The Iterator Protocol

for...of, spread and array destructuring all work through one agreement, the iterator protocol. An object is iterable when it has a [Symbol.iterator]() method that returns an iterator: an object whose next() returns { value, done }. Arrays, strings, Maps and Sets all follow it, and any class can join in. A generator method, *[Symbol.iterator](), is the shortest way to implement it.

How it reads

• [Symbol.iterator]() is what for...of looks for
• next() returns done: true to say the sequence is over
• Destructuring only pulls as many values as it needs

Cloud tip: Implementing the protocol makes your data structures feel native: every tool that loops just works.