SOURCE

const arr=[1,2,3];
function iterator(arr){
  let index=0;  //定变量
  return{   // 返回值就是对象指针
    next:function(){  //next方法
      return index<arr.length?  //next的返回值要做判断
      {value:arr[index++],done:false}:
      {value:undefined,done:true};
    }
  }
}
const it=iterator(arr);
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());