ReactiveX ?

ReactiveX는 Observer 패턴과 Iterator 패턴 그리고 함수형 프로그래밍(functional programming)을 사용한 최선의 조합 이라고 ReactiveX공식 사이트에 기재되어있다.

RxJS ?

Reactive Extensions Library for JavaScript

RxJS is a library for reactive programming using Observables, to make it easier to compose asynchronous or callback-based code. This project is a rewrite of Reactive-Extensions/RxJS with better performance, better modularity, better debuggable call stacks, while staying mostly backwards compatible, with some breaking changes that reduce the API surface