var obj = {

        get a() {

            return 2;

        },

        get c() {

            return 34

        }

    }

    Object.defineProperty(

        obj,

        'b',

        {

            get: function () {

                return this.a * 2

            },

            enumerable: true

        }

    )

    console.log(obj);

    /*

    * a: 2

    * b: 4

    * c: 34

    * get a: ƒ a()

    * get b: ƒ ()

    * get c: ƒ c()

    //有set就得有get 不然会出现去怪问题

    var obj1 = {

        get a() {

            return this._a_;

        },//有点像初始化变量

        set a(val) {

            this._a_ = val + 4;

        }//类似于变量的赋值 如果没有set obj1.a为undefined 就算下面赋值也不行

    };

    obj1.a = 3;

    console.log(obj1.a)

    /*对象的属性操作符 是否可以枚举*/

    var obj2 = {name:'hxq'};

    Object.defineProperty(

        obj2,

        'a',

        {

            value:2,

            enumerable:true

        }

    )

    Object.defineProperty(

        obj2,

        'b',

        {

            value:3,

            enumerable:false

        }

    )

    console.log(obj2);

    /*for in 遍历下标*/

    for (let i in [1,1,1,3,232,3]){

        console.log(i)  //可以通过遍历下标来遍历值

    }

//    for of 直接遍历值

    for  (let i of [21312,,1,31,3,123,1]){

        console.log(i)

    }

    /*for of 原理内置的@@iterator来手动遍历数组*/

    var arr=[12,231,32,32];

    var it=arr[Symbol.iterator]();

    console.log(it)

    console.log(it.next());//{value: 12, done: false}

    console.log(it.next());//{value: 231, done: false}

    console.log(it.next());//{value: 32, done: false}

    console.log(it.next());//{value: 32, done: false}

    console.log(it.next());//{value: undefin, done: true}