Jan 5, 2026.

We are pleased to announce the release of Scala.js 1.20.2!

This release comes with even more performance improvements to the WebAssembly and JavaScript backends alike.

The Scala standard library has been upgraded to match Scala 2.12.21 and 2.13.17.

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 1.20.1 can be used with 1.20.2 without change.
• It is forward binary compatible with 1.20.0 and 1.20.1: libraries compiled with 1.20.2 can be used with previous 1.20.x versions without change.
• It is backward source compatible with 1.20.0 and 1.20.1: source code that used to compile with previous 1.20.x versions should compile as is when upgrading to 1.20.2.

In addition, like Scala.js 1.20.0 and 1.20.1:

• It is not forward binary compatible with 1.19.x: libraries compiled with 1.20.2 cannot be used with 1.19.x or earlier.
• It is not entirely backward source compatible with 1.19.x: it is not guaranteed that a codebase will compile as is when upgrading from 1.19.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.

Enhancements

Performance improvements

Scala.js 1.20.2 brings a number of performance improvements compared to 1.20.1. These come in addition to the improvements shipped in 1.20.1.

For both JavaScript and WebAssembly:

• Micro-optimizations in low-level methods in java.lang.Math, Float, and Double.
• Various new micro-optimizations performed by the optimizer, among which:
  • Divisions and remainders by constants.
  • Constant range checks of the form x >= c1 && x <= c2.

For JavaScript:

• Performance improvements for Long:
  • They are not boxed anymore when stored in fields, stored in arrays, or passed as method arguments. They are still boxed when returned from methods.
  • Division and remainder operations received a 3x speedup thanks to a new, state-of-the-art algorithm.

For WebAssembly, performance improvements in the following areas:

• java.lang.Long.parseLong, parseUnsignedLong.
• Faster allocation of arrays of primitive types.
• Fewer Wasm-to-JS calls for JavaScript methods that return primitive values.

Miscellaneous

New JDK APIs

This release adds support for the following JDK methods:

• In java.lang.CharSequence:
  • isEmpty
• In java.lang.Math:
  • scalb
• In java.util.regex.Matcher:
  • appendReplacement and appendTail with a StringBuilder argument.
  • replaceFirst and replaceAll with a replacer: Function argument.
• In java.lang.IndexOutOfBoundsException:
  • constructors with an index argument (Int or Long).

Bug fixes

The following bugs have been fixed in 1.20.2:

• #5287 MatchError.getMessage() requires getClass(), which JS classes do not provide.
• #5246 Optimizer crash with Labeled+Return of inline class, discarded in statement position.
• #5180 Rare AssertionError when linking a corrupted codebase with a cyclic inheritance chain.

You can find the full list on GitHub.