在红宝书第六章开头部分，提到了JavaScript对象的属性分为数据属性和访问器属性，其中有两种定义属性的方法 ：Object.defineProperty()和Object.defineproperties(),对着书上的代码写了一遍之后，出现了奇怪的结果：

Object.defineProperty()

var book = {
    _year: 2014,
    edition: 1
};
Object.defineProperty(book,"year",{
    get: function() {
        return this._year;
    },
    set: function(newValue){
        if (newValue > 2014) {
            this._year = newValue;
            this.edition += newValue - 2014;
        };
    }
});
book.year = 2015;
console.log(book.edition); //结果为2

Object.defineProperties

var book = {};
Object.defineProperties(book,{
    _year: {
        value: 2014
    },
    edition: {
        value: 1
    },
    year: {
        get: function() {
            return this._year;
        },
        set: function(newValue) {
            if (newValue > 2014) {
                this._year = newValue;
                this.edition += newValue - 2014
            };
        }
    }
});
book.year = 2015;
console.log(book.edition);  //结果为1

请问这是为什么呢？