Array.prototype.keys()

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

Try it

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 Language Specification
# sec-array.prototype.keys

Browser compatibility

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See also