MongoDB is a schemaless NoSQL database system. It saves data in binary JSON format which makes it easier to pass data between client and server. It provides high performance, high availability, and easy scalability because it is a cross-platform, document-oriented database.
ExpressJS is lightweight framework used to build web applications in Node. It is inspired by the popular Ruby framework, Sinatra. It provides a number of robust features for building single and multi page web application.