Announcing Scala.js 1.13.1

Apr 10, 2023.

We are pleased to announce the release of Scala.js 1.13.1!

This release mostly contains bug fixes and optimizations.

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 in #scala-js on Discord 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 patch release:

  • It is backward binary compatible with all earlier versions in the 1.x series: libraries compiled with 1.0.x through 1.13.0 can be used with 1.13.1 without change.
  • It is forward binary compatible with 1.13.0: libraries compiled with 1.13.1 can be used with 1.13.0 without change.
  • It is backward source compatible with 1.13.0: source code that used to compile with 1.13.0 should compile as is when upgrading to 1.13.1.

In addition, like Scala.js 1.13.0:

  • It is not forward binary compatible with 1.12.x: libraries compiled with 1.13.1 cannot be used with 1.12.x or earlier.
  • It is not entirely backward source compatible with 1.12.x: it is not guaranteed that a codebase will compile as is when upgrading from 1.12.x or earlier (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.



This release adds support for the following JDK methods:

  • In the classes java.util.concurrent.atomic.Atomic{Integer,Long,Reference}, the methods getAndUpdate, updateAndGet, getAndAccumulate and accumulateAndGet.


Scala.js 1.13.1 brings significant improvements to the performance of the linker (the fastLinkJS task). Most of these improvements apply to incremental scenarios, starting from the second invocation of fastLinkJS for a given project in the same build tool session. As a result, repeated invocations of fastLinkJS (possibly in watch mode, like ~fastLinkJS in sbt) should feel much snappier than with previous versions.

Bug fixes

Among others, the following bugs have been fixed in 1.13.1:

  • #4801 Delegating to the implementation of a calculated super class causes Internal error
  • #4841 Repeat calls of fastLinkJS sometimes cause IllegalStateException on Windows and JDK 17+
  • #4833 High memory consumption during linking when using ModuleSplitStyle.SmallModulesFor(...)
  • #4835 SmallModulesFor splitting style can cause circular dependencies between emitted modules

You can find the full list on GitHub.