Get started with Scala

First things first, in order to use OutWatch you will need to have Java and sbt installed. Check out this guide for Java and this guide for sbt if you’re unsure how to proceed.

Using sbt new

The fastest and easiest way to create a new OutWatch application is by using sbt new. Note that this requires sbt version 0.13.13, so update to that version if you don’t have it yet.

$ sbt new outwatch/seed.g8

This will create a prompt to create your first project. It will be in a new directory of your given name. To compile your Scala code to JavaScript simply run:

$ cd <your-project-name>
$ sbt fastOptJS::webpack

Then open the index.html file in your browser to see what you just created.

Opening your project in IntelliJ

To open your project in IntelliJ IDEA (Community or Ultimate Edition doesn’t matter), simply go to File -> Open and then choose the directory you just created. IntelliJ should now start indexing the files and be ready shortly.

Recompiling your files as needed

Retyping fastOptJS every time we want to compile can get quite annoying and inefficient. Luckily, in sbt, we can run every command in watch mode by simply adding a tilde ~ in front of it.

$ sbt
> ~fastOptJS::webpack
[success] (...)
1. Waiting for source changes... (press enter to interrupt)

You also have the option automatically reload your page on code changes. To do that, first you’ll need to install webpack-dev-server:

$ npm install -g webpack-dev-server

Then, while in sbt, start up the server process:

> fastOptJS::startWebpackDevServer

Then instruct sbt to recompile on source changes:

> ~fastOptJS

And now your server should compile your code and serve it on port 8080. Next up, you’ll want to tell webpack to make a page reload on every change. You can do this by adding the following to your build.sbt:

webpackDevServerExtraArgs := Seq("--inline")

Now if you want to stop the background server process simply do this:

> fastOptJS::stopWebpackDevServer

And voila, you’ve created your first working developer environment. You can skip the rest of this chapter if everything worked for you.

Creating an OutWatch project from scratch

Create a new SBT project and add the ScalaJS plugin to your plugins.sbt. Then add the following line to your build.sbt.

libraryDependencies += "io.github.outwatch" %%% "outwatch" % "0.10.2"

Great, we’ve created our first OutWatch Project!

Now we’ll create a small Hello World app to get you started. First we’ll create a new Scala file HelloOutWatch.scala in our main directory. Inside we’ll want to import the framework by specifying import outwatch.dom._ at the top of our file. We also need to import the Scala.js main class: import scala.scalajs.js.JSApp Now we’re ready to create our main entry point:

import scala.scalajs.js.JSApp
import outwatch.dom._

object HelloOutWatch extends JSApp {
  def main(): Unit = {


Inside our main function we will want to write the following line:

OutWatch.render("#app", h1("Hello World"))

This will render an h1 element into the element with the id app.

Next we’ll need to create an index.html file at your project’s root. Inside we’ll want to add a reference to our js files and also add a node in which we want to display our app:

  <div id="app"></div>
  <script type="text/javascript" src="./target/scala-2.12/scalajs-bundler/main/<your-project-name>-fastopt-bundle.js"></script>

We’re basically done here. We created our first app. Now we only need to run it.

To run this, simply launch sbt and then use the fastOptJS::webpack command to compile your code to JavaScript.

$ sbt
> fastOptJS::webpack

And with that, we’re done, check your browser, to see what we just created (it’s not that special, but it’s something).