Does it mutate 😱
Array.prototype.keys()
No mutation
Description
The keys() method returns a new Array Iterator that contains the keys for each index in the array.
arr.keys()https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/keys
Example
var array1 = ['a', 'b', 'c'];
var iterator = array1.keys();
for (let key of iterator) {
console.log(key); // expected output: 0 1 2
}