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JavaScript has a formal iterator protocol: any object with a next() method that returns { value, done } is iterable.

The [Symbol.iterator] is the magic protocol method. for...of, spread, destructuring all use it.

Generators are a much easier way to make iterators:

The function* declares a generator. yield pauses, returning a value; the function resumes on the next .next() call.

Generators have state:

Async iteration with for await:

This is what powers things like Node's fs.readFile streams, GraphQL subscriptions, and message queue consumers.