Background: RxJS & Observables

What We'll Cover

  • (Very Brief) Conceptual Roots
  • What is RxJS?
  • What are observables?
  • How are they used in Angular?

Don't let observables scare you away from Angular.

Functional and Reactive Programming (Really) Oversimplified

  • Functional programming prefers:
    • Immutability over state (data doesn't change)
    • Results over steps¬†
    • Composition over structure (lots of pieces)
    • Declarative over imperative style
  • Reactive programming:
    • Asynchronous and event-driven
    • Think of an Excel spreadsheet
    • But what kinds of events?


  • A stream is a sequence of events over time
  • Anything¬†can be a stream, from mouse clicks to data from a server
  • Streams can then be thought of as arrays of values and operated on as such.
    • For example, mouse click coordinates: [(100,200), (300,400), (110,350)]

Put it Together: Functional Reactive Programming (FRP)

  • Around for 20+ years
  • Entire applications can be built around streams
  • Create or identify the streams in your application, then combine and subscribe to them


  • Allows us to build in a declarative style by defining streams, how they are connected, and what happens as new values arrive over time
  • With little to no application state (state is typically stored on certain streams or the DOM)


  • RxJS is the JavaScript version of Microsoft's Reactive Extensions, which enable the creation of reactive applications with something called observables

What are Observables?

  • A data type introduced by RxJS to help us create, subscribe to, and react to streams
  • Asynchronous, push-based sequences of data that can be subscribed to
  • Think of them like an API for the stream, not the stream itself
Single Return Value Multiple Return Values
Object Iterables (Array/Set/ Map)
Push/Asynchronous/Reactive Promise Observable

Common RxJS Operators

var source$ = Rx.Observable.range(1,4); //1,2,3,4

//map & flatMap: change each value
source$.map(x => x*2); //2, 4, 6, 8

//filter: return only selected values based on custom logic
source$.filter(x => x % 2 === 0); //2, 4

//reduce: do a computation on the stream and output final value
source$.reduce((prev, curr) => prev + curr); //10

//scan: do a computation on the stream but output intermittment values
source$.scan((prev, curr) => prev + curr); //1, 3, 6, 10

//take: take the first value
source$.take(1) //1

RxJS in Angular

  • Angular 2+ uses observables in two ways:
    • Internally, to implement core logic like the EventEmitter
    • In the API, specifically in forms and HTTP, to help data flow between components
  • Practically, we use RxJS in three places:
    • Services, where set up our REST calls
    • Components, where we subscribe and use our data.
    • Route resolvers, if we need data to load ahead of time.

We can still use promises.

(I'll show you how.)

Let's get started!