function makeIterator(array) {
  let nextIndex = 0;
  return {
    next: function() {
      return nextIndex < array.length ?
      {value: array[nextIndex++], done: false} :
      {done: true};
    }
  };
}

const it = makeIterator(['yo', 'lo']);
console.log(it.next().value); // 'yo'
console.log(it.next().value); // 'lo'
console.log(it.next().done);  // true