Scala.js is a compiler that compiles Scala source code to equivalent Javascript code. It can be seen as the start of a revolution in developing web application. In this talk, I'll present Scala.js project, common libraries for having a pleasure web development (such as scalatags, autowire and upickle) and integration with well known javascript libraries such as AngularJS and React.js.

1. MILAN - 08TH OF MAY - 2015
PARTNERS
Hands on ScalaJS
Alberto Paro

2. Engineer from Politecnico of Milan
Working at Big Data Technologies + Consulting
Author of two ElasticSearch books.
Mainly working in Scala (Akka, Spray.io, Playframework, Apache Spark)
Developed big application with Scala.JS
Alberto Paro

3. Table Of Contents
Background information
Why Scala.JS
SBT
Libraries
Tricks and Tips
Questions

4. 1. Why Scala.Js?

5. Why I don’t like Javascript
• A lot of language “defects”
• It’s a script language, not designed to scale large applications (SPA or many modules)
• IDEs are good, but no refactory available
• Hard to be used for large projects
• Verbose if you want safe code.
• Coffescript and similar (Any => Js) try to simplify it.
• A lot of language compiles to javascript (Coffeescript, Microsoft typescript, …)
https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS

6. What’s Scala.JS
"Scala.js is a compiler that converts Scala code into the equivalent, executable Javascript”
Write Scala, not Javascript
The compiler handles the rest
It brings the advantages of Scala into client web development.
Support of “all” Scala (also macros)
Easy interoperability with Javascript
SBT integration
IDE support as “standard” Scala language
Compiled code is reasonable small (GCC => Google Closure Compiler)
SourceMaps for easy debug in Web browser
Interoperability with Javascript (JS => ScalaJS , ScalaJS => JS)
Scala.js code calls Javascript
Scala.js can export classes and methods to Javascript

7. Scala.js Compiler Pipeline
Main.class
Main$.class
Main.Scala
Main.sjsir
Main$.sjsir
script- fastopt.js
500k - 3000Kb
script- opt.js
100k - 1000kb
Scalac
Scala.js
Plugin
Optimizer
Renderer
Google C
C

8. On developing Web Apps
No code reuse between client and server
A least you must learn two languages (Scala + JS)
Algorithms and models must be rewritten
RCP/Ajax calls with “no type”
No compiler checks
Javascript issues:
Forgot a “var”
Js => [“10”, “10”, “10”, “10”].map(parseInt)
[10, NaN, 2, 3]
['10','10','10','10','10'].map(function(x){return
parseInt(x);})

9. On developing Web Apps with Scala.js
Application in only one language
and the language is Scala
Strong typing in the application and RCP layer
Autowire library
Scala typesafe
document.getElementByld(“foo”)
“Standard” behavior
scala.js> List("10", "10", "10", "10").map(parseInt)
List(10, 10, 10, 10)
…

10. Some Semantic differences from JS
• Division by 0 doesn’t throw exception
• Checking the type of number is based on its numeric value, not class instance.
• Scala.Unit is represented as undefined in Javascript
• JavaScript regular expressions are slightly different from Java regular expressions.
• Different behavior of some exceptions
Javascript VM != Java VM

11. 2. SBT

12. name := "Foo root project”
lazy val root = project.in(file(".")).
aggregate(fooJS, fooJVM).
settings(publish := {},
publishLocal := {})
lazy val foo = crossProject.in(file(".")).
settings(
name := "foo",
version := "0.1-SNAPSHOT",
scalaVersion := "2.11.6”,
libraryDependencies += "com.lihaoyi" %%% "scalatags" % "0.4.3"
).
jvmSettings(
// Add JVM-specific settings here
).
jsSettings(
// Add JS-specific settings here )
lazy val fooJVM = foo.jvm
lazy val fooJS = foo.js
Simple multi target JS/JVM
SBT configuration for ScalaJS
<project root>
+- jvm
| +- src/main/scala
+- js
| +- src/main/scala
+- shared
+- src/main/scala

13. 3. Libraries

14. Type safe RPC between client and server
API trait in shared code
Server code implement the backend code
Client calls in a type safe way
// shared interface
trait Api{
def add(x: Int, y: Int, z: Int): Int
}
Safe Server Client Communication
AutoWire
// server-side router and implementation
object Server extends autowire.Server...
object ApiImpl extends Api
def add(x: Int, y: Int, z: Int) = x + y + z
}
// client-side callsite
import autowire._ // needed for correct macro application
object Client extends autowire.Client...
Client[Api].add(1, 2, 3).call(): Future[Int]
// | | |
// | | The T is pickled and wrapped in a Future[T]
// | The arguments to that method are pickled automatically
// Call a method on the `Api` trait

