var book = {};
    
    Object.defineProperties(book, {
        _year: {
            value: 2004
        },
        
        edition: {
            value: 1
        },
        
        year: {            
            get: function(){
                return this._year;
            },
            
            set: function(newValue){
                if (newValue > 2004) {
                    this._year = newValue;
                    this.edition += newValue - 2004;
                }                  
            }            
        }        
    });
       
    var descriptor = Object.getOwnPropertyDescriptor(book, "_year");
    alert(descriptor.value);          //2004
    alert(descriptor.configurable);   //false
    alert(typeof descriptor.get);     //"undefined"
    
    var descriptor = Object.getOwnPropertyDescriptor(book, "year");
    alert(descriptor.value);          //undefined
    alert(descriptor.enumerable);     //false
    alert(typeof descriptor.get);     //"function"
    

    以下这两行看不懂
    alert(descriptor.configurable);   //false
    alert(descriptor.enumerable);     //false
    对象默认的可配置属性和可枚举属性是true吧
    示例代码未设置这两个属性，默认的应该为true吧
    
    PS：红宝书6.1.3（24印143页）