RxJS

RxJS is a library for composing asynchronous and event-based programs by using observable sequences. It is a JavaScript implementation of the Reactive Extensions (ReactiveX) standard. At its core are Observables, which represent streams of values that can emit items over time. Observers subscribe to observables to receive next values, errors, and completion signals. Observables are lazy and cancellable: execution starts when subscribed and can be stopped by unsubscribing.

RxJS provides a wide collection of operators to create, transform, filter, combine, and time-shift observable sequences. Operators are typically used through a pipe mechanism, enabling readable, declarative data-flow pipelines. Common creation operators include of, from, fromEvent, interval, and timer. Transforming operators such as map, filter, and tap, and combination operators such as merge, concat, zip, combineLatest, and switchMap enable complex asynchronous logic.

Subjects are both an observable and an observer, allowing multicasting to multiple subscribers. Variants such as BehaviorSubject, ReplaySubject, and AsyncSubject offer different replay behavior and lifecycle semantics. Schedulers provide control over when a subscription executes and on which execution context, aiding in controlling concurrency and timing.

RxJS is widely used in web development and is a core part of Angular, though it can be used with plain JavaScript or TypeScript and with other frameworks. It originated from the Reactive Extensions family and has evolved through modern versions. The library is open source under the MIT license and emphasizes composability, declarative data flow, and robust error handling. Marble diagrams are often used to illustrate timing in testing and documentation.