Reactive programming is a programming paradigm oriented around data flows and the propagation of change.
Reactive Programming raises the level of abstraction of your code so you can focus on the interdependence of events that define the business logic, rather than having to constantly fiddle with a large amount of implementation details. Code in RP will likely be more concise.
MobX is a battle tested library that makes state management simple and scalable by transparently applying functional reactive programming.
Most.js is a toolkit for reactive programming. It helps you compose asynchronous operations on streams of values and events, e.g. WebSocket messages, DOM events, etc.