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!