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Developer documentation

Very important notice about the Javalib

If you haven't read it, read the very important notice about the Javalib in the Javalib documentation .


Scala.js is entirely built with sbt, and also requires Node.js to be installed. For complete support, Node.js >= 13.2.0 is required.

The first time, or in the rare events where package.json changes (history), you need to run

$ npm install

from your shell. If you really do not want to do this, you can avoid that step, but you will need to use set MyScalaJSPlugin.wantSourceMaps in testSuite := false within sbt to by-pass source-map-related tests. In addition, bootstrap tests will not pass.

Otherwise, everything happens within sbt.

Run the normal test suite using the entire Scala.js toolchain using

> testSuite2_12/test

In order to test the tests themselves, run the cross-compiling tests on the JVM with:

> testSuiteJVM2_12/test

If you have changed the IR or the compiler, you typically need to

> clean

before testing anew.

If you have changed the logging API, the linker interface, the JS environments, the test adapter or the sbt plugin, you typically need to

> reload

To test in fullOpt stage:

> set scalaJSStage in Global := FullOptStage

There are also a few additional tests in a separate testing project:

> testSuiteEx2_12/test

The compiler tests (mostly verifying expected compile error messages) can be run with

> compiler2_12/test

The full partest suite (tests of the Scala language, ported in Scala.js) are run with:

> partestSuite2_12/test

or, more typically,

> partestSuite2_12/testOnly -- --fastOpt

The JUnit tests from scala/scala can be run with

> scalaTestSuite2_12/test

Metals-based IDEs

We recommend Metals-based IDEs such as VS Code to develop Scala.js itself. It can import the Scala.js build out-of-the-box.

After importing the build in Metals, you will need to run clean in sbt before normal sbt commands can correctly work. Metals will continue to provide all its features after cleaning.


If you want to develop in Eclipse, use sbteclipse. Projects as created by the build by default are not suited for Eclipse. You can create somewhat appropriate projects with:

$ GENERATING_ECLIPSE=true sbt "eclipse with-source=true"

You will still have to fix a few things:

  • Uncheck the "Allow output directories per source directory" in Build path
  • Add transitive project dependencies in Build path

Preparing a Pull Request

One common build failure is code styling. Reproduce results locally with:

$ sbt scalastyleCheck

Organization of the repository

The repository is organized as follows:

Compilation pipeline

  • ir/ The Intermediate Representation, produced by the compiler and consumed by the linker
  • compiler/ The scalac compiler plugin
  • linker-private-library/ Some Scala.js files whose compiled .sjsir files are used as resources of the linker (2.12 only)
  • linker-interface/ The linker interface, without its implementation
  • linker/ The linker, optimizer, verifier, etc.: everything that happens at link time


  • library/ The Scala.js standard library (everything inside scala.scalajs.*)
  • javalanglib/ Implementation in Scala.js of the classes in java.lang.*
  • javalib/ Implementation in Scala.js of other classes in java.*
  • scalalib/ Almost void project for recompiling the Scala library for Scala.js
  • library-aux/ A few files of the Scala library that need to be compiled separately

All of these are packaged in scalajs-library.jar.

Testing infrastructure

There is a generic infrastructure that maps the sbt-testing-interface API across the JVM/JS boundary, so that Scala.js testing frameworks can be piloted from JVM processes such as sbt.

  • test-interface/ the JS definition of the sbt-testing-interface API
  • test-bridge/ JS side of the bridge
  • test-adapter/ JVM side of the bridge
  • test-common/ Code common between test-bridge and test-adapter

This repository also contains a specific implementation of JUnit:

  • junit-runtime/ The run-time library for JUnit
  • junit-plugin/ The JUnit compiler plugin

sbt plugin

  • sbt-plugin/ The sbt plugin itself (2.12 only)

Testing projects

  • test-suite/ The main test suite of Scala.js
  • test-suite-ex/ Additional tests
  • partest-suite/ The partest suite of Scala
  • scala-test-suite/ The JUnit suite of Scala

Example projects/sandboxes

  • examples/helloworld/ A simple Hello World, typically used as sandbox for quick testing
  • examples/reversi/ The historical Reversi demo - we use it to track the impact of changes on the emitted code size
  • examples/testing/ A simple project with tests using the DOM, mostly used to test testHtml with DOM interaction

The helloworld and reversi also have HTML pages to run them in real browsers.

The build itself

The build itself contains the entire sbt plugin (and all its dependencies) as part of its sources. If you change any of the linker interface, linker, test adapter, or the sbt plugin itself, chances are you need to reload the build for your changes to take effect.

Publish locally

To publish your changes locally to be used in a separate project, use the following incantations. SCALA_VERSION refers to the Scala version used by the separate project.

> ;ir2_12/publishLocal;linkerInterface2_12/publishLocal;linker2_12/publishLocal;testAdapter2_12/publishLocal;sbtPlugin/publishLocal
> ;compiler2_12/publishLocal;library2_12/publishLocal;testInterface2_12/publishLocal;testBridge2_12/publishLocal;jUnitRuntime2_12/publishLocal;jUnitPlugin2_12/publishLocal

If using a non-2.12.x version for the Scala version, the 2_12 suffixes must be adapted in the last command (not in the first command).