Announcing Scala.js 1.10.1

Jun 25, 2022.

We are pleased to announce the release of Scala.js 1.10.1!

This release mostly contains bug fixes and optimizations. It also updates the version of the Scala standard library to 2.12.16 for 2.12.x versions.

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 1.10.0 can be used with 1.10.1 without change.
  • It is forward binary compatible with 1.10.0: libraries compiled with 1.10.1 can be used with 1.10.0 without change.
  • It is backward source compatible with 1.10.0: source code that used to compile with 1.10.0 should compile as is when upgrading to 1.10.1.

In addition, like Scala.js 1.10.0:

  • It is forward binary compatible with 1.8.x and 1.9.x, but not 1.7.x: libraries compiled with 1.10.1 can be used with 1.8.x and later, but not with 1.7.x or earlier.
  • It is not entirely backward source compatible with 1.9.x: it is not guaranteed that a codebase will compile as is when upgrading from 1.9.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.



Scala.js 1.10.0 brought some optimizations with respect to closures. Scala.js 1.10.1 expands on these optimizations with the following improvements:

  • When a closure captures a variable that ends up being constant-folded, the constant is propagated inside the closure, and the capture parameter is removed.
  • Most closures that are not declared within a loop do not need an IIFE (Immediately Invoked Function Expression) anymore, even when they actually capture some variables.

Bug fixes

Among others, the following bugs have been fixed in 1.10.1:

  • #4684 Unit JS default param makes compiler crash with “Found a dangling UndefinedParam”
  • #4675 ES2021 not supported in fullLinkJS

You can find the full list on GitHub.