Announcing Scala.js 1.0.0-M1

Jul 3, 2017.

We are very excited to announce the first milestone of Scala.js 1.0.0, aka 1.0.0-M1!

This development release is mostly intended for testing purposes, and as a synchronization point with library authors so that they can start upgrading in preparation for the final release.

As the change in “major” version number witnesses, this release is not binary compatible with 0.6.x. Libraries need to be recompiled and republished using this milestone to be compatible.

Moreover, this release is not entirely source compatible with 0.6.x either. We expect, however, that further milestones for 1.0.0 will stay backward source compatible with this first milestone.

Please report any issues on GitHub.

The following libraries and testing frameworks have already been upgraded and published for 1.0.0-M1:

Preparations before upgrading from 0.6.x

Before upgrading to 1.0.0-M1, we strongly recommend that you upgrade to Scala.js 0.6.18, and address all deprecation warnings. Since Scala.js 1.0.0-M1 removes support for all the deprecated features in 0.6.x, it is easier to see the deprecation messages guiding you to the proper replacements.

Additionally to the explicitly deprecated things, make sure to use scalaJSLinkerConfig instead of the following sbt settings:

  • scalaJSSemantics
  • scalaJSModuleKind
  • scalaJSOutputMode
  • emitSourceMaps
  • relativeSourceMaps
  • scalaJSOutputWrapper
  • scalaJSOptimizerOptions

Upgrade to 1.0.0-M1 from 0.6.18

As a first approximation, all you need to do is to update the version number in project/plugins.sbt:

addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.0.0-M1")

In addition, if you use some of the components that have been moved to separate repositories, you will need to add some more dependencies in project/plugins.sbt:

If you use jsDependencies (or rely on the jsDependencies of your transitive dependencies):

  • Add addSbtPlugin("org.scala-js" % "sbt-jsdependencies" % "1.0.0-M1") in project/plugins.sbt
  • Add .enablePlugins(JSDependenciesPlugin) to Scala.js projects
  • Add .jsConfigure(_.enablePlugins(JSDependenciesPlugin)) to crossProjects

If you use the Node.js with jsdom environment:

  • Add libraryDependencies += "org.scala-js" %% "scalajs-env-nodejs" % "1.0.0-M1" in project/plugins.sbt

If you relied on the automatic selection of the above environment due to jsDependencies += RuntimeDOM, you have to explicitly select it:

  • Add jsEnv := new org.scalajs.jsenv.jsdomnodejs.JSDOMNodeJSEnv() to the relevant Scala.js project settings

If you use the PhantomJS environment:

  • Add addSbtPlugin("org.scala-js" % "sbt-scalajs-env-phantomjs" % "1.0.0-M1") in project/plugins.sbt

This should get your build up to speed to Scala.js 1.0.0-M1. From there, you should be able to test whether things go smoothly, or whether you are affected by the breaking changes detailed below.

Breaking changes

This section discusses the backward incompatible changes, which might affect your project.

Access to the global scope instead of the global object

This is the only major breaking change at the language level. In Scala.js 1.x, and @JSGlobalScope objects refer to the global scope of JavaScript, rather than the global object. Concretely, this has three consequences, which we outline below. Further information can be found in the documentation about the global scope in Scala.js.

Members can only be accessed with a statically known name which is a valid JavaScript identifier

For example, the following is valid:


but the following variant, where the name Math is only known at run-time, is not valid anymore:

val mathName = "Math"

The latter will cause a compile error. This is because it is not possible to perform dynamic lookups in the global scope. Similarly, accessing a member whose name is statically known but not a valid JavaScript identifier is also prohibited:


Global scope objects cannot be stored in a separate val

For example, the following is invalid and will cause a compile error:

val g =

as well as:

def foo(x: Any): Unit = println(x)

This follows from the previous rule. If the above two snippets were allowed, we could not check that we only access members with statically known names.

The first snippet can be advantageously replaced by a renaming import:

import js.Dynamic.{global => g}

Accessing a member that is not declared causes a ReferenceError to be thrown

This is a run-time behavior change, and in our experience the larger source of breakages in actual code.

Previously, reading a non-existent member of the global object, such as


would evaluate to undefined. In Scala.js 1.x, this throws a ReferenceError. Similarly, writing to a non-existent member, such as = 42

would previously create said global variable. In Scala.js 1.x, it also throws a ReferenceError.

