tags.findLastIndex((t) => t === "js");

if (i !== -1) {
  tags.splice(i, 1);
}

console.log(tags);
// ["js", "html", "css", "node"]

["js", "html", "css", "node"]

🚀 Common Use Cases

• Remove last duplicate — splice the final matching tag or category.
• Update trailing record — replace the most recent matching row.
• Deduplication — keep first occurrence via findLastIndex in filters.
• Log analysis — index of the latest error for detail panels.
• Form arrays — focus the last invalid field in a list.
• Undo stacks — find the last reversible action marker.

📝 Notes

• ES2023—pair with feature detection or polyfills for old browsers.
• Returns -1 on failure, not undefined.
• Index 0 is valid; use index !== -1 for found checks.
• For primitive equality only, lastIndexOf may suffice.
• Short-circuits at the first match from the tail.
• Works on typed arrays with the same signature.

💡 Best Practices