Array.prototype.keys()
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
Thekeys() method ofArray instances returns a newarray 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: 2Syntax
keys()Parameters
None.
Return value
A newiterable iterator object.
Description
When used onsparse arrays, thekeys() method iterates empty slots as if they have the valueundefined.
Thekeys() method isgeneric. It only expects thethis value to have alength property and integer-keyed properties.
Examples
Using keys() on sparse arrays
UnlikeObject.keys(), which only includes keys that actually exist in the array, thekeys() iterator doesn't ignore holes representing missing properties.
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
Thekeys() method reads thelength property ofthis and then yields all integer indices between 0 andlength - 1. No index access actually happens.
const arrayLike = { length: 3,};for (const entry of Array.prototype.keys.call(arrayLike)) { console.log(entry);}// 0// 1// 2Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-array.prototype.keys |