Built-in keys iterator in JavaScript

All iterables have the built-in keys iterator that allows you to iterate over keys. Let's test it on an array:

javascriptlet arr = ['a', 'b', 'c'];

We get an iterator:

javascriptlet iter = arr.keys();

And loop through it:

javascriptfor (let elem of iter) {
  console.log(elem); // 0, 1, 2
}