Announcing Scala.js 1.2.0

Sep 9, 2020.

We are pleased to announce the release of Scala.js 1.2.0!

The highlight of this release is the addition of the JavaScript types js.Map[K, V] and js.Set[T] in the standard library. Thanks to @exoego for this contribution!

In addition, this release contains a number of bug fixes and small improvements. The version of the Scala standard library has been upgraded to Scala 2.12.12 and 2.13.3.

Read on for more details.

Getting started

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

If you need help with anything related to Scala.js, you may find our community on Gitter and on Stack Overflow.

Bug reports can be filed on GitHub.

Release notes

If upgrading from Scala.js 0.6.x, make sure to read the release notes of Scala.js 1.0.0 first, as they contain a host of important information, including breaking changes.

This is a minor release:

  • It is backward binary compatible with all earlier versions in the 1.x series: libraries compiled with 1.0.x and 1.1.x can be used with 1.2.0 without change.
  • It is not forward binary compatible with 1.1.x: libraries compiled with 1.2.0 cannot be used with 1.1.x or earlier.
  • It is not entirely backward source compatible: it is not guaranteed that a codebase will compile as is when upgrading from 1.1.x (in particular in the presence of -Xfatal-warnings).

As a reminder, libraries compiled with 0.6.x cannot be used with Scala.js 1.x; they must be republished with 1.x first.

Known source breaking changes


The type of the space argument has been narrowed from js.Any in Scala.js <= 1.1.1 to Int | String in Scala.js 1.2.0. This may cause code that uses this method not to compile anymore. For example:

def configString(config: js.Dictionary[String], mySpace: js.Any): String =
  js.JSON.stringify(config, space = mySpace)

will stop compiling because js.Any is not a subtype of Int | String. Ideally, this issue would be addressed by changing the type of mySpace itself and the callers of configString. If that is not possible, for example due to binary compatibility constraints, it is always possible to use a cast:

def configString(config: js.Dictionary[String], mySpace: js.Any): String =
  js.JSON.stringify(config, space = mySpace.asInstanceOf[Int | String])


js.Map[K, V] and js.Set[T]

The standard library now contains definitions for the JavaScript built-in classes Map and Set, as js.Map[K, V] and js.Set[T], respectively. Like js.Array, they support the full Scala collection API.

ECMAScript module support in testHtml

The task testHtml creates a .html page that runs the unit tests of a codebase in a browser. Until Scala.js 1.1.1, it only supported Scripts (i.e., with the default ModuleKind.NoModule). Starting with v1.2.0, it also supports ECMAScript modules (ModuleKind.ESModule).

It still does not support CommonJS modules, since those are not supported by browsers at all.

Export abstract JS classes

It is now possible to export abstract JS classes (that extend js.Any), for example:

abstract class AbstractBase extends js.Object

These can be extended by classes written in JavaScript.



  • java.util.Date.toInstant() and java.util.Date.from(Instant), although they will only transitively link if support for java.time APIs is enabled (e.g., using scala-java-time)
  • java.lang.Character.codePointCount

Upgrade to GCC v20200719

The Google Closure Compiler used internally by Scala.js for fullOptJS has been upgraded to v20200719.

Bug fixes

Among others, the following bugs have been fixed in 1.2.0:

  • #4131 getTime on java.util.Date returns 0 when initialized with a value outside the ECMAScript Time Range
  • #4114 Compiler crash for JS class containing nested JS objects
  • #4171NoSuchElementException: key not found: LabelName<matchAlts1>” with dotty 0.27.0-RC1
  • #4174 fullOptJS shouldn’t add a sourceMap references when sourceMap’s are disabled
  • #4151 Very Long String in source results in cryptic error message
  • #4148 and #4168 Unable to find extant inner trait IR during fastopt

You can find the full list on GitHub.