Reactive Programming with JavaScript

Reactive Programming
with JavaScript
@giorgionatili

@giorgionatili // #mobiletea
$ whoami
• Lead Software Engineer (McGraw-Hill Education)
• Community fellow since 2004 (Codeinvaders)
• Open source fanatic
• Founder of Mobile Tea

Mobile Tea
• A free community
• A movement around the two continents
• A place to have fun while learning

Supporters
http://careers.mheducation.com https://dev.twitter.com

Agenda
• An Overview of JavaScript past and future
• Reactive Programming in a Nutshell
• What is Functional Programming
• Asynchronous JavaScript Applications
• Redux, Angular 2 and Reactive Programming

JavaScript

Why people hate it
• Lack of code organization, spaghetti code
everywhere and abuse of globals
• Weird inheritance chain (aka prototype property)
• Callbacks are not straight forward to digest
• The mutability of the this keyword

Why people love it
• It has a rather low entry barrier; people can start
coding and get results quickly
• It’s full of (legacy) examples to start with
• JavaScript is suited for the rest of us, continuously
learning web hackers not having a PhD in CS

It’s your choice

JS, the New Assembly
• Everybody loves it but nobody wants really write it
• Some of the internals are hidden by popular
frameworks (and people is happy about it)
• Only recently syntax and features get a relevant
improvement (ES2015)

JS, What’s Next

Long Story Short
• ECMAScript (ES) standardization started in 1996
• ES 3 was published in 1999 (the age of browsers war)
• ES 5 was published in 2009 (adoption started late
2011)
• ES6 was published in 2015 (early adoption)
• ES Next https://babeljs.io/docs/plugins/

What’s Changing
• The way of thinking in JavaScript (modules, scopes,
arrow functions, classes, etc.)
• The support for asynchronous programming (async
and await, promises, yield, etc.) tasks
• Math, Number, String, Array and object APIs re-
evolution

Array & Arrow Functions
Array.from(document.querySelectorAll('*')) // Returns a real Array
Array.of(1, 2, 3) // Similar to new Array(…),
[0, 0, 0].fill(7, 1) // [0,7,7]
[1, 2, 3].find(x => x == 3) // 3
[1, 2, 3].findIndex(x => x == 2) // 1 
[1, 2, 3, 4, 5].copyWithin(3, 0) // [1, 2, 3, 1, 2]
["a", "b", "c"].entries() // iterator [0, "a"], [1,"b"], [2,"c"]
["a", "b", "c"].keys() // iterator 0, 1, 2
["a", "b", "c"].values() // iterator "a", "b", "c"

Classes & Inheritance
// Pseudo-code of Array
class Array {
constructor(...args) { /* ... */ }
static [Symbol.create]() {
// Install special [[DefineOwnProperty]]
// to magically update 'length'
}
}
// User code of Array subclass
class MyArray extends Array {
constructor(...args) { super(...args); }
}

Proxy Objects
var target = {};
var handler = {
get: function (receiver, name) {
return `Hello, ${name}!`;
}
};
var p = new Proxy(target, handler);
p.world === 'Hello, world!';
custom behavior for fundamental operations

Huge Impact
• Many new features and emerging standards
• Early browsers adoptions through transpilers (babeljs,
traceur, etc.)
• Frameworks are adopting it (e.g. Angular 2, EmberJS,
ReactJS, etc.)

Now Everything is Possible

Reactive Programming

In a Nutshell
• Is a programming paradigm oriented around data
ﬂows
• Variables and property values get updated at runtime
and the system is notiﬁed about changes
• Is similar to the well known observer pattern
• Supports different programming paradigms like
Imperative, Object Oriented and Functional

Why It Matters
• Enables developers creating data streams of
anything, not just from click and hover events
• Data streams emit events that can be handled
asynchronously
• Data streams are data structures and then can be
ﬁltered
• Extends the asynchronous event engine of JavaScript
to data ﬂows

Reactive Systems

Reactive Systems
• Responsive: the system responds in a timely manner
to data stream changes
• Elastic: the system stays responsive under varying
workload
• Message Driven: the system relies on asynchronous
not blocking message-passing
• Resilient: the system stays responsive in the face of
failure (error handling)

