Iterators & the Iteration Protocol

The Iterator Protocol

Any object implementing Symbol.iterator() that returns an object with next() method is iterable. This protocol powers for...of, spread, destructuring, and Array.from().

// Custom iterable object:
const counter = {
  from: 1,
  to: 5,
  [Symbol.iterator]() {
    let current = this.from;
    const last = this.to;
    return {
      next() {
        return current <= last
          ? { value: current++, done: false }
          : { value: undefined, done: true };
      }
    };
  }
};

for (const n of counter) console.log(n);  // 1 2 3 4 5
console.log([...counter]);  // [1,2,3,4,5]
const [first, ...rest] = counter;  // destructuring

// Built-in iterables: String, Array, Map, Set, NodeList
const str = "hello";
console.log([...str]);  // ["h","e","l","l","o"]

const map = new Map([["a",1],["b",2]]);
for (const [key, value] of map) {
  console.log(key, value);
}

// Infinite iterator:
function naturals() {
  let n = 0;
  return {
    [Symbol.iterator]() { return this; },
    next() { return { value: n++, done: false }; }
  };
}
function take(iter, n) {
  return Array.from({ length: n }, () => iter.next().value);
}
console.log(take(naturals(), 5));  // [0,1,2,3,4]