Thekeys() method ofArray instances returns a newarray 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 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// 2

Specifications

Browser compatibility

See also

• Polyfill ofArray.prototype.keys incore-js
• es-shims polyfill ofArray.prototype.keys
• Indexed collections guide
• Array
• Array.prototype.entries()
• Array.prototype.values()
• Array.prototype[Symbol.iterator]()
• TypedArray.prototype.keys()
• Iteration protocols