Three built-in iterators in JavaScript

In fact, each iterable object contains not one iterator, but three: values, keys, and entries. Each object type has its own iterator by default. For arrays, this is values, and, for example, for a Map collection, this is entries.

With this knowledge, we can now apply this principle to any collection. For example, let's say we have a Map collection:

javascriptlet map = new Map();
map.set('a', 1);
map.set('b', 2);
map.set('c', 3);

The default iterator for this collection is entries:

javascriptfor (let elem of map) {
  console.log(elem); // ['a', 1], ['b', 2], ['c', 3]
}

But we can easily get the keys of our collection:

javascriptfor (let elem of map.keys()) {
  console.log(elem); // 'a', 'b', 'c'
}

We can also get values:

javascriptfor (let elem of map.values()) {
  console.log(elem); // 1, 2, 3
}