JavaScript Array lastIndexOf() Method

What You’ll Learn

The lastIndexOf() method returns the last index where a value appears, searching from the end toward the start. It returns -1 if missing. This tutorial covers syntax, five examples, backward fromIndex, comparison with indexOf(), and practical removal of the last duplicate.

Introduction

When duplicates exist, indexOf() finds the first match. lastIndexOf() finds the final one—useful for trimming trailing duplicates, parsing paths from the end, or removing the most recent occurrence of a tag or token.

Like indexOf(), it uses strict equality and returns a numeric index or -1. The array is not modified.

Understanding the lastIndexOf() Method

array.lastIndexOf(searchElement, fromIndex?) scans backward from fromIndex (default: last element) toward index 0. The first match encountered in that direction is the last occurrence in the array (within the search range).

Index 0 is valid, so always test with !== -1, not a truthy check on the index.

📝 Syntax

General form of Array.prototype.lastIndexOf:

array.lastIndexOf(searchElement, fromIndex)

Parameters

• searchElement — value to locate.
• fromIndex — optional; index to start searching backward (default length - 1).

Return value

• Index of the last matching element (0 or greater).
• -1 if no match.
• Original array unchanged.

Common patterns

• arr.lastIndexOf(x) — last occurrence index.
• arr.lastIndexOf(x, i) — search backward from index i.
• arr.indexOf(x) !== arr.lastIndexOf(x) — has duplicates?
• arr.splice(arr.lastIndexOf(x), 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, 2, 5, 2];

console.log(numbers.lastIndexOf(2));
// 6

6

const colors = ["red", "blue", "green", "blue", "yellow"];

console.log(colors.indexOf("blue"));
// 1

console.log(colors.lastIndexOf("blue"));
// 3

1
3

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

console.log(numbers.lastIndexOf(2));
// 6 (last "2")

console.log(numbers.lastIndexOf(2, 5));
// 4 (search only up to index 5)

6
4

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

const index = numbers.lastIndexOf(7);

console.log(index);
// -1

if (index !== -1) {
  console.log("Found at", index);
} else {
  console.log("7 not in array");
}
// 7 not in array

-1
7 not in array

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

const lastIdx = numbers.lastIndexOf(target);

if (lastIdx !== -1) {
  numbers.splice(lastIdx, 1);
}

console.log(numbers);
// [1, 2, 3, 4, 2, 5]

[1, 2, 3, 4, 2, 5]

🚀 Common Use Cases

• Remove last duplicate — splice at lastIndexOf.
• Path segments — find last slash or delimiter index.
• Detect duplicates — compare with indexOf.
• Scoped backward search — limit with fromIndex.
• Undo stack logic — find most recent matching entry.
• Tag lists — remove the last instance of a label.

📝 Notes

• Returns last match only—use indexOf for first.
• Not found → -1 (index 0 is valid).
• Strict ===; does not find NaN.
• Searches single values, not sub-array sequences.
• For custom tests, use findLastIndex() (ES2023).
• ES5—excellent support including IE9+.

💡 Best Practices