Javascript访问器属性实例分析

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 = 2006;
console.log(book.edition);