Announcing Scala.js 1.12.0

Nov 23, 2022.

We are excited to announce the release of Scala.js 1.12.0!

This release brings a number of bug fixes and enhancements. The highlights are:

  • Support for the JavaScript operator **, introduced in ECMAScript 2016.
  • Checked exceptions for ArrayStoreExceptionss and NegativeArraySizeExceptions

The Scala standard library was upgraded to version 2.12.17 and 2.13.10.

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 minor release:

  • It is backward binary compatible with all earlier versions in the 1.x series: libraries compiled with 1.0.x through 1.11.x can be used with 1.12.0 without change.
  • It is not forward binary compatible with 1.11.x: libraries compiled with 1.12.0 cannot be used with 1.11.x or earlier.
  • It is not entirely backward source compatible: it is not guaranteed that a codebase will compile as is when upgrading from 1.11.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.

Enhancements with compatibility concerns

Checked exceptions for ArrayStoreException and NegativeArraySizeException

As documented in the semantics of Scala.js, ArrayStoreExceptions and NegativeArraySizeExceptions are Undefined Behavior in Scala.js, similarly to ClassCastExceptions and ArrayIndexOutOfBoundsExceptions. Prior to Scala.js 1.12.0, we made no effort internally to provide decent error messages, or any other kind of checks.

Starting with this release, erroneous conditions leading to one of these two exceptions are checked in development (fastLink) mode. They will be reported as UndefinedBehaviorErrors in fastLink mode, and unchecked in fullLink mode.

In some rare situations, this may turn code that appeared to work into actively throwing an exception.

Like other checked behaviors, it is now possible to configure the linker so that these exceptions are compliant. In that case, they will be thrown as specified for the JVM, in both fastLink and fullLink. You may enable them with the following sbt settings:

scalaJSLinkerConfig ~= {
  import org.scalajs.linker.interface.CheckedBehavior

This may have significant performance impact in fullLink, like other compliant behaviors.

js.Dynamic.** now refers to the JavaScript ** operator

Previously, dynamic1 ** dynamic2 would be equivalent to the JavaScript expression dynamic1["**"](dynamic2). When recompiled, ** will now resolve the JavaScript ** operator (see below for details).

If calling a method named "**" was intended, the above should be rewritten to dynamic1.applyDynamic("**")(dynamic2).

New deprecations

@JSOperator is expected on JavaScript operator methods

When defining a facade type, we can model JavaScript operators using methods with specific names. For example, the facade for js.BigInt contains:

import scala.scalajs.js
import scala.scalajs.js.annotation._

@js.native @JSGlobal
final class BigInt extends js.Object {
  def +(other: BigInt): BigInt = js.native

Starting from Scala.js 1.12.0, such operator methods should be annotated with @JSOperator, as follows:

  @JSOperator def +(other: BigInt): BigInt = js.native

A compiler warning will be emitted if the annotation is missing.


Support for the JavaScript operator **

ECMAScript 2016 introduced the ** operator in JavaScript. a ** b computes a raised to the power b, and is applicable to numbers and bigints.

Before Scala.js 1.12.0, it was not possible to write Scala.js code calling that operator. Scala.js 1.12.0 now supports it, through the definition of an @JSOperator def **, as follows:

import scala.scalajs.js
import scala.scalajs.js.annotation._

@js.native @JSGlobal
final class BigInt extends js.Object {
  @JSOperator def **(other: BigInt): BigInt = js.native

Without the annotation, a compiler warning will be emitted, and the definition will behave as a call to a method named "**" instead, for backward compatibility. If that is actually the intended semantics, the warning can be silenced with an explicit @JSName("**").

Bug fixes

The following bugs have been fixed in 1.12.0:

  • #4739 Incorrect codegen for singleton case from Scala 3 enum in Scala 2.13 codebase
  • #4734 j.u.ArrayDeque#pollFirst is O(n)
  • #4731 j.u.Arrays.equals(Array[AnyRef]) doe not handle null values.
  • #4753 The optimizer thinks that x.getClass() is non-nullable.
  • #4755 The optimizer does not respect eval order of generic array get/set with nulls

You can find the full list on GitHub.