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 May 2018.

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 array1 = ["a", "b", "c"];
const iterator = array1.keys();

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

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

Syntax

js
keys()

Parameters

None.

Return value

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® 2025 Language Specification
# sec-array.prototype.keys

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
keys

Legend

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Full support
Full support

See also