Angular Observables and Promises – How to Use Them

Angular, as we all know, is a JavaScript framework that makes it easy to build web applications. RxJS is really about bringing the principles of functional programming to JavaScript.
Functional Programming in JavaScript?
It all started with the rise of asynchronicity or asynchronous JavaScript, where we have data that is flowing through our application coming from different sources, for example, APIs, Web sockets and User-triggered events and timers. We have a situation where we can’t control when any of this data is sent through our application. Neither can we decide the order in which it happens, so having a more functional paradigm would help resolve that. You want your application to be more predictable, but the scaling up of applications that run client side means less predictability.

Link: https://dzone.com/articles/angular-observables-and-promises-how-to-use-them