array.keys

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 }

arr.keys()

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