//iterator  for ...of循环

 {

     let arr=['hello','world'];

     let map=arr[Symbol.iterator]();

     console.log(map.next())

     console.log(map.next())

     console.log(map.next())

 }

 {

     let obj={

         start:[1,3,2],

         end:[7,9,8],

         [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)

     }

 }

 {

     let arr=['hello','world'];

     for(let value of arr){

         console.log('value',value)

     }

 }