ECMAScript5新特性

数据属性特性：[[value]] [[writable]] [[enumerable]] [[configurable]]
访问器属性特性：[[enumerable]] [[configurable]] [[set]] [[get]]
要想修改属性默认的特性，就得用Object.defineProperty() 和 Object.defineProperties();

var book = {
    _year: 2004,
    edition: 1
};
Object.defineProperty(book, "year", {
    get: function() {
        return this._year;
    },
    set: function(newValue) {
    if (newValue > 2004) {
    this._year = newValue;
    this.edition += newValue -2004;
    }
    }
});
book.year = 2005;
alert(book.edition);

var book = {};
Object.defineProperties(book, {
    _year: {
        value: 2004
    },
    edition: {
        value: 1,
        writable: true
    },
    year: {
        get: function() {
            return this._year;
        },
        set: function(newValue) {
            if(newValue > 2004) {
                this._year = newValue;
                this.edition += newValue - 2004;
            }
        }
    }

});
book.year = 2005;
alert(book.edition);

上面两者等同

好像实现了类似public private那种修改和访问的权限