MongoDB dynamic variables in MapReduce

I have node.js router for mongodb mapreduce:

app.get('/api/facets/:collection/:groupby', function(req, res) {
    var collection = db.collection(req.params.collection);
    var groupby = req.params.groupby;
    var map = function() {
      if (!this.region) {
        return;
      }
      for (index in this.region) {
        emit(this.region[index], 1);
      }
    }
    var reduce = function(previous, current) {
      var count = 0;

      for (index in current) {
        count += current[index];
      }
      return count;
    }
    var options = {out: groupby + '_facets'};
    collection.mapReduce(map, reduce, options, function (err, collection) {
    collection.find(function (err, cursor) {
        cursor.toArray(function (err, results) {
                res.send(results);
            });            
    })
    })
});

This works good. But I want to use my groupby param. When I try to do something like this:

  var map = function() {
    if (!this[groupby]) {
            return;
        }
    for (index in this[groupby]) {
            emit(this[groupby][index], 1);
        }
  }

I receive TypeError: Cannot call method 'find' of undefined. Is there any way to create such dynamic mapreduce function?

Thanks.

Edited:

Wow! I do it myself. Just pass scope param to mapreduce argument like so scope:{keys: groupby} and then I was able to do var key = this[keys] inside map function and use key variable instead this.region. Great!