Functional Reactive Programming with Bacon.js

Hubchat Tech & Beer 21.4.2016

Olli Mahlamäki / Hubchat


  • Quick introduction
  • Workshop
    • 5 exercises
    • Concrete API tips inbetween

Functional Programming

Important parts

  • Pure functions
  • Avoid state
  • Immutable data
  • Higher order functions


var messages = [
  { from: "olli",  to: "pekka", text: "Hi Pekka" },
  { from: "pekka", to: "olli",  text: "What's up?" },
  { from: "olli",  to: "pekka", text: "Doing a tech an beer!" },
  { from: "pekka", to: "olli",  text: "Ok cool" },

  .filter(function(message) { return isDirectedToMe(message) })
  .map(function(message) { return message.text })

FRP (with bacon)

var messages = [
  { from: "olli",  to: "pekka", text: "Hi Pekka" },
  { from: "pekka", to: "olli",  text: "What's up?" },
  { from: "olli",  to: "pekka", text: "Doing a tech an beer!" },
  { from: "pekka", to: "olli",  text: "Ok cool" },

// Simulate messages coming from server
var messagesE = Bacon.sequentially(1000, messages)

  .filter(function(message) { return isDirectedToMe(message) })
  .map(function(message) { return message.text })


Bacon.js logo

Javascript FRP library

Used in production

Bacon vs. Promises

  • Both solve async cases
  • Promise: 0-1 values
  • EventStream: 0-n values
  • Combo: Bacon.fromPromise(...)

Other FRP libraries

  • RxJS - very similar to Bacon.js, but more popular
  • Reactive extensions (C#)
  • RxJava
  • React ? nothing to do with FRP, but it does work great together with Bacon or RxJS


  • Stream of events with value
  • From user input
  • From other EventStreams


var mappedStream =
  • Creates new stream "mappedStream"
    • Gets a value every time there is a value in stream
    • If the value of stream is x, the value of mappedStream is f(x)
    • f should be a pure function
    • f can be as simple as function() { return true }


var filteredStream = stream.filter(f)
  • Creates new stream "filteredStream"
    • Every time there is a value x in stream, f(x) gets called
    • If the function f returns true, the value goes to filteredStream
    • If not, the value is skipped
    • f should be a pure function

Simple map filter exercise


  • Take 2 (or more) streams as input, return a single stream as output
  • This is the beef that makes FRP more powerful than promises


Merges events from both streams a and b into the result stream.

Result stream ends when both streams have ended.


  • Add listener to the stream
  • This is the place for side effects

myStream.onValue(function(value) {
  // Called every time myStream gets a value

jQuery helpers

// Standard jquery
$("#myButton").on("click", handlerFunction)

// Bacon extension
var clickStream = $("#myButton").asEventStream("click")

Bacon and jQuery


Combines the latest values of property a and stream or property b using a given function. In this example, a self-explanatory plus function is used.

First output event is generated when both a and b have produced a value. The combined stream ends when both the inputs have ended or one has ended without having produced a value.

Calculator Exercise

Bacon <3 React

Bacon vs. Flux

  • You can implement flux in Bacon
  • Actions -> Bacon.Bus()
  • Store -> Bacon.combineTemplate() or Bacon.update()

Bacon & React demo


Creates a new stream for each value in the source stream, using the given function f. Merges the events from all created streams into the result stream.

Result stream ends when both the source stream and all the so-far created streams have ended.


Like flatMap, flatMapLatest creates new streams for each source event. Instead of merging all created streams, it "switches" (hence the former name "switch") between them so that when a new stream is created, the earlierly created stream is no longer listened to.

Result stream ends when source stream has ended and the latest created stream has ended.

Don't worry

  • When you truly understand flatMap & flatMapLatest, everything in Bacon.js becomes simple
  • In the next example, you just need to know that flatMapLatest is used when doing ajax requests

Last exercise

What next


Olli Mahlamäki @omahlama