getter与setter

 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}