Functional Programming

FP definition relies on foreboding statements like
“functions as first-class objects” and “eliminating side
effects”
Definition

• All of your functions must accept at least one
argument
• All of your functions must return data or another
function
• No loops! (What???)
Getting Started

In Practice
function totalForArray(currentTotal, arr) {
currentTotal += arr[0];
// I am not using Array.shift on because
// we're treating arrays as immutable.
var remainingList = arr.slice(1);
// This function calls itself with the remainder of the list, and the
// current value of the currentTotal variable
if(remainingList.length > 0) {
return totalForArray(currentTotal, remainingList);
}
// Unless the list is empty, in which case we return
// the currentTotal value
else {
return currentTotal;
}
}

• First Class Functions: functions treated as objects,
can be passed as arguments and returned as values
• Pure Functions: a function that doesn’t depend on
and doesn’t modify the states of variables out of its
scope
• Immutable Variables: a variable that preserves its
original value after a mutation
Things I Like

First Class Functions
function analyticsHandler(e) {
...//perform some analytics.
}
$('form').on('submit',analyticsHandler);
You really used jQuery? Yes, I am sorry! :)

Pure Functions
var values = { a: 1 };
function impureFunction ( items ) {
var b = 1;
items.a = items.a * b + 2;
return items.a;
}
var c = impureFunction( values );
// Now `values.a` is 3
function pureFunction ( a ) {
var b = 1;
a = a * b + 2;
return a;
}
var d = pureFunction( values.a );
// Values.a is not modified

Immutable Variables
var arr = new ImmutableArray([1, 2, 3, 4]);
var v2 = arr.push(5);
arr.toArray(); // [1, 2, 3, 4]
v2.toArray(); // [1, 2, 3, 4, 5]

• The data should be immutable (creating new data
structures instead of modifying the ones that already
exist)
• The app and its components should be stateless (no
memory of previous execution)
App Perspective

It’s Not a Buzzword

• This book gives a lucid and thorough
account of the concepts and techniques
used in modern functional programming
languages. Standard ML is used for
notation, but the examples can be easily
adapted to other functional languages.
• Originally published: January 1, 1989
• Author: Chris Reade
It’s a Science

Functional Reactive
Programming
• Is a programming paradigm for reactive programming
to which are applied the building blocks of functional
programming (e.g. map, reduce, ﬁlter, merge, etc.)
• It’s implemented in JavaScript by popular libraries
such as bacon.js and RxJS (sounds familiar Java
developers?)

Bacon.js
var up = $('#up').asEventStream('click');
var down = $('#down').asEventStream('click');
var counter =
// map up to 1, down to -1
up.map(1).merge(down.map(-1))
// accumulate sum
.scan(0, function(x,y) { return x + y });
// assign the observable value to jQuery property text
counter.assign($('#counter'), 'text');

Asynchronous

The Callback Hell
var p_client = new Db('integration_tests_20', new Server("127.0.0.1", 27017, {}),
{'pk':CustomPKFactory});
p_client.open(function(err, p_client) {
p_client.dropDatabase(function(err, done) {
p_client.createCollection('test_custom_key', function(err, collection) {
collection.insert({'a':1}, function(err, docs) {
collection.find({'_id':new ObjectID("aaaaaaaaaaaa")}, function(err, cursor) {
cursor.toArray(function(err, items) {
test.assertEquals(1, items.length);
// Let's close the db
p_client.close();
});
});
});
});
});
});

ES6 Promises
var promise = new Promise(function(resolve, reject) {
// do a thing, possibly async, then…
if (/* everything turned out fine */) {
resolve("Stuff worked!");
}
else {
reject(Error("It broke"));
}
});
promise.then(function(result) {
console.log(result); // "Stuff worked!"
}, function(err) {
console.log(err); // Error: "It broke"
});

Async and Await
var request = require('request');
function getQuote() {
var quote;
return new Promise(function(resolve, reject) {
request('http://ron-swanson-quotes.com/v2/quotes', function(error, response, body) {
quote = body;
resolve(quote);
});
});
}
async function main() {
var quote = await getQuote();
console.log(quote);
}
main();
console.log('Ron once said,');

Asynchronous
(made cool!)

