Array.prototype.keys()

const array = ["a", "b", "c"];
const iterator = array.keys();

for (const key of iterator) {
  console.log(key);
}

// : 0
// : 1
// : 2

keys()

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

const arrayLike = {
  length: 3,
};
for (const entry of Array.prototype.keys.call(arrayLike)) {
  console.log(entry);
}
// 0
// 1
// 2