.keys()

Returns a new Array Iterator object that contains the keys for each index in the array.

Examples:

let fruits = ['apple', 'banana', 'mango'];
let keys = fruits.keys();
for (let key of keys) {
  console.log(key);
}
// 0
// 1
// 2

let items = ['item1', 'item2', 'item3'];
let itemKeys = items.keys();
console.log(Array.from(itemKeys)); // [0, 1, 2]