RxJS is the reactive programming library that ports the Reactive Extensions (originally from Microsoft's .NET world) to JavaScript. It models async data — events, HTTP responses, websocket messages, user input — as Observables, which can be composed, filtered, mapped, debounced, and combined with operators like map, switchMap, combineLatest, and scan.
Angular ships RxJS as a core dependency, which is most of why the library still has significant production presence. React projects rarely use RxJS directly anymore — TanStack Query and Zustand cover what teams used to reach for RxJS for. The mental model (declarative async pipelines) is valuable beyond the specific library.
