Announcing Scala.js 1.4.0

Jan 12, 2021.

We are excited to announce the release of Scala.js 1.4.0!

This release complements the module splitting support, introduced in Scala.js 1.3.0, with support for dynamic module loading. It is now possible to structure an application such that specific features are only loaded if necessary, the first time they are used.

In addition, this release significantly improves the performance of scala.Array. In particular, arrays of numeric types (except Longs) are implemented with JavaScript typed arrays under the hood.

This release also contains a number of bug fixes. The version of the Scala standard library for 2.13.x was upgraded to 2.13.4.

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 minor release:

  • It is backward binary compatible with all earlier versions in the 1.x series: libraries compiled with 1.0.x through 1.3.x can be used with 1.4.0 without change.
  • It is not forward binary compatible with 1.3.x: libraries compiled with 1.4.0 cannot be used with 1.3.x or earlier.
  • It is not entirely backward source compatible: it is not guaranteed that a codebase will compile as is when upgrading from 1.3.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.

Fixes with compatibility concerns


java.lang.Class.isAssignableFrom had some serious issues, which we fixed in Scala.js 1.4.0. It now completly follows its specification on the JVM. However, it is possible that some code was relying on the broken behavior, which was, in some cases, closer to how isInstance and isInstanceOf works on Scala.js.

In particular, the following idiom, which we have seen before, is now broken more often than before:

def validate[T: ClassTag](x: Any): Option[T] = {
  if (classTag[T].runtimeClass.isAssignableFrom(x.getClass()))

Previously, if the runtimeClass was java.lang.Integer, and the value of x was the integer 5, the above code would successfully return Some(5). With the fixed isAssignableFrom, it will return None on Scala.js 1.4.0.

This happens because x.getClass() returns the smallest numeric type that can hold the value 5, i.e., java.lang.Byte. The previous implementation of isAssignableFrom would return true for classOf[j.l.Integer].isAssignableFrom(classOf[j.l.Byte]), but it now returns false.

The above idiom can be fixed by replacing isAssignableFrom by isInstance:

def validate[T: ClassTag](x: Any): Option[T] = {
  if (classTag[T].runtimeClass.isInstance(x))

Dynamic module loading

In Scala.js 1.3.0, we had introduced module splitting support. Scala.js 1.4.0 goes one step further with dynamic module loading. Using this feature requires to use fastLinkJS/fullLinkJS instead of fastOptJS/fullOptJS, since it generates several .js files for one project. It is supported with any splitting mode and with any number of entry points.

Here is an example:

class HeavyFeature {
  def doHeavyFeature(x: Int): Int =
    x * 2

class MyApp {
  def useHeavyFeature(): Unit = {
    val input: Int = getInput()
    val resultPromise: js.Promise[Int] = js.dynamicImport {
      new HeavyFeature().doHeavyFeature(input)
    for (result <- resultPromise.toFuture)

The js.dynamicImport method has the following signature:

def dynamicImport[A](body: => A): js.Promise[A]

Semantically, it will evaluate body asynchronously and return a Promise of the result. More importantly, it acts as a border for the Scala.js linker to split out a module that will be dynamically loaded. Without going into too many details, the above Scala.js would generate something like the following JavaScript modules:

// heavyfeature.js

class HeavyFeature {
  doHeavyFeature(x) {
    return x * 2;

export function HeavyFeatureEntryPoint(x) {
  return new HeavyFeature().doHeavyFeature(x);
// main.js

class MyApp {
  useHeavyFeature() {
    const input = getInput()
    const resultPromise = import("./heavyfeature.js")
      .then(mod => mod.HeavyFeatureEntryPoint(input));
    resultPromise.then(result => updateUIWithOutput(result));

In other words, the content of the js.dynamicImport {} block is extracted in a separate module heavyfeature.js, along with all its dependencies. The call is replaced by a dynamic import() call, followed by an invocation of the main entry point.

When the main.js application is loaded, we do not need to load, nor download, the heavyfeature.js file. It will only be loaded dynamically the first time we actually call useHeavyFeature(). This reduces initial download times for users.

A word of caution: Scala.js only splits into modules along class boundaries. Therefore, do not put the heavy feature implementation in another method of MyApp, that is called from the js.dynamicImport block. That would defeat the purpose, as the heavy feature would have to be put inside the same module as MyApp. Always make sure that the code called by js.dynamicImport bocks lives in separate classes or objects.



The following APIs were added in the JS types:

  • Added js.RegExp.ExecResult.groups, introduced in ECMAScript 2018 (thanks to @vhiairrassary)
  • Added js.TypedArray.fill, introduced in ECMAScript 2015

Tools API

Users of the scalajs-ir artifact in Scala.js itself may now use the functionality in ir.Hashers. Previously, trying to do so would fail to link with missing

Upgrade to GCC v20210106

We upgraded to the Google Closure Compiler v20210106.

Bug fixes

Among others, the following bugs have been fixed in 1.4.0:

  • #4295 {fast,full}OptJS is still failing for empty reports in 1.3.1, with a NoSuchElementException.
  • #4292 Base64.DecodingInputStream returns 0 instead of -1
  • #4350 Different behavior in Regex#replaceAllIn w.r.t. non-matching groups
  • #4328 j.l.Class.isAssignableFrom has some serious issues
  • #4278 Local JS classes inside anonymous classes cause IR checking errors
  • #4322 Default parameters in non-native JS traits cause IR checking errors
  • #4362 JS constructors that require new cannot be called with expanded varargs (: _*).
  • #3667 RPCCore$RPCException: encoded string too long
  • #4325 withPrettyPrint(true) causes NullPointerException in fullOptJS
  • #4212 Compiled JS files permissions are too restrictive

You can find the full list on GitHub.