This step-by-step tutorial where we start with the setup of a Scala.js sbt project and end up having some user interaction and unit testing. The code created in this tutorial is available with one commit per step in the scalajs-tutorial repository on GitHub.
Note for Scala.js 1.x users: The present tutorial is targeted at the latest stable version of Scala.js, i.e., 0.6.32. Some details may vary if you try to follow along with Scala.js 1.x. Please consult relevant pages of the documentation for any discrepancies.
To go through this tutorial, you will need to download & install sbt. Note that no prior sbt knowledge (only a working installation) is required to follow the tutorial.
You will also need to download & install Node.js.
First create a new folder where your sbt project will go.
To setup Scala.js in a new sbt project, we need to do two things:
- Add the Scala.js sbt plugin to the build
- Enable the plugin in the project
Adding the Scala.js sbt plugin is a one-liner in
project/plugins.sbt (all file names we write in this tutorial are relative to the project root):
We also setup basic project settings and enable this plugin in the sbt build file (
build.sbt, in the project root directory):
Last, we need a
project/build.properties to specify the sbt version (you can find the latest version here):
That is all we need to configure the build.
If at this point you prefer to use an IDE, you can import the build into VS Code with Metals (or any other editor supported by Metals) or IntelliJ IDEA (see “Installation” here). Note that for compiling and running your application, you will still need to use sbt from the command line.
For starters, we add a very simple
TutorialApp in the
tutorial.webapp package. Create the file
As you expect, this will simply print “HelloWorld” when run. To run this, simply launch
sbt and invoke the
$ sbt sbt:Scala.js Tutorial> run [info] Compiling 1 Scala source to (...)/scala-js-tutorial/target/scala-2.13/classes ... [info] Fast optimizing (...)/scala-js-tutorial/target/scala-2.13/scala-js-tutorial-fastopt.js [info] Running tutorial.webapp.TutorialApp Hello world! [success] (...)
Source maps in Node.js: To get your stack traces resolved on Node.js, you will have to install the
$ npm install source-map-support
- Create an HTML page which includes that file
> fastOptJS [info] Fast optimizing (...)/scala-js-tutorial/target/scala-2.13/scala-js-tutorial-fastopt.js [success] (...)
This will perform some fast optimizations and generate the
(It is possible that the
[info] does not appear, if you have just run the program and not made any change to it.)
Create the HTML Page
scalajs-tutorial-fastopt.html (or whatever name you prefer, for example
index-dev.html) in the project root with the following content. We will go in the details right after.
The script tag simply includes the generated code (attention, you might need to adapt the Scala version from
2.12 (or even
2.11) here if you are using an older version of Scala).
Since we have set
scalaJSUseMainModuleInitializer := true in the build, the
TutorialApp.main(args: Array[String]) method is automatically called at the end of the
-fastopt.js file (with an empty array as argument).
If you now open the newly created HTML page in your favorite browser, you will see … nothing. The
println in the
Adding the DOM Library
To use the DOM, it is best to use the statically typed Scala.js DOM library. To add it to your sbt project, add the following line to your
sbt-savvy folks will notice the
%%% instead of the usual
%%. It means we are using a Scala.js library and not a
normal Scala library. Have a look at the Dependencies guide for details. Don’t forget
to reload the build file if sbt is still running:
sbt:Scala.js Tutorial> reload [info] Loading settings for project global-plugins from plugins.sbt ... [info] Loading global plugins from (...)/.sbt/1.0/plugins [info] Loading settings for project scalajs-tutorial-build from plugins.sbt ... [info] Loading project definition from (...)/scala-js-tutorial/project [info] Loading settings for project scala-js-tutorial from build.sbt ... [info] Set current project to Scala.js Tutorial (in build file:(...)/scala-js-tutorial/)
If you are using an IDE plugin, you will also have to reimport the build for autocompletion to work.
Using the DOM Library
Now that we added the DOM library, let’s adapt our HelloWorld example to add a
<p> tag to the body of the page, rather than printing to the console.
First of all, we import a couple of things:
We additionally import
document (which corresponds to
We now create a method that allows us to append a
<p> tag with a given text to a given node:
Replace the call to
println with a call to
appendPar in the
sbt:Scala.js Tutorial> fastOptJS [info] Compiling 1 Scala source to (...)/scala-js-tutorial/target/scala-2.13/classes ... [info] Fast optimizing (...)/scala-js-tutorial/target/scala-2.13/scala-js-tutorial-fastopt.js [success] (...)
As you can see from the log, sbt automatically detects that the sources must be recompiled before fast optimizing.
You can now reload the HTML in your browser and you should see a nice “Hello World” message.
fastOptJS each time you change your source file is cumbersome. Luckily sbt is able to watch your files and recompile as needed:
sbt:Scala.js Tutorial> ~fastOptJS [success] (...) [info] 1. Monitoring source files for scalajs-tutorial/fastOptJS... [info] Press <enter> to interrupt or '?' for more options.
From this point in the tutorial we assume you have an sbt with this command running, so we don’t need to bother with rebuilding each time.
This step shows how you can add a button and react to events on it by still just using the DOM (we will use jQuery in the next step). We want to add a button that adds another
<p> tag to the body when it is clicked.
We start by adding a method to
TutorialApp which will be called when the button is clicked:
You will notice the
onclick attribute (make sure to add the button before the
Reload your HTML page (remember, sbt compiles your code automatically) and try to click the button. It should add a new paragraph saying “You clicked the button!” each time you click it.
Depending on jQuery
Just like for the DOM, there is a typed library for jQuery available in Scala.js: scalajs-jquery (there is a livelier fork which you may prefer if it already supports the version of Scala you are using).
