Announcing Scala.js 1.20.0.

Author: sjrd

_config.yml

@@ -64,8 +64,8 @@ colors:  #in hex code if not noted else
 
 ### VERSIONS ###
 versions:
-  scalaJS: 1.19.0
+  scalaJS: 1.20.0
   scalaJSBinary: 1
   scalaJS06x: 0.6.33
   scalaJS06xBinary: 0.6
-  scalaJSDOM: 2.8.0
+  scalaJSDOM: 2.8.1

_data/library/versions.yml

@@ -36,3 +36,4 @@
 - 1.17.0
 - 1.18.0
 - 1.19.0
+- 1.20.0

_posts/news/2025-09-05-announcing-scalajs-1.20.0.md

@@ -0,0 +1,145 @@
+---
+layout: post
+title: Announcing Scala.js 1.20.0
+category: news
+tags: [releases]
+permalink: /news/2025/09/05/announcing-scalajs-1.20.0/
+---
+
+
+We are pleased to announce the release of Scala.js 1.20.0!
+
+This release mainly comes with many performance improvements to the WebAssembly and JavaScript backends alike.
+
+As of this writing, the latest versions of Firefox (since v131), Safari (since v18.4) and Chrome (since v137) support all the WebAssembly features required to run Scala.js-on-Wasm.
+
+Read on for more details.
+
+<!--more-->
+
+## Getting started
+
+If you are new to Scala.js, head over to [the tutorial]({{ BASE_PATH }}/tutorial/).
+
+If you need help with anything related to Scala.js, you may find our community [in `#scala-js` on Discord](https://discord.com/invite/scala) and [on Stack Overflow](https://stackoverflow.com/questions/tagged/scala.js).
+
+Bug reports can be filed [on GitHub](https://github.com/scala-js/scala-js/issues).
+
+## Release notes
+
+If upgrading from Scala.js 0.6.x, make sure to read [the release notes of Scala.js 1.0.0]({{ BASE_PATH }}/news/2020/02/25/announcing-scalajs-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 1.19.x can be used with 1.20.0 without change.
+* It is *not* forward binary compatible with 1.19.x: libraries compiled with 1.20.0 cannot be used with 1.19.x or earlier.
+* It is *not* entirely backward source compatible: it is not guaranteed that a codebase will compile *as is* when upgrading from 1.19.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
+
+### Link-time conditional branching
+
+Thanks to our optimizer's ability to inline, constant-fold, and then eliminate dead code, we have been able to write link-time conditional branches for a long time.
+Typical examples include polyfills, as illustrated in the documentation of [`scala.scalajs.LinkingInfo`](https://javadoc.io/static/org.scala-js/scalajs-library_2.13/1.19.0/scala/scalajs/LinkingInfo$.html#esVersion:Int):
+
+{% highlight scala %}
+if (esVersion >= ESVersion.ES2018 || featureTest())
+  useES2018Feature()
+else
+  usePolyfill()
+{% endhighlight %}
+
+which gets folded away to nothing but
+
+{% highlight scala %}
+useES2018Feature()
+{% endhighlight %}
+
+when optimizing for ES2018+.
+
+However, this only works because both branches can *link* during the initial reachability analysis. We cannot use the same technique when one of the branches would refuse to link in the first place.
+The canonical example is the usage of the JS `**` operator, which does not link below ES2016. The following snippet produces good code when linking for ES2016+, but does not link at all for ES2015:
+
+{% highlight scala %}
+def pow(x: Double, y: Double): Double = {
+  if (esVersion >= ESVersion.ES2016) {
+    (x.asInstanceOf[js.Dynamic] ** y.asInstanceOf[js.Dynamic]) // does not link!
+      .asInstanceOf[Double]
+  } {
+    Math.pow(x, y)
+  }
+}
+{% endhighlight %}
+
+Scala.js 1.20.0 introduces `scala.scalajs.LinkingInfo.linkTimeIf`, a conditional branch that is *guaranteed* by spec to be resolved at link-time.
+Using a `linkTimeIf` instead of the `if` in `def pow`, we can successfully link the fallback branch on ES2015, avoiding the linking issue in the then branch.
+
+{% highlight scala %}
+import scala.scalajs.LinkingInfo.linkTimeIf
+
+def pow(x: Double, y: Double): Double = {
+  linkTimeIf(esVersion >= ESVersion.ES2016) {
+    // this branch is only *linked* if we are targeting ES2016+
+    (x.asInstanceOf[js.Dynamic] ** y.asInstanceOf[js.Dynamic])
+      .asInstanceOf[Double]
+  } {
+    Math.pow(x, y)
+  }
+}
+{% endhighlight %}
+
+The condition of `linkTimeIf` must be a link-time constant expression.
+It can only contain:
+
+* int and boolean literals and constants,
+* boolean operators (`!`, `&&` and `||`),
+* int comparison operators (such as `==` and `<`),
+* link-time properties in the `LinkingInfo` object, notably `esVersion`, `productionMode` and `isWebAssembly`.
+
+### Performance improvements
+
+Scala.js 1.20.0 brings a number of performance improvements.
+
+For both JavaScript and WebAssembly:
+
+* Many low-level methods in `java.lang.Integer`, `Long`, `Float`, `Double`, `Math` and `System` have been micro-optimized.
+* Scala `Range`s have a faster initialization path (this particular enhancement is also coming in Scala 2.13.17 for the JVM).
+
+For JavaScript:
+
+* `Long` performance was significantly enhanced, notably additive operations, conversions to strings and conversions to floating-point numbers.
+
+For WebAssembly, performance improvements in the following areas:
+
+* Varargs, when compiled with Scala.js 1.20.0+ on Scala 2, and with the upcoming Scala 3.8.0+ on Scala 3
+* `java.util.ArrayList`, `ArrayDeque`, `PriorityQueue` and `java.util.concurrent.CopyOnWriteArrayList`
+
+## Miscellaneous
+
+### New JDK APIs
+
+This release adds support for the following JDK methods:
+
+* In `java.lang.Math`:
+  * `multiplyFull`
+  * `multiplyHigh`
+  * `unsignedMultiplyHigh`
+* In `java.lang.Double`:
+  * `doubleToRawLongBits`
+* In `java.lang.Float`:
+  * `floatToRawIntBits`
+* In `java.lang.Integer` and `java.lang.Long`:
+  * `compress`
+  * `expand`
+
+## Bug fixes
+
+The following bugs have been fixed in 1.20.0:
+
+* [#5159](https://github.com/scala-js/scala-js/issues/5159) Regression in v1.19.0: `AssertionError` with module splitting on and optimizer off.
+* [#5165](https://github.com/scala-js/scala-js/issues/5165) Wasm: `try..finally` with non-nullable reference type produces invalid code.
+* [#5208](https://github.com/scala-js/scala-js/issues/5208) `doubleToLongBits` can observe non-canonical NaN bit patterns.
+
+You can find the full list [on GitHub](https://github.com/scala-js/scala-js/issues?q=is%3Aissue+milestone%3Av1.20.0+is%3Aclosed).

