mongo db group by count

mongodb_groupby.js

db.messenger_rooms.group({
    keyf: function(doc){
        var date = new Date(doc.created_at);
        date.setHours(date.getHours() + 9);
        var dateKey = (date.getFullYear()+"-"+(date.getMonth()+1)+"-"+date.getDate());
        return {'day':dateKey};
    },
    cond: {created_at: {$gte: ISODate("2014-04-01"), $lte: ISODate("2014-04-03")}},
    initial: { count: 0, reserved_count: 0},
    reduce: function(doc, aggregator){
        aggregator.count += 1;
        if(doc.reservation_status !== undefined && doc.reservation_status !== null) aggregator.reserved_count++;
    }
});