15. https://github.com/lihaoyi/scalatags
libraryDependencies += "com.lihaoyi" %%% "scalatags" % "0.5.1”
ScalaTags is a small XML/HTML construction library for Scala.
Since ScalaTags is pure Scala, any editor that understands Scala will understand scalatags.
Not only do you get syntax highlighting, you also get code completion:
• Autocomplete
• Error Highlighting
• In-editor documentation:
And all the other good things (jump to definition, extract method, etc.) you're used to in a statically typed
language.
No more digging around in templates which mess up the highlighting of your HTML editor, or waiting
months for the correct plugin to materialize.
Strong Typed HTML Syntax for JVM/JS
Scala Tags

16. Strong Typed HTML Syntax for JVM/JS
Scala Tags - Example
html(
head(
script(src:="..."),
script(
"alert('Hello World')"
)
),
body(
div(
h1(id:="title", "This is a title"),
p("This is a big paragraph of text")
)
)
)
<html>
<head>
<script src="..."></script>
<script>alert('Hello World')</script>
</head>
<body>
<div>
<h1 id="title">This is a title</h1>
<p>This is a big paragraph of text</p>
</div>
</body>
</html>

17. https://github.com/greencatsoft/scalajs-angular
libraryDependencies += "com.greencatsoft" %%% "scalajs-angular" % "0.4”
“scalajs-angular aims to help developers build AngularJS based applications in type safe manner with
Scala language.”
Schermata 2015-04-26 alle 10.56.47.png
AngulaJS - scalajs-angular 1/2

18. It provides annotation helper for AngularJS Services, Controllers, Directives, and Filters.
It wraps ngRoute for routing in your SPA.
Schermata 2015-04-26 alle 10.56.47.png
AngulaJS - scalajs-angular 2/2

19. https://github.com/jokade/scalajs-angulate
libraryDependencies += "biz.enef" %%% "scalajs-angulate" % "0.1”
“scalajs-angulate is a small library to simplify developing AngularJS applications in Scala (via Scala.js).
To this end it provides:
• façade traits for the Angular core API
• macros for defining controllers, services and directives in a more natural Scala style”
val module = angular.createModule("counter")
module.controllerOf[CounterCtrl]
class CounterCtrl extends Controller {
var count = 0
def inc() = count += 1
def dec() = count -= 1
// private properties and functions are not exported to the controller scope
private def foo() : Unit = ...
}
AngulaJS - scalajs-angulate 1/2

20. It can be called in a similar HTML fragment:
<html ng-app="counter">
<body>
<div ng-controller="App.CounterCtrl as ctrl">
Count: {{ctrl.count}} <button ng-click="ctrl.inc()">+</button> <button ng-click="ctrl.dec()">&ndash;</button>
</div>
<!-- ... -->
</body>
</html>
Schermata 2015-04-26 alle 10.56.47.png
AngulaJS - scalajs-angulate 2/2

21. I love ReactJS because:
• Your code is clear. It is arranged into components, each with its own defined responsibility. (Web
component, better dividi-et-impera approach)
• Your app is predictable. It’s very clear where data flows, and what happens when a user does
something.
• Your app is fast. React is really, really fast, creating a better experience for users, even if you have a
ton of data. (Virtual DOM, …)
• Your app is standards-based. React adds layers only when it needs to. You feel like you are actually
writing JavaScript and HTML, not some magic template language.
• Surprise, you’re an app developer. React breaks down barriers across platforms, by applying its
same model across the board. This means that once you learn the React way of structuring an web
application, you have a huge head start on developing a native iOS or Android app, thanks to react-
native. Surely this will happen to other platforms.Schermata 2015-04-26 alle 10.56.47.png
Scala Js - React
http://aspiringwebdev.com/why-react-js-matters-for-developers/

