Announcing Scala.js 0.5.3

Jul 30, 2014.

We are excited to announce the release of Scala.js 0.5.3!

This release was focused mostly on performance, bringing speedups from 1.3x to 3.3x to your applications. In some cases, Scala.js becomes slightly faster than JavaScript!

Scala.js 0.5.3 is backward binary compatible with older versions of the 0.5.x branch. However, it is not forward binary compatible. This means:

  • You don’t need to re-publish libraries
  • You must upgrade to Scala.js 0.5.3 if any library you depend on uses Scala.js 0.5.3

If you choose to re-publish a library, make sure to bump its version.

Please report any issues on GitHub.

Improvements in the 0.5.3 release

For changes introduced in 0.5.0, how to upgrade, getting started etc. have a look at the 0.5.0 announcement (see also the announcements for 0.5.1 and 0.5.2).

Performance improvements

Scala.js 0.5.3 is the first release to include an actual optimizer specific to Scala.js. This optimizer runs as part of the fastOptJS task, and consistently brings speedups to all applications from 1.3x to 3.3x (Rough benchmarks), along with code size reduction (15 % for the fastOpt version and 6 % for the fullOpt on our demo application) and less memory usage. Because it runs at link time, the optimizer also applies to libraries you depend on that have been compiled with Scala.js 0.5.0 to 0.5.2 (although a few optimizations won’t be as effective).

The optimizer is incremental in the same sense as the incremental compilation of sbt: on each run, it will reoptimize only the parts of your application that need reoptimizing. This means that it will run much faster starting from the second run within an sbt session (within 200 ms in typical scenarios).

Should you experience any issue (e.g., your code broke), please report them on GitHub. You can also disable the optimizer with the sbt setting

ScalaJSKeys.inliningMode := scala.scalajs.sbtplugin.InliningMode.Off

Alternatively, you can force it to run in batch mode (non incremental) on every run with the following setting:

ScalaJSKeys.inliningMode := scala.scalajs.sbtplugin.InliningMode.Batch

New parts of the Java standard library

The following classes from the Java standard library are now available:

  • InputStream, FilterInputStream, DataInput, ByteArrayInputStream
  • scala.scalajs.js.typearray.ArrayBufferInputStream, an implementation of InputStream reading a JavaScript TypedArray

These classes are automatically available in all your Scala.js projects.

Other, additional Java classes are also available in the javalib-ex package. These classes require some features of ECMAScript 6 to be implemented by the JavaScript engine, and must therefore be enabled explicitly with this dependency:

libraryDependencies += "org.scala-lang.modules.scalajs" %% "scalajs-javalib-ex" % scalaJSVersion

Currently, the only additional class is

JavaScript libraries in Node.js

Until 0.5.2, the Node.js runner had trouble running JavaScript libraries that were “too” smart about being run as a Node.js module (bug #706). To fix this issue, the jsDependencies mechanism has been augmented with an optional commonJSName directive, to be used as:

jsDependencies += "org.webjars" % "mustachejs" % "0.8.2" / "mustache.js" commonJSName "Mustache"

The commonJSName directive should be set to the name used by the library to export itself when run in a CommonJS environment (such as Node.js). You can typically figure that out from the library’s documentation.


The following bugs have been fixed in 0.5.3:

  • #820 Generated html for phantomjs on windows fails to load scripts
  • #843 js.Array.toList (and others) fails with a ClassCastException
  • #865 PhantomJS doesn’t use existing shell environment
  • #872 string.split('\n') does not work

Changes to the IR

Some (more) minor changes have been made to the IR to better accommodate the optimizer. This is the reason for the lack of forward binary compatibility in this release.