array.keys

The keys() method returns a new Array Iterator that contains the keys for each index in the array.

var arr = ['a', 'b', 'c'];
var iterator = arr.keys();

console.log(iterator.next()); // { value: 0, done: false }
console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: undefined, done: true }

Syntax

arr.keys()

Return value

A new Array iterator object.

Examples

Key iterator doesn't ignore holes

var arr = ['a', , 'c'];
var sparseKeys = Object.keys(arr);
var denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys);  // [0, 1, 2]

Specifications

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.keys' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of 'Array.prototype.keys' in that specification.	Draft

Browser compatibility

See also

• Array.prototype.values()
• Array.prototype.entries()
• Iteration protocols