Iterables: how for...of really works

The iteration protocol

for...of works on arrays, strings, Maps, Sets, and even your own objects. How does it know how to walk them all? A shared contract with two halves:

• An iterator is any object with a next() method returning { value, done }. Each call hands over the next item, and done: true means finished.
• An iterable is any object that can produce an iterator, via a method stored under the special key Symbol.iterator.

About that key: a symbol is a guaranteed-unique value used as a hidden property key. The language uses well-known symbols like Symbol.iterator so these hook methods can never collide with your normal property names.

The protocol exists to avoid an N×M explosion: the language has many consuming constructs (for...of, destructuring, the Map and Set constructors...) and many collection types. Instead of every construct knowing every type, they all speak { value, done }. Implement the contract once on your own class and all of those constructs work on it for free.