Announcing Scala.js 1.7.1

Oct 7, 2021.

We are pleased to announce the release of Scala.js 1.7.1!

This is mostly a bugfix release. It also updates the version of the Scala standard library to 2.12.15 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 on Gitter 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 1.7.0 can be used with 1.7.1 without change.
  • It is forward binary compatible with 1.7.0: libraries compiled with 1.7.1 can be used with 1.7.0 without change.
  • It is backward source compatible with 1.7.0: source code that used to compile with 1.7.0 should compile as is when upgrading to 1.7.1.

In addition, like Scala.js 1.7.0:

  • It is not forward binary compatible with 1.6.x: libraries compiled with 1.7.1 cannot be used with 1.6.x or earlier.
  • It is not entirely backward source compatible with 1.6.x: it is not guaranteed that a codebase will compile as is when upgrading from 1.6.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.

Fixes with compatibility concerns

Default parameter values of @js.native defs are now ignored


In native JS types, default parameters of methods are only meaningful by their presence, but not by their actual values. When calling a method without specifying one of its default parameters, the parameter is actually omitted in JavaScript, and therefore it is up to the called method to fill in its own default behavior. This notably allows to use the pseudo-value = js.native. For example:

@js.native @JSGlobal
object Foo extends js.Object {
  def bar(x: Int, y: Int = js.native): Int = js.native

could be an accurate description of the following JavaScript definition:

const Foo = {
  bar(x, y) {
    return x + (y === undefined ? 1 : y);

When calling that method from Scala.js as


it is translated as, without second parameter, and it is therefore the JavaScript condition on y === undefined that is used.

This behavior applies regardless of the value used in the facade. If we pretend in the facade that the default value is 2, as follows:

  def bar(x: Int, y: Int = 2): Int = js.native

it will have no impact on the behavior of the code. The parameter will still be omitted when calling the JavaScript function, which will still use 1 as the actual default value.

Issue in previous versions

The above behavior has always been specified as is. It was however previously incorrectly implemented for @js.native defs. For example, if bar was a top-level function, we could have defined is as

object Foo {
  @js.native @JSGlobal("bar")
  def bar(x: Int, y: Int = 2): Int = js.native

In that case, until Scala.js 1.7.0, calling was erroneously compiled to bar(5, 2). This is inconsistent with other native JS methods.

That also prevented such definitions to use = js.native, since it would report that js.native cannot be compiled to actual code for run-time execution.

Fix in Scala.js 1.7.1

In Scala.js 1.7.1, the behavior of default parameters in @js.native defs has been aligned with other native JS methods. Now, the call is correctly compiled to bar(5), and therefore uses the JS-defined default value of 1.

This change may break some code when compiling the call site bar(5) with Scala.js 1.7.1, if the code relied on the buggy previous behavior. That may only happen if the default value specified in the facade type is not an accurate representation of what the JavaScript code actually uses.

The fix also allows = js.native to be used in the default parameters of @js.native defs.

Bug fixes

Among others, the following bugs have been fixed in 1.7.1:

  • #4542 Many dynamicImports with FewestModules causes the Linker to GC itself to death
  • #4545 JDK 12+ bug: Referring to non-existent class java.lang.constant.ConstantDesc
  • #4548 Referencing only the class data of an imported native JS class causes a GCC error
  • #4553 js.native as default arg doesn’t compile for top-level method facades
  • #4554 Default values for params of @js.native defs are actually used, instead of being replaced by <UndefinedParam>

You can find the full list on GitHub.