Array.prototype.keys()

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.

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.

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