A realtime Database for JavaScript Applications
RxDB (short for Reactive Database) is a NoSQL-database for JavaScript Applications like Websites, hybrid Apps, Electron-Apps and NodeJs. 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 useful for UI-based apps that always display the realtime state to the user. RxDB can do a realtime replication with any CouchDB compliant endpoint and also with GraphQL endpoints.
If you are new to RxDB, you should start here.