{

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

    let map=arr[Symbol.iterator]();  //返回false时继续执行，true停止执行！

    console.log(map.next());

    console.log(map.next());

    console.log(map.next());

}

{

    let obj={

        start:[,,],

        end:[,,],

        [Symbol.iterator](){

            let self=this;

            let index=;

            let arr=self.start.concat(self.end);

            let len=arr.length;

            return {

                next(){

                    if(index<len){

                        return {

                            value:arr[index++],

                            done:false  //fasle会继续执行

                        }

                    }else {

                        return {

                            value:arr[index++],

                            done:true //truee会结束

                        }

                    }

                }

            }

        }

    }

    for(let key of obj){

        console.log(key);

    }

}