assets/badges/scalajs-1.20.0.svg

@@ -5,6 +5,16 @@ title: All previous versions of the Scala.js API
 
 ## All previous versions of the API
 
+### Scala.js 1.20.0
+* [1.20.0 scalajs-library]({{ site.production_url }}/api/scalajs-library/1.20.0/scala/scalajs/js/index.html)
+* [1.20.0 scalajs-test-interface]({{ site.production_url }}/api/scalajs-test-interface/1.20.0/)
+* [1.20.0 scalajs-javalib-intf]({{ site.production_url }}/api/scalajs-javalib-intf/1.20.0/)
+* [1.20.0 scalajs-ir]({{ site.production_url }}/api/scalajs-ir/1.20.0/org/scalajs/ir/index.html)
+* [1.20.0 scalajs-linker-interface]({{ site.production_url }}/api/scalajs-linker-interface/1.20.0/org/scalajs/linker/interface/index.html) ([Scala.js version]({{ site.production_url }}/api/scalajs-linker-interface-js/1.20.0/org/scalajs/linker/interface/index.html))
+* [1.20.0 scalajs-linker]({{ site.production_url }}/api/scalajs-linker/1.20.0/org/scalajs/linker/index.html) ([Scala.js version]({{ site.production_url }}/api/scalajs-linker-js/1.20.0/org/scalajs/linker/index.html))
+* [1.20.0 scalajs-test-adapter]({{ site.production_url }}/api/scalajs-sbt-test-adapter/1.20.0/org/scalajs/testing/adapter/index.html)
+* [1.20.0 sbt-scalajs]({{ site.production_url }}/api/sbt-scalajs/1.20.0/#org.scalajs.sbtplugin.package)
+
 ### Scala.js 1.19.0
 * [1.19.0 scalajs-library]({{ site.production_url }}/api/scalajs-library/1.19.0/scala/scalajs/js/index.html)
 * [1.19.0 scalajs-test-interface]({{ site.production_url }}/api/scalajs-test-interface/1.19.0/)

doc/all-api.md

@@ -5,6 +5,7 @@ title: Version history
 
 ## Version history of Scala.js
 
+- [1.20.0](/news/2025/09/05/announcing-scalajs-1.20.0/)
 - [1.19.0](/news/2025/04/21/announcing-scalajs-1.19.0/)
 - [1.18.2](/news/2025/01/23/announcing-scalajs-1.18.2/)
 - [1.18.1](/news/2025/01/09/announcing-scalajs-1.18.1/)

doc/internals/version-history.md

@@ -32,8 +32,8 @@ The Wasm backend emits code with the following requirements:
   * Exception handling, including the latest `exnref`-based variant
 * The `ESModule` module kind (see [emitting modules](./module.html))
 
-Supported engines include Node.js 23, Chrome, Firefox and Safari.
-Node.js and Chrome currently require using some experimental flags (see below).
+Supported engines include Node.js 23, Chrome 137, Firefox 131 and Safari 18.4.
+Node.js currently requires using some experimental flags (see below).
 
 ## Language semantics
 
@@ -102,9 +102,7 @@ As mentioned above, Node.js 23 and above requires the following flags:
 
 ### Chrome
 
-In `chrome://flags/`, enable ["Experimental WebAssembly"](chrome://flags/#enable-experimental-webassembly-features).
-
-For `js.async`/`js.await` support, also enable ["Experimental WebAssembly JavaScript Promise Integration (JSPI)"](chrome://flags/#enable-experimental-webassembly-jspi).
+Firefox supports all the features we use since v137.
 
 ### Firefox
 
@@ -123,7 +121,7 @@ Safari supports all the "always required" features since v18.4.
 
 If the performance of your application depends a lot on JavaScript interop, the Wasm output may be (significantly) slower than the JS output.
 
-For applications whose performance is dominated by computions inside Scala code, the Wasm output should be significantly faster than the JS output (geomean 15% lower run time across our benchmarks).
+For applications whose performance is dominated by computions inside Scala code, the Wasm output should be significantly faster than the JS output (geomean 30% lower run time across our benchmarks).
 
 Further work on improving performance is ongoing.
 Keep in mind that performance work on the Wasm backend is a few months old, compared to a decade of optimizations in the JS backend.