Announcing Scala.js 0.6.20

Sep 1, 2017.

We are pleased to announce the release of Scala.js 0.6.20!

This release is mostly intended to bridge the gap between the 0.6.x and 1.x branches, to make it easier to cross-compile and/or migrate.

Read on for more details.

Getting started

If you are new to Scala.js, head over to the tutorial.

Release notes

If upgrading from Scala.js 0.6.14 or earlier, make sure to read the release notes of 0.6.15, which contain important migration information.

As a minor release, 0.6.20 is backward source and binary compatible with previous releases in the 0.6.x series. Libraries compiled with earlier versions can be used with 0.6.20 without change. 0.6.20 is also forward binary compatible with 0.6.{17-19}, but not with earlier releases: libraries compiled with 0.6.20 cannot be used by projects using 0.6.{0-16}.

Please report any issues on GitHub.

Breaking changes

sbt 0.13.16 or above is required (or sbt 1.x)

The sbt plugin of Scala.js 0.6.20 starts using features of sbt 0.13.16. If you are using an older version of sbt 0.13.x, you will have to upgrade to 0.13.16 or later.

Since 0.6.19, Scala.js also supports sbt 1.0.0+.

scalajs-env-selenium has to be upgraded to 0.2.0

Scala.js 0.6.20 contains internal changes to improve the so-called test adapter, which is the mechanism used by sbt to communicate with testing frameworks. These changes are invisible to you, but they broke scalajs-env-selenium 0.1.x. If you use it, you will have to upgrade it to 0.2.0.


Scala.js 0.6.20 introduces more aggressive deprecations for features that will disappear in 1.x. Note that those features have already had better replacements since Scala.js 0.6.18; we are just making it more obvious that these new features should be used to be compatible with 1.x.


This annotation is now deprecated. Instead of using it, you should add the following to your project settings:

scalacOptions += "-P:scalajs:sjsDefinedByDefault"

and remove the @ScalaJSDefined annotations everywhere in your codebase. The semantics of your codebase will be unchanged.


js.JSApp has traditionally provided two services to an object Foo that extends it. These two services are replaced by two different features.

Discoverability by sbt as main object

Since Scala.js 0.6.18, the sbt plugin can recognize “standard” main methods of the form

def main(args: Array[String]): Unit = ...

in objects, even if they do not extend js.JSApp. Use such a main method to replace js.JSApp in the context of discoverability by sbt.

To enable it as main method, make sure you also set

scalaJSUseMainModuleInitializer := true

in your project settings.

Automatic export to JavaScript


package bar

object Foo extends js.JSApp {
  def main(): Unit = println("Hello world!")

the object Foo and its main method are automatically exported such that JavaScript code can call


To achieve exactly the same behavior without js.JSApp, define Foo as

package bar

object Foo {
  protected def getInstance(): this.type = this

  def main(): Unit = println("Hello world!")

Alternatively, you can define it as

package bar

object Foo {
  def main(): Unit = println("Hello world!")

but in that case, the JavaScript code will have to be changed to


jsDependencies += RuntimeDOM and requiresDOM := true

These settings will not be supported by sbt-jsdependencies 1.x, the new home of jsDependencies and related features.

Instead of relying on them to configure a JS environment equipped with the DOM, you should explicitly do so. For example, to use Node.js with jsdom, use:

jsEnv := new org.scalajs.jsenv.jsdomnodejs.JSDOMNodeJSEnv()

New features

Improved support for cross-compilation with 1.x and jsDependencies

The release notes of Scala.js 1.0.0-M1 detail how to cross-compile between 0.6.x and 1.x. One aspect of it was particularly painful: handling the new JSDependenciesPlugin. Scala.js 0.6.20 makes this easier by introducing a shim of JSDependenciesPlugin inside sbt-scalajs. It is now sufficient to add the following to your project/plugins.sbt:

// For jsDependencies
  if (scalaJSVersion.startsWith("0.6.")) Nil
  else Seq(addSbtPlugin("org.scala-js" % "sbt-jsdependencies" % "1.0.0-M1"))

and add enablePlugins(JSDependenciesPlugin) to the projects that require it. There is no need for the hacky project/JSDependenciesCompat.scala anymore.


The following JDK API class has been added:

  • java.util.SplittableRandom

Bug fixes

The following bugs have been fixed in 0.6.20:

  • #3107 ByteArrayOutputStream (and in general all Closeables) should be an AutoCloseable
  • #3082 Incorrect handling of main generates broken code

You can find the full list on GitHub.