Object.keys（obj），返回一个数组，数组里是该obj可被枚举的所有属性。请看示例：
示例一：
function Pasta(grain, width, shape) {
this.grain = grain;
this.width = width;
this.shape = shape;
this.toString = function () {
return
(this.grain + ", " + this.width +
", " + this.shape);
}
}
console.log(Object.keys(Pasta));
//console: []
var spaghetti = new Pasta("wheat", 0.2,
"circle");
console.log(Object.keys(spaghetti));
//console: ["grain", "width", "shape",
"toString"]
示例二：
var arr = ["a", "b",
"c"];
console.log(Object.keys(arr)); // console: ["0", "1", "2"]
var obj = { 0 :
"a", 1 : "b",
2 : "c"};
console.log(Object.keys(obj)); // console: ["0", "1", "2"]
var an_obj = { 100: "a",
2: "b", 7:
"c"};
console.log(Object.keys(an_obj)); // console: ["2", "7", "100"]
var my_obj = Object.create({}, { getFoo : { value : function () { return
this.foo } } });
my_obj.foo = 1;
console.log(Object.keys(my_obj)); // console: ["foo"]
