RxDB (short for Reactive Database) is an offline-first, NoSQL-database for JavaScript Applications like Websites, hybrid Apps, Electron-Apps, Progressive Web Apps and Node.js. Reactive means that you can not only query the current state, but subscribe to all state changes like the result of a query or even a single field of a document. This is great for UI-based realtime applications in a way that makes it easy to develop and also has great performance benefits but can also be used to create fast backends in Node.js. RxDB provides an easy to implement protocol for realtime replication with your existing infrastructure or any compliant CouchDB endpoint.