one:PRIMARY> use local
switched to db local
one:PRIMARY> db.foo.insert({_id:'filler'})
WriteResult({ "nInserted" : 1 })
one:PRIMARY> x = db.foo.mapReduce(function() {emit(this._id, ":(")}, (key, values)=> values[0], {out: {merge:'oplog.rs'}})
{
	"result" : "oplog.rs",
	"timeMillis" : 12,
	"counts" : {
		"input" : 1,
		"emit" : 1,
		"reduce" : 0,
		"output" : 17
	},
	"ok" : 1
}
one:PRIMARY> db.oplog.rs.find({_id: 'filler'})
{ "_id" : "filler", "value" : ":(" }