fnes_1_이터러블_이터레이터_프로토콜.js

log(String.prototype[Symbol.iterator]);
// ƒ [Symbol.iterator]() { [native code] }
log(Array.prototype[Symbol.iterator]);
// ƒ values() { [native code] }

const iterable1 = [1, 2];
const iterator1 = iterable1[Symbol.iterator]();

log(iterator1.next()); // {value: 1, done: false}
log(iterator1.next()); // {value: 2, done: false}
log(iterator1.next()); // {value: undefined, done: true}
log(iterator1.next()); // {value: undefined, done: true}

for (const val of iterable1) console.log(val);
// 1
// 2

function reverseIterator(list) {
  var cur = list.length;
  return {
    next: () => cur-- > 0 ?
      { value: list[cur], done: false } :
      { value: undefined, done: true }
  }
}

const iterator2 = reverseIterator([1, 2]);

log(iterator2.next()); // {value: 2, done: false}
log(iterator2.next()); // {value: 1, done: false}
log(iterator2.next()); // {value: undefined, done: true}

for (const val of reverseIterator([1, 2])) log(val);
// Uncaught TypeError: reverseIterator(...) is not a function or its return value is not iterable