Express + Node.js + Mongodb: get parametter attribute list and search by $in operator

routes.js

var express = require('express');
var router = express.Router();

/**
  * Get events from the REST webservice
  * [GET] url: https://mydomain.com/event/params?eventType=festival,gallery,show
  */
router.get('/event', function(req, res) {
    var query = {}; //query for MongoDB
    var arr,subq;
    
    var eventType;
    if (req.query.eventType) {
      eventType = req.query.eventType; //get value of eventType param from the request object.
      //Transform tCSV received to Array
	  subq = {};
      subq["$in"] = [];
      arr = eventType.split(',');
      //Create subquery
	  subq["$in"] = arr; 
	  //Add the subquery to the query object  
      query["eventType"]= subq;
    }
    
	//Get the DB and then the collection:
    var db = req.db;
    var collection = db.collection('events');
  
	//Find and return the response.
    collection.find(query).toArray(function(err, docs) {
        if( err || !docs) {
			console.log("No events found");
			res.end("No events found for: "+query);
		} 
		else {
		    console.log(docs);
			res.json({ events: docs });
		}
    });
});
module.exports = router;

server.js

/*
 * A simple Nodejs and Express server using MongoDB.
 */
var http = require('http');
var path = require('path');
var cors = require('cors');
var bodyParser = require('body-parser');
var express = require('express');

//DB init (MongoDB)
var db = {};
var MongoClient = require('mongodb').MongoClient
  , assert = require('assert');

// Connection URL
var url = 'mongodb://myhost:27017/mydatabase';

// Use connect method to connect to the server
MongoClient.connect(url, function(err, thisdb) {
  db = thisdb;
  assert.equal(null, err);
  console.log("Connected successfully to server");
});

var app = express(); //Creates an Express application
var server = http.createServer(app); //calls http.Server() internally and returns the resulting instance.

//Express configuration:
app.use(express.static(path.resolve(__dirname, 'client')));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
  next();
});
app.use(cors());

// Make our db accessible to our router
app.use(function(req,res,next){
    req.db = db;
    next();
});

//Routes
var events = require('./routes/events');
app.use('/', events)

//Bind and listen for connections on the given host and port
server.listen(process.env.PORT || 3000, process.env.IP || "0.0.0.0", function(){
  var addr = server.address();
  console.log("Swingnow server listening at", addr.address + ":" + addr.port);
});

module.exports = app;