Protokół iterowalności i for...of

// Arrays and strings are iterable
const arr = [10, 20, 30];
for (const n of arr) {
  console.log("arr item:", n);
}

const word = "JS";
for (const ch of word) {
  console.log("char:", ch);
}

// A tiny iterable that counts 0..max-1
const Counter = {
  make: function (max) {
    return {
      max: max,
      // Symbol.iterator returns an iterator object
      [Symbol.iterator]: function () {
        let i = 0;
        return {
          next: function () {
            if (i < max) {
              const out = { value: i, done: false };
              i = i + 1;
              return out;
            }
            return { value: undefined, done: true };
          }
        };
      }
    };
  }
};

for (const x of Counter.make(3)) {
  console.log("count:", x);
}

// You can drive the iterator manually
const iterable = Counter.make(2);
const it = iterable[Symbol.iterator]();

console.log(it.next()); // { value: 0, done: false }
console.log(it.next()); // { value: 1, done: false }
console.log(it.next()); // { value: undefined, done: true }

// Spread (...) and Array.from consume iterables
const small = Counter.make(4);
const a1 = [...small];
const a2 = Array.from(Counter.make(3));

console.log("spread:", a1);
console.log("from :", a2);