RxJS is a library in the ReactiveX family of functional reactive programming libraries. It allows you to develop programs that respond to asynchronous events by composing sequences of observable streams.

Observable streams can send zero or more “data” notifications and terminate with either an “completed” or an “error” notification. Subsequent streams can combine and remix these notifications into new observable patterns, making it possible to compose rich sequences of asynchronous events that clean up resources appropriately when done.

RxJS is available under an Apache license. It is developed by Microsoft Open Source and numerous third-party contributors.