JS-对象属性的get与set

    var book = {
        _year: 2004,
        edition: 1
    };
    // 参考网址：
    Object.defineProperty(book, "year", {
        // writable : false, // 不能重写 book.year 的属性
        get: function(){
            return this._year;
        },
        set: function(newValue){
            if (newValue > 2004) {
                this._year = newValue;
                this.edition += newValue - 2004;
            }
        }
    });
    book.year = 2005;
    console.log(book.edition);   //2

    // 旧版本支持的语法 - 不能与上面的同时使用，会返回 error
    book.__defineGetter__("year", function(){
        return this._year;
    });
    book.__defineSetter__("year", function(newValue){
        if (newValue > 2004) {
            this._year = newValue;
            this.edition += newValue - 2004;
        }
    });
    book.year = 2005;
    console.log(book.edition);   //2