Tidy and professionalise existing MEAN + socket.io code

We have about 300 lines of Node.js with Express, Mongoose, [login to view URL], (and maybe nodemon and pm2 - not sure) which were written by a hobbyist. It is basically a chat which people can view anonymously or login to participate, and also downloads details on each room. It works perfectly but needs to be made more professional and robust. For instance, it is missing appropriate error handling. A professional experienced coder would be able to correct bad practices, for instance, the connecting code to MongoDb may be outdated (we are getting a message to pass the option {useNewUrlParser: true} to MongoClient). Also, there are some dependencies we do not think are being used, such as nodemon and pm2. We would like advice on those, and are open to suggestions on further improvements and deployment to cope with varying audience size.. For an experienced MEAN coder (+[login to view URL]), this is a straightforward tidy-up.