Array.prototype.keys()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The keys() method of Array instances returns a new array iterator object that contains the keys for each index in the array.

Try it

const array = ["a", "b", "c"];
const iterator = array.keys();

for (const key of iterator) {
  console.log(key);
}

// Expected output: 0
// Expected output: 1
// Expected output: 2

Syntax

keys()

Parameters

None.

Return value

A new iterable iterator object.

Description

When used on sparse arrays, the keys() method iterates empty slots as if they have the value undefined.

The keys() method is generic. It only expects the this value to have a length property and integer-keyed properties.

Examples

Using keys() on sparse arrays

Unlike Object.keys(), which only includes keys that actually exist in the array, the keys() iterator doesn't ignore holes representing missing properties.

js
const arr = ["a", , "c"];
const sparseKeys = Object.keys(arr);
const denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys); // [0, 1, 2]

Calling keys() on non-array objects

The keys() method reads the length property of this and then yields all integer indices between 0 and length - 1. No index access actually happens.

js
const arrayLike = {
  length: 3,
};
for (const entry of Array.prototype.keys.call(arrayLike)) {
  console.log(entry);
}
// 0
// 1
// 2

Specifications

Specification

ECMAScript® 2027 Language Specification
# sec-array.prototype.keys

Browser compatibility

Enable JavaScript to view this browser compatibility table.

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This page was last modified on Jul 20, 2025 by MDN contributors.

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Standard built-in objects
Array
Constructor
1. Array()
Static methods
Static properties
1. [Symbol.species]
Instance methods
Instance properties
1. length
2. [Symbol.unscopables]
Inheritance
Object/Function
Static methods
Static properties
Instance methods
Instance properties
1. __proto__
2. constructor