Iterators and Iterables

An iterator’s next() returns { value, done }. An iterable provides [Symbol.iterator]() returning an iterator. Arrays, strings, maps, sets, spread, destructuring, and for...of use this protocol.

Custom iterable

const countdown = {
  from: 3,
  *[Symbol.iterator]() {
    for (let value = this.from; value > 0; value -= 1) yield value;
  }
};
console.log([...countdown]);

Return a fresh iterator when multiple independent traversals are expected. Iterators may implement return for early-exit cleanup and throw for injected failure. Async iterables implement Symbol.asyncIterator and are consumed with for await...of.