Javascript用定义多个属性方法却赋不了值

var book={};
Object.defineProperties(book,{

_year:{
    value:2004
},
edition:{
    value:1
},
year:{
    get:function () {
        return this._year;
        // body...
    },
    set:function(newValue){
        if (newValue>2004) {
            this._year=newValue;
            this.edition+=newValue-2004;
        }
    }
}
});

   book.year=2005;
   console.log(book.year);