1  var human = {
 2         _age:18,//下划线是一种常用的记号.用于只能通过对象方法访问的属性
 3         role:"大人"
 4     };
 5     Object.defineProperty(human, "age", {
 6         get: function () {//放回age属性值
 7             return this._age;
 8         },
 9         set: function (ageValue) {//设置对象中的属性值
10             if (ageValue >= 18)
11                 this.role = "大人";            
12             else
13                 this.role = "小孩";
14         }
15     });
16     var descriptor = Object.getOwnPropertyDescriptor(human, "_age");//访问数据属性
17     alert(descriptor.value);//18
18     alert(descriptor.configurable); //true
19     alert(typeof descriptor.get);//undefined
20     descriptor = Object.getOwnPropertyDescriptor(human, "age");//访问访问器属性
21     alert(descriptor.value);//undefined
22     alert(descriptor.enumerable);//false
23     alert(typeof descriptor.get);//function