SOURCE

{
  let arr = ['hello', 'world'];
  let map1 = arr[Symbol.iterator]();
  // console.log(map1.next());
  // console.log(map1.next());
} {
  let obj = {
    start: [1, 2, 3],
    end: [7, 8, 9],
    [Symbol.iterator]() {
      let self = this;
      let index = 0;
      let arr = self.start.concat(self.end);
      let len = arr.length;
      return {
        next() {
          if (index < len) {
            return {
              value: arr[index++],
              done: false
            }
          } else {
            return {
              value: arr[index++],
              done: true
            }
          }
        }
      }
    }
  }
  for (let key of obj) {
    // console.log(key);
  }
} {
  var it = makeIterator(['a', 'b']);
  // console.log(it.next()); // { value: "a", done: false }
  // console.log(it.next()); // { value: "b", done: false }
  function makeIterator(array) {
    var nextIndex = 0;
    return {
      next: function() {
        return nextIndex < array.length ? {
          value: array[nextIndex++],
          done: false
        }: {
          value: undefined,
          done: true
        };
      }
    };
  }
} {
  let generator = function * () {
    yield 1;
    yield * [2, 3, 4];
    yield 5;
  };
  var iterator = generator();
  console.log(iterator.next());
  console.log(iterator.next());
  console.log(iterator.next());
  console.log(iterator.next());
  console.log(iterator.next());
} {
  var str = new String("hi"); 
  console.log([...str]);
  str[Symbol.iterator] = function() {
    return {
      next: function() {
        if (this._first) {
          this._first = false;
          return {
            value: "bye",
            done: false
          };
        } else {
          return {
            done: true
          };
        }
      },
      _first: true
    };
  };
console.log([...str]);
  console.log(str);
}