Redux in a Nutshell
• Redux is a predictable state container for JavaScript
applications
• Instead of mutating the state directly, you specify the
mutations you want to happen with plain objects called
actions
• Then you write a special function called a reducer to
decide how every action transforms the entire
application’s state
• You should always return the state of the app

Redux Actions
“Actions are payloads of information that send data
from your application to your store”
{ type: 'ADD_TODO', text: 'Use Redux' }
{ type: 'REMOVE_TODO', id: 42 }
{ type: 'LOAD_ARTICLE', response: { ... } }

Redux Reducers
“Actions describe the fact that something happened,
then a reducer handle the event and eventually
update the app state”

Redux Reducers
import * as TodoActions from './todoActions';
const initialState = {
todos: [],
currentFilter: 'SHOW_ALL'
}
export function rootReducer(state = initialState, action){
switch (action.type) {
case TodoActions.ADD_TODO:
return {
todos: state.todos.concat({
id: action.id,
text: action.text,
completed: action.completed
}),
currentFilter: state.currentFilter
};
case TodoActions.TOGGLE_TODO:
return {};
// Continue...
}
}

Redux Stores
“A Redux store is the object that brings together actions
and reducers offering a centralized way to dispatch
actions and access the app state”
import { createStore } from 'redux'
import todoApp from './reducers'
import { addTodo} from './actions'
let store = createStore(todoApp)
// Log the initial state
console.log(store.getState())
// Dispatch some actions
store.dispatch(addTodo('Learn about actions'))

Redux Data Flow
• You call store.dispatch(action)
• The store calls the reducer function you gave it
• The root reducer may combine the output of multiple
reducers into a single state tree
• The store saves the complete state tree returned by
the root reducer

How it Sounds?

Angular 2

Tech Stack
• TypeScript
• RxJS
• JSPM or WebPack
• Grunt (seriously?!?) or Gulp
• Karma + Jasmine or (Mocha + Chai)

TypeScript
• A super set of JavaScript developed by Microsoft
• Implements classes, interfaces, inheritance, strict
data typing, private properties and methods, etc.
• Angular Team choice for the next version of Angular
(Microsoft and Google are now buddies!)

RxJS
• A reactive streams library that allows you to work with
asynchronous data streams
• Combines Observables and Operators so we can
subscribe to streams and react to changes using
composable operations

RxJS, hello world!
$scope.counter = 0;
rx.Observable
.interval(1000)
.take(3)
.safeApply($scope, function(x) {
$scope.counter = x;
})
.subscribe(); // shows 0, 1, 2

Integrating Redux
$ npm install angular2-redux
import {AppStore} from "angular2-redux";
import {bootstrap} from "angular2/platform/browser";
// create factory to be called once angular has been bootstrapped
const appStoreFactory = () => {
let reduxAppStore;
// create redux store
// ...
return new AppStore(reduxStore);
};
// bootstrap angular
bootstrap(MyAppComponent,[provide(AppStore, { useFactory:  
appStoreFactory })]);

Demo
https://plnkr.co/edit/3mhKoOOAKJp27E2FpIOq

Wait a second…

Reactive Programming, functional
programming and OOP?

Using Functions Properly
• Keep your functions pure
• Use your functions as arguments
• Write small reusable functions
• Return functions if needed
• Always return a value

Taking Advantages from RxJS
import {Http} from 'angular2/http';
import {Observable} from 'rxjs/Observable';
import {Observer} from 'rxjs/Observer';
import 'rxjs/add/operator/share';
import {Todo} from 'app/interfaces';
export class TodosService {
todos$: Observable<Todo[]>;
private _todosObserver: Observer<Todo[]>;
private _dataStore: {
todos: Todo[]
};
constructor(private _http: Http) {
// Create Observable Stream to output our data
this.todos$ = new Observable(observer =>
this._todosObserver = observer).share();
this._dataStore = { todos: [] };
}
}

Embracing Redux Data Flows
• Decouple your dependencies by implementing event
driven architecture
• Keep the app state predictable by returning a new
state

Questions and Answers

–Giorgio Natili
“Keep calm and
rock ‘n roll the Codemotion!”
Thanks!

Reactive Programming with JavaScript

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