Add the following line in your
Don’t forget to reload the sbt configuration now:
- Hit enter to abort the
Again, make sure to update your IDE project files if you are using a plugin.
TutorialApp.scala, remove the imports for the DOM, and add the import for jQuery:
This allows you to easily access the
jQuery main object of jQuery in your code (also known as
We can now remove
appendPar and replace all calls to it by the simple:
[message] is the string originally passed to
appendPar, for example:
If you try to reload your webpage now, it will not work (typically a
TypeError would be reported in the console). The
An option is to include
jquery.js from an external source, such as jsDelivr.
After reloading and rerunning
fastOptJS, this will create
Setup UI in Scala.js
We still want to get rid of the
onclick attribute of our
<button>. After removing the attribute, we add the
setupUI method, in which we use jQuery to add an event handler to the button. We also move the “Hello World” message
into this function.
Since we do not call
@JSExportTopLevel annotation (and the corresponding import).
Finally, we add a last call to
jQuery in the main method, in order to execute
setupUI, once the DOM is loaded:
Again, since we are not calling
jQuery will call it through that callback).
We now have an application whose UI is completely setup from within Scala.js. The next step will show how we can test this application.
In this section we will show how such an application can be tested using uTest, a tiny testing framework which compiles to both Scala.js and Scala JVM. As a note aside, this framework is also a good choice to test libraries that cross compile. See our cross compilation guide for details.
Supporting the DOM
Before we start writing tests which we will be able to run through the sbt console, we first have to solve another
issue. Remember the task
run? If you try to invoke it now, you will see something like this:
> run [info] Running tutorial.webapp.TutorialApp [error] TypeError: (0 , $m_Lorg_scalajs_jquery_package$(...).jQuery$1) is not a function [error] at $c_Ltutorial_webapp_TutorialApp$.main__AT__V (.../TutorialApp.scala:9:11) [error] ... [trace] Stack trace suppressed: run last compile:run for the full output. [error] (compile:run) org.scalajs.jsenv.ExternalJSEnv$NonZeroExitException: Node.js exited with code 1 [error] Total time: 1 s, completed Oct 13, 2016 3:06:00 PM
What basically happens here is that jQuery (which is automatically included because of
jsDependencies) cannot properly load, because there is no DOM available in Node.js.
To make the DOM available, add the following to your
This will use the
jsdom library to simulate a DOM in Node.js.
Note that you need to install it separately using
$ npm install jsdom
After reloading, you can invoke
> run [info] Running tutorial.webapp.TutorialApp [success] (...)
Using a testing framework in Scala.js is not much different than on the JVM.
It typically boils down to two sbt settings in the
For uTest, these are:
We are now ready to add a first simple test suite (
This test uses jQuery to verify that our page contains exactly one
<p> element which contains the text “Hello World”
after the UI has been set up.
To run this test, simply invoke the
> test [info] Compiling 1 Scala source to (...)/scalajs-tutorial/target/scala-2.13/test-classes... [info] Fast optimizing (...)/scalajs-tutorial/target/scala-2.13/scala-js-tutorial-test-fastopt.js -------------------------------- Running Tests -------------------------------- + tutorial.webapp.TutorialTest.HelloWorld 2ms Tests: 1, Passed: 1, Failed: 0 [success] Total time: 14 s, completed 16-mars-2018 20:04:28
We have successfully created a simple test.
test task uses Node.js to execute your tests.
A more complex test
We also would like to test the functionality of our button. For this we face another small issue: the button doesn’t
exist when testing, since the tests start with an empty DOM tree. To solve this, we create the button in the
method and remove it from the HTML:
This brings another unexpected advantage: We don’t need to give it an ID anymore but can directly use the jQuery object to install the on-click handler.
We now define the
ButtonClick test just below the
After defining a helper method that counts the number of messages, we retrieve the button from the DOM and verify we have exactly one button and no messages. In the loop, we simulate a click on the button and then verify that the number of messages has increased.
You can now call the
test task again:
> test [info] Compiling 1 Scala source to (...)/scalajs-tutorial/target/scala-2.13/test-classes... [info] Fast optimizing (...)/scalajs-tutorial/target/scala-2.13/scala-js-tutorial-test-fastopt.js -------------------------------- Running Tests -------------------------------- + tutorial.webapp.TutorialTest.HelloWorld 3ms + tutorial.webapp.TutorialTest.ButtonClick 6ms Tests: 2, Passed: 2, Failed: 0 [success] Total time: 15 s, completed 16-mars-2018 20:07:33
This completes the testing part of this tutorial.
Here we show a couple of things you might want to do when you promote your application to production.
uses the advanced optimizations of the Google Closure Compiler. To run
full optimizations, simply use the
> fullOptJS [info] Full optimizing (...)/scala-js-tutorial/target/scala-2.12/scala-js-tutorial-opt.js [info] Closure: 0 error(s), 0 warning(s) [success] (...)
Note that this can take a while on a larger project (tens of seconds), which is why we typically don’t use
during development, but
fastOptJS instead. If you want to
test the full-optimized version from sbt,
you need to change the stage using the following sbt setting:
> set scalaJSStage in Global := FullOptStage
(by default, the stage is
We also need to create our final production HTML file
scalajs-tutorial.html which includes the fully optimized code:
If you serve your Scala.js application from a web server, you should additionally
gzip the resulting
.js files. This step might reduce the size of your application down
to 20% of its original size.
This completes the Scala.js tutorial. Refer to our documentation page for deeper insights into various aspects of Scala.js.