A typical use case of the previous behavior was to test whether a global variable was defined or not, e.g.,

if (js.isUndefined( {
  // Promises are not supported
} else {
  // Promises are supported

This idiom is broken in Scala.js 1.x, and needs to be replaced by an explicit use of js.typeOf:

if (js.typeOf( != "undefined")

The js.typeOf “method” is magical when its argument is member of a global scope object.

extends js.JSApp does not export the object to JavaScript anymore

Consider the following object definition:

package bar

import scala.scalajs.js

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

In 0.6.x, extending js.JSApp has two consequences:

  • Foo is recognized by the sbt plugin as a main object, and it can be used by scalaJSUseMainModuleInitializer := true (as well as in deprecated launchers)
  • Foo is exported to JavaScript as a 0-argument function bar.Foo(), and its main() method as well

In Scala.js 1.x, the second bullet is not true anymore, which constitutes a run-time behavior change. If you need to preserve this behavior, you need to explicitly export Foo and its main() method as follows:

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

  def jsAccessor(): Foo.type = this

In addition, js.JSApp itself is deprecated, so you should use a def main(args: Array[String]): Unit method instead. js.JSApp will be removed in Scala.js 1.0.0-RC1.

js.UndefOr[A] is now an alias for A | Unit

Instead of defining js.UndefOr[+A] as its own type, it is now a simple type alias for A | Unit:

type UndefOr[+A] = A | Unit

The Option-like API is of course preserved.

We do not expect this to cause any significant issue, but it may impact type inference in subtle ways that can cause compile errors for previously valid code. You may have to adjust some uses of js.UndefOr due to these changes, especially if using Scala 2.10.

testHtml replaces both testHtmlFastOpt and testHtmlFullOpt

The separation of testHtmlFastOpt and testHtmlFullOpt, which were independent of the value of scalaJSStage, caused significant unfixable issues in 0.6.x. In Scala.js 1.x, both are replaced by a single task, testHtml. It is equivalent to the old testHtmlFastOpt if the value of scalaJSStage is FastOptStage (the default), and to testHtmlFullOpt if it is FullOptStage. This makes it more consistent with other tasks such as run and test.

scalajs-javalib-ex was removed

The artifact scalajs-javalib-ex is removed in 1.x. It only contained a partial implementation of java.util.ZipInputStream. If you were using it, we recommend that you integrate a copy of its source code from Scala.js 0.6.x into your project.

js.use(x).as[T] was removed

The use cases for js.use(x).as[T] have been dramatically reduced by non-native JS classes (previously known as Scala.js-defined JS classes). This API seems virtually unused on the visible Web. Moreover, it was the only macro in the Scala.js standard library.

We have therefore removed it from the standard library, and it is not provided anymore. On demand, we can republish it as a separate library, if you need it.

Scala 2.12.0 is not supported anymore

A severe regression in Scala 2.12.0 upstream, affecting js.UndefOr, forced us to drop support for Scala 2.12.0. Scala 2.12.1+ is supported.


There are very few enhancements in Scala.js 1.0.0-M1. Scala.js 1.0.0 is focused on simplifying Scala.js, not on adding new features. Nevertheless, here are a few enhancements.

Non-native JS classes by default (previously known as Scala.js-defined)

In Scala.js 0.6.x, we can declare a so-called Scala.js-defined JS class as follows:

class Foo extends js.Object

In Scala.js 1.x, “Scala.js-defined” is the default, so the above snippet would be simply written as

class Foo extends js.Object

Consequently, we also introduce a shift of terminology. Such a class is now called a non-native JS class (by opposition to a native JS class with @js.native). Both native and non-native JS classes (resp. traits, objects) are called JS classes (resp. traits, objects). All of them are called JS types.

Also note that the annotation @ScalaJSDefined is deprecated, and will be removed in Scala.js 1.0.0-RC1.

Scala.js can access require and other magical “global” variables of special JS environments

The changes from global object to global scope mean that magical “global” variables provided by some JavaScript environments, such as require in Node.js, are now visible to Scala.js. For example, it is possible to dynamically call require as follows in Scala.js 1.x:

val pathToSomeAsset = "assets/logo.png"
val someAsset =

We still recommend to use @JSImport and CommonJSModule for statically known imports.

The sbt plugin builds on top of sbt-crossproject

Scala.js 0.6.x defined itself the notion of crossProject and the %%% operator for cross-platform dependencies. In Scala.js 1.x, we standardize on sbt-crossproject, which provides a unified API that can also be used by Scala Native.

The changes are entirely backward source compatible, because sbt-crossproject was designed with that specific goal. However, the traditional cross-project constructor:

lazy val foo =

is now deprecated, as it hard-codes the JVM × JS pair of platforms. Instead, you should use

lazy val foo = crossProject(JVMPlatform, JSPlatform).in(...)...


