Iterables and Iterators

Iterables and Iterators

Iterables are data structures you can loop through with for...of.

Common iterables

• Arrays
• Strings
• Sets
• Maps
• NodeLists

for (let item of ["a", "b"]) {
  console.log(item);
}

What makes something iterable?

An iterable has a special method called Symbol.iterator. The for...of loop uses it behind the scenes.

Iterators

An iterator is an object that walks through an iterable one step at a time. It has a next() method that returns:

{ value: ... , done: false } // more items to come
{ value: ..., done: true }   // finished

Manual iteration

const it = [10, 20][Symbol.iterator]();
it.next(); // { value: 10, done: false }
it.next(); // { value: 20, done: false }
it.next(); // { value: undefined, done: true }

TL;DR

• Iterables are things you can loop with for...of.
• Arrays, strings, Sets, and Maps are iterable.
• Iterators have a next() method.
• next() returns { value, done }.