code improvements

backend/server.js

@@ -20,7 +20,7 @@ app.set ('view engine', 'ejs');
 app.use (require ('./middleware')); //common middleware for all requests
 app.use (require ('./controllers')); //all routes
 
-MongoClient.connect (config.dbURL)
+MongoClient.connect (config.DB_URL)
   .then (database => {
     databaseHelper.initModule (database);