22. I love ScalaJS ReactJS because:
• It composed by simple concepts:
• Properties (Props) are immutable
• State is mutable
• a render method
• Everything is strong typed: VDOM,
Components
• Easy to extend components• Schermata 2015-04-26 alle 10.56.47.png
Scala Js - React
Javascript
var HelloMessage = React.createClass({displayName:
'HelloMessage',
render: function() {
return React.createElement("div", null, "Hello ",
this.props.name);
}
});
React.render(React.createElement(HelloMessage, {name:
”Alberto"}), mountNode);• Schermata 2015-04-26 alle 10.56.47.png
Scala
val HelloMessage =
ReactComponentB[String]("HelloMessage")
.render(name => <.div("Hello ", name))
.build
React.render(HelloMessage(”Alberto"), mountNode)Schermata 2015-04-26 alle 10.56.47.png
https://github.com/japgolly/scalajs-react
http://japgolly.github.io/scalajs-react/

23. Scala
Scala Js - React
https://github.com/japgolly/scalajs-react
http://japgolly.github.io/scalajs-react/
ScalaJS React provide also many extra:
• Scalaz support ("com.github.japgolly.scalajs-react" %%% "ext-scalaz71" % "0.8.4")
• Monocle support (Lens for case class) ("com.github.japgolly.scalajs-react" %%% "ext-monocle" %
"0.8.4")
• Testing support ("com.github.japgolly.scalajs-react" %%% "test" % "0.8.4" % "test")
val s = Simulation.focus >> ChangeEventData("hi").simulation >> Simulation.blur
• Module extra ("com.github.japgolly.scalajs-react" %%% "extra" % "0.8.4") with
 Router
 Component Mixins:
 Broadcaster and Listenable
 ExternalVar
 LogLifecycle
 OnUmount
 SetInterval

24. Scala
Scala CSS - CSS Type Safe!
https://github.com/japgolly/scalacss
http://japgolly.github.io/scalacss/book/
ScalaCSS aims to bring type-safety and clarity to:
• creating CSS
• using CSS
• maintaining CSS
• correctness of CSS
You can create standalone CSS like SCSS/LESS, or you can create inline styles to be applied to
directly without the need to manually manage class names.
Being 100% Scala, you benefit from all the normal type-aware advantages like jump-to-definition of a
style, type-safe keys and values, worry-free refactoring, etc.
Example: http://japgolly.github.io/scalacss/book/quickstart/standalone.html

25. Scala
Other Libraries Links
A lot of libraries are linked in Scala.js Homepage (http://www.scala-js.org/)
For a complete projects, the best resources are:
- SPA Tutorial (https://github.com/ochrons/scalajs-spa-tutorial) with doc
(http://ochrons.github.io/scalajs-spa-tutorial/css-in-scala.html):
- Spray
- Autowire
- Scala.JS React
- Scala CSS
- Deploy
- Querki (https://github.com/jducoeur/Querki)
- Scala.Js jquery
- Facades
- Play Scala.Js Example (https://github.com/hussachai/play-scalajs-showcase)
- Gitter Channels

26. 4. Tips & Tricks

27. To manage the javascript library, Scala needs the code signature of the Javascript methods.
First search on scala.js site or github scalajs
trait JQuery extends js.Object {
/**
* Adds the specified class(es) to each of the set of matched elements.
*/
def addClass(classNames:String):JQuery = js.native
def addClass(func:js.ThisFunction2[Element, Int, String, String]):JQuery = js.native
@JSName("after")
def afterInternal(content:js.Any):JQuery = js.native
@JSName("append")
def appendInternal(content:js.Any*):JQuery = js.native
@JSName("appendTo")
def appendToInternal(target:js.Any):JQuery = js.native
@JSName("attr")
def attrInternal(attributeName:String):UndefOr[String] = js.native
Facading Javascript Libraries
$(“div#myId”).addClass("m
yclass")

28. “Automatically” bootstrap your façade from a typescript definition
(https://github.com/borisyankov/DefinitelyTyped): ~900 library already “typed”.
The process is not 100 % accurate, so manual editing is often needed afterwards. This can be improved,
but not to perfection, because the features offered by the type systems of TypeScript and Scala.js differ in
some subtle ways.
Usage
$ sbt 'run somelib.d.ts SomeLib.scala'
Facading Javascript Libraries
https://github.com/sjrd/scala-js-ts-importer

29. 5. Questions

30. MILAN - 08TH OF MAY - 2015
PARTNERS
THANK YOU!
Alberto Paro
alberto.paro@gmail.com
@aparo77
PARTNERS