Jun 20, 2026.

We are pleased to announce the release of Scala.js 1.22.0!

This release adds support for sbt 2.x, and officializes the support for WebAssembly as stable.

This release also comes with performance improvements and bug fixes.

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 Scala Users.

Bug reports can be filed on GitHub.

Release notes

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 through 1.21.x can be used with 1.22.0 without change.
• It is not forward binary compatible with 1.21.x: libraries compiled with 1.22.0 cannot be used with 1.21.x or earlier.
• It is not entirely backward source compatible: it is not guaranteed that a codebase will compile as is when upgrading from 1.21.x (in particular in the presence of -Xfatal-warnings/-Werror).

Changes with compatibility concerns

Configuration for WebAssembly

The configuration for WebAssembly now requires to explicitly target ECMAScript 2022+. Moreover, the method .withExperimentalUseWebAssembly of StandardConfig is deprecated in favor of ESFeatures.withUseWebAssembly.

The minimal configuration for WebAssembly is now:

// Emit ES modules with the Wasm backend
scalaJSLinkerConfig := {
  scalaJSLinkerConfig.value
    .withModuleKind(ModuleKind.ESModule)
    .withESFeatures(_.withESVersion(ESVersion.ES2022).withUseWebAssembly(true))
},

Using js.async/js.await requires the following additional config, which requires that the Wasm engine support JSPI:

  scalaJSLinkerConfig.value
    ...
    .withWasmFeatures(_.withUseJSPI(true))

We have also removed the polyfills for Wasm JS String Builtins. The WebAssembly backend now expects the engine to support Wasm 3.0.

Enhancements

Support for sbt 2.x

The sbt plugin sbt-scalajs is now published for sbt 2.x in addition to sbt 1.x.

As part of that effort, the following artifacts are published for Scala 3: scalajs-ir, scalajs-linker-interface, scalajs-linker and scalajs-test-adapter. Authors of non-sbt tooling may find those useful as well.

The WebAssembly backend is now officially stable

Concretely, this means that we will keep supporting it as part of the core Scala.js repository. There will be no more breaking changes during the lifetime of 1.x, similarly to the JS backend.

The Wasm backend emits code with the following requirements:

• A JavaScript host (i.e., we do not currently generate standalone Wasm), supporting ECMAScript 2022+
• A Wasm engine with support for Wasm 3.0
• The ESModule module kind (see emitting modules)

Supported engines include Node.js 25, Chrome 137, Firefox 134 and Safari 26. As part of our commitment to stability, this baseline for requirements will remain the default going forward.

In order to use js.async/js.await, we also require the JavaScript Promise Integration (JSPI) proposal. It has reached Stage 4 (standardized) but is not part of Wasm 3.0, which is why it requires an explicit opt-in.

See the documentation on the WebAssembly backend for more details.

Code size improvements

When you use methods of Character such as getType, isLetter or isUnicodeIdentifierStart (among many others), the library embeds information about the Unicode database into the generated bundle. This embedded database is now smaller, thanks to a better internal encoding.

More generally, starting with v1.22.0, the JavaScript backend aggressively optimizes code size for arrays of numeric literals. For example, if you have a big Array[Int] with literals in your codebase, its generated code will be smaller.

Performance improvements

There were performance improvements in the following areas.

For both the JS and Wasm backends:

• Unicode database-related methods
• Bounds checks in the JDK collections

For the JS backend:

• Even faster Longs: addition, subtraction, division (with constant and variable divisors), toString and absolute value

Miscellaneous

Unicode database-related methods updated for Unicode v15.0

All Unicode-related methods have been updated to Unicode v15.0, which is in sync with JDK 21. Previously, there were inconcistencies.

Moreover, some methods have been fixed to take so-called contributory properties into account. For example, before v1.22.0, isLowerCase only returned true for characters with general category Ll. Now, it also returns true for characters that have the Other_Lowercase contributory property.

So yes, the Unicode-related methods are smaller, faster, and more accurate in Scala.js 1.22.0!

Bug fixes

The following bugs have been fixed in 1.22.0:

• #5345 java.math.BigInteger.xor throws ArrayIndexOutOfBoundsException for mixed-sign inputs
• #5347 Error with m_sjsr_PrivateFieldsSymbolHolder and java.lang.ClassCastException
• #5354 Linker cache growing with fullLinkJS used repeatedly from sbt

You can find the full list on GitHub.