All iterables have the built-in keys
iterator that allows you to iterate over
keys. Let's test it on an array:
let arr = ['a', 'b', 'c'];
We get an iterator:
let iter = arr.keys();
And loop through it:
for (let elem of iter) {
console.log(elem); // 0, 1, 2
}
Test the work of this iterator
on a Map
collection.
Test the work of this iterator
on a Set
collection.