为Object实现iterator接口

let obj = {a:1, b:2, c:3};

// method 1
obj[Symbol.iterator] = function() {
  let index = 0,
      list = Object.keys(this).map(e=>this[e]);
  return {
    next: () => {
      return {
        value: list[index++],
        done: index > list.length,
      };
    },
  };
}

// method 2
obj[Symbol.iterator] = function* () {
  let list = Object.keys(this).map(e=>this[e]);
  for(let v of list) {
    yield v;
  }
}

for(let item of obj) {
  console.log(item); // 1 2 3
}