Array.prototype.keys()

const array1 = ["a", "b", "c"];
const iterator = array1.keys();
for (const key of iterator) {
  console.log(key);
}
// Expected output: 0
// Expected output: 1
// Expected output: 2

arr.keys();

var arr = ["a", "b", "c"];
var itérateur = arr.keys();
console.log(itérateur.next()); // { value: 0, done: false }
console.log(itérateur.next()); // { value: 1, done: false }
console.log(itérateur.next()); // { value: 2, done: false }
console.log(itérateur.next()); // { value: undefined, done: true }

var arr = ["a", , "c"];
var clésCreuses = Object.keys(arr);
var clésDenses = [...arr.keys()];
console.log(clésCreuses); // ["0", "2"]
console.log(clésDenses); // [0, 1, 2]