Amongst others, the following bugs have been fixed since 0.6.18:

  • #2800 Global lets, consts and classes cannot be accessed by Scala.js
  • #2382 Name clash for $outer pointers of two different nesting levels (fixed for Scala 2.10 and 2.11; 2.12 did not suffer from the bug in 0.6.x)

See the full list on GitHub.

Cross-building for Scala.js 0.6.x and 1.x

If you want to cross-compile your libraries for Scala.js 0.6.x and 1.x (which you definitely should), here are a couple tips.

Dynamically load a custom version of Scala.js

Since the version of Scala.js is not decided by an sbt setting in build.sbt, but by the version of the sbt plugin in project/plugins.sbt, standard cross-building setups based on ++ cannot be applied. We recommend that you load the version of Scala.js from an environment variable. For example, you can do this in your project/plugins.sbt file:

val scalaJSVersion =

addSbtPlugin("org.scala-js" % "sbt-scalajs" % scalaJSVersion)

You can then launch

$ SCALAJS_VERSION=1.0.0-M1 sbt

from your command line to start up your build with Scala.js 1.0.0-M1.

Extra dependencies for JS environments

You can further build on the above val scalaJSVersion to dynamically add dependencies on scalajs-env-phantomjs and/or scalajs-env-jsdom-nodejs if you use them:

// For Node.js with jsdom
libraryDependencies ++= {
  if (scalaJSVersion.startsWith("0.6.")) Nil
  else Seq("org.scala-js" %% "scalajs-env-jsdom-nodejs" % "1.0.0-M1")

// For PhantomJS
  if (scalaJSVersion.startsWith("0.6.")) Nil
  else Seq(addSbtPlugin("org.scala-js" % "sbt-scalajs-env-phantomjs" % "1.0.0-M1"))

In both cases, you can then use the source-compatible API in build.sbt to select your JS environment of choice.

Extra dependencies for jsDependencies

The case of sbt-jsdependencies is trickier, because it defines an AutoPlugin that needs to be enabled on your projects. This means that there is no source-compatible way to write your build.sbt. The trick is to provide a fake JSDependenciesPlugin when compiling for 0.6.x.

First add the following to project/plugins.sbt:

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

Then create a file project/JSDependenciesCompat.scala with the following content:

package jsdependenciescompat

import sbt._

object FakeJSDependenciesPlugin extends AutoPlugin

object FakeJSDependenciesPluginProvider extends AutoPlugin {
  import Compat._

  object autoImport {
    import org.scalajs._

    val JSDependenciesPluginCompat = jsdependencies.sbtplugin.JSDependenciesPlugin

  object Compat {
    object jsdependencies {
      object sbtplugin {
        val JSDependenciesPlugin = FakeJSDependenciesPlugin

You can now write enablePlugins(JSDependenciesCompat) in your build.sbt, and otherwise use jsDependencies and other keys as usual.

Understanding how the trick works is left as an exercise for the reader.

Warning-free cross-compilation of @ScalaJSDefined

In Scala.js 1.x, @ScalaJSDefined is deprecated because it is the default. However, by default, in Scala.js 0.6.x, it is required. Scala.js 0.6.17 introduced the compiler option -P:scalajs:sjsDefinedByDefault to allow warning-free cross-compilation. Add the following setting to your build.sbt:

scalacOptions ++= {
  if (scalaJSVersion.startsWith("0.6.")) Seq("-P:scalajs:sjsDefinedByDefault")
  else Nil

And then remove all your @ScalaJSDefined annotations. This will be source-compatible between both versions, and not produce any warnings.

Enabling sbt-crossproject even with Scala.js 0.6.x

This should only be necessary if your codebase also cross-compiles with Scala Native, in which case you need your build to depend on sbt-crossproject’s sbt-scalajs-crossproject in the 0.6.x build. However, you must not depend on it in the 1.x build, because it conflicts with the now built-in support of sbt-crossproject in Scala.js 1.x. Therefore, here is what you should put in your project/plugins.sbt:

// For sbt-crossproject support even with Scala.js 0.6.x
  if (scalaJSVersion.startsWith("0.6."))
    Seq(addSbtPlugin("org.scala-native" % "sbt-scalajs-crossproject" % "0.2.0"))

In that case, you still need to apply the shadowing import described in sbt-crossproject’s readme, i.e., in your build.sbt:

// shadow sbt-scalajs' crossProject and CrossType from Scala.js 0.6.x (no-op with Scala.js 1.x)
import sbtcrossproject.{crossProject, CrossType}