Object.keys

传入对象，返回属性名组成的数组

    var obj = {"a":123,"b":456};

    console.log(Object.keys(obj))------["a","b"]



    var obj1 = {100:"jdsi","a":3998,9:"dedc"};

    console.log(Object.keys(obj1))-----["100","a","9"]



    var obj2 = Object.create({},{getFoo:{value:function(){
                    return this.foo;
                }}})
    obj2.foo =1;
    console.log(Object.keys(obj2))-----["foo"]

传入字符串，返回索引组成的数组

    var str = "abck123";
    console.log(Object.keys(str))-------[0,1,2,3,4,5,6]

传入构造函数，返回空数组或者属性名

    function Person(name,age,gender){
        this.name = name;
        this.age = age;
        this.gender = gender
        this.toString = function(){
            return (this.name+","+this.age+","+this.gender);
        }
    }

    console.log(Object.keys(Person))----[];

    var spa = new Person("Tom",20,"male");
    console.log(Object.keys(spa))----["name","age","gener","toString"]

传入数组，返回索引组成的数组

    var arr = ["a", "b", "c"];
    console.log(Object.keys(arr)); ---- ["0","1", "2"]