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