var map = function() {
        if(myMap.has(this.user_id)
            myMap.set(this.user_id, myMap.get(this.user_id) + 1)
        else
            myMap.set(this.user_id, 1)

        emit(this.user_id, 1);
    };

    var reduce = function(k,vals) { return 1; };


    collection.mapReduce(map, reduce, {
        scope: {
            myMap: new Map() // is this possible?
        }
    });