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Announcing Scala.js 1.17.0, and add doc for the Wasm backend.
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‎_config.yml

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### VERSIONS ###
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versions:
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scalaJS: 1.16.0
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scalaJS: 1.17.0
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scalaJSBinary: 1
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scalaJS06x: 0.6.33
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scalaJS06xBinary: 0.6

‎_data/doc.yml

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url: /doc/project/module.html
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- text: JavaScript Environments
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url: /doc/project/js-environments.html
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- text: Emitting WebAssembly
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url: /doc/project/webassembly.html
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- text: Cross-building
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url: /doc/project/cross-build.html
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- text: Testing

‎_data/library/versions.yml

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- 1.12.0
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- 1.13.0
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- 1.16.0
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- 1.17.0

‎_posts/news/2024-09-28-announcing-scalajs-1.17.0.md

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---
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layout: post
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title: Announcing Scala.js 1.17.0
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category: news
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tags: [releases]
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permalink: /news/2024/09/28/announcing-scalajs-1.17.0/
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---
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We are excited to announce the release of Scala.js 1.17.0!
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This release comes with a brand new, shiny, experimental WebAssembly backend.
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You can now, under certain conditions, take your existing Scala.js application and compile it to WebAssembly instead.
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There were also some bug fixes.
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Despite the abnormally long release cycle (v1.16.0 was released 6 months ago), the only external bug report came in 3 weeks ago.
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As far as we can tell, nobody was blocked waiting for a bugfix for this long.
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Read on for more details.
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<!--more-->
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## Getting started
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If you are new to Scala.js, head over to [the tutorial]({{ BASE_PATH }}/tutorial/).
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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).
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Bug reports can be filed [on GitHub](https://github.com/scala-js/scala-js/issues).
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## Release notes
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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.
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This is a **minor** release:
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* It is backward binary compatible with all earlier versions in the 1.x series: libraries compiled with 1.0.x through 1.16.x can be used with 1.17.0 without change.
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* It is *not* forward binary compatible with 1.16.x: libraries compiled with 1.17.0 cannot be used with 1.16.x or earlier.
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* It is *not* entirely backward source compatible: it is not guaranteed that a codebase will compile *as is* when upgrading from 1.16.x (in particular in the presence of `-Xfatal-warnings`).
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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.
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## Enhancements with compatibility concerns
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### Changes to the IR and linker APIs
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For tooling authors who directly manipulate the IR and linker APIs, there have been some breaking changes in that area.
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This is in line with our version policy for the linker APIs.
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The most likely changes you may hit are:
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* The reference types in the IR, such as `ClassType` and `ArrayType`, now have a `nullable: Boolean` flag.
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There is also a new type `AnyNotNullType`.
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* The `NewArray` node does not accept multiple dimensions anymore.
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If you want to emit a multi-dimensional array creation, emit a call to `java.lang.reflect.Array.newInstance` instead.
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## Enhancements
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### Experimental WebAssembly backend
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Starting with this release, Scala.js ships with an *experimental* WebAssembly backend.
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Under some conditions, you may use it as a drop-in replacement for the usual JavaScript backend.
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#### Minimal setup
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You can set it up as follows:
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{% highlight scala %}
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// Emit ES modules with the Wasm backend
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scalaJSLinkerConfig := {
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scalaJSLinkerConfig.value
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.withExperimentalUseWebAssembly(true) // use the Wasm backend
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.withModuleKind(ModuleKind.ESModule) // required by the Wasm backend
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},
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// Configure Node.js (at least v22) to support the required Wasm features
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jsEnv := {
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val config = NodeJSEnv.Config()
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.withArgs(List(
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"--experimental-wasm-exnref", // required
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"--experimental-wasm-imported-strings", // optional (good for performance)
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"--turboshaft-wasm", // optional, but significantly increases stability
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))
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new NodeJSEnv(config)
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},
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{% endhighlight %}
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Make sure `node -v` reports at least v22.0.0.
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If not, install a newer version.
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You are then set up to `run` and `test` your codebase with the WebAssembly backend from sbt.
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#### Limitations
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Note that the WebAssembly backend *silently ignores* all the `@JSExport` and `@JSExportAll` annotations.
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It is never possible to call methods of Scala classes from JavaScript, which includes `toString()`, even through string concatenation.
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JavaScript code may still call all public members of JavaScript classes (classes that inherit from `js.Any`).
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Moreover, `@JSExportTopLevel` is supported, as well as all the other `@JS...` annotations.
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The WebAssembly backend does not yet support emitting multiple modules.
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The module split style must be set to `ModuleSplitStyle.FewestModules` (which is the default).
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Moreover, the codebase must not contain any feature that require emitting multiple modules: `@JSExportToplevel` annotations with several module names, or `js.dynamicImport`.
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We expect to lift that limitation in the future.
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Other than that, we expect the WebAssembly backend to support all Scala.js semantics.
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Please report any issues you may find.
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Stack traces are currently suboptimal.
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#### Use in browsers
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If you want to use it in browsers, you will need:
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* For Firefox: in `about:config`, enable `javascript.options.wasm_exnref`.
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Also make sure to *disable* `javascript.options.wasm_js_string_builtins`: Firefox has two issues with it that break Scala.js ([1919901](https://bugzilla.mozilla.org/show_bug.cgi?id=1919901) and [1920337](https://bugzilla.mozilla.org/show_bug.cgi?id=1920337))
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* For Chrome: in `chrome://flags/`, enable ["Experimental WebAssembly"](chrome://flags/#enable-experimental-webassembly-features).
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#### More information
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Read more detailed information about [the WebAssembly backend in the docs]({{ BASE_PATH }}/doc/project/webassembly.html).
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## Miscellaneous
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### New JDK APIs
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This release adds support for the following JDK methods:
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* In `java.lang.Character`: `codePointAt`, `codePointBefore`, `codePointCount` and `offsetByCodePoints`
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* In `java.util.concurrent.ConcurrentHashMap`: `forEach`, `forEachKey` and `forEachValue`
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### Unicode version
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The Unicode database used by the methods of `java.lang.Character` was updated to Unicode v15.0.
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## Bug fixes
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Among others, the following bugs have been fixed in 1.17.0:
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* [#5026](https://github.com/scala-js/scala-js/issues/5026) Output .js file names can be too long on Windows, esp. for non-ASCII class names.
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* [#5044](https://github.com/scala-js/scala-js/issues/5044) `jl.reflect.Array.newInstance()` does not throw the `IllegalArgumentException`s it is supposed to.
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You can find the full list [on GitHub](https://github.com/scala-js/scala-js/issues?q=is%3Aissue+milestone%3Av1.17.0+is%3Aclosed).

‎assets/badges/scalajs-1.17.0.svg

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‎doc/all-api.md

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## All previous versions of the API
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### Scala.js 1.17.0
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* [1.17.0 scalajs-library]({{ site.production_url }}/api/scalajs-library/1.17.0/scala/scalajs/js/index.html)
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* [1.17.0 scalajs-test-interface]({{ site.production_url }}/api/scalajs-test-interface/1.17.0/)
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* [1.17.0 scalajs-javalib-intf]({{ site.production_url }}/api/scalajs-javalib-intf/1.17.0/)
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* [1.17.0 scalajs-ir]({{ site.production_url }}/api/scalajs-ir/1.17.0/org/scalajs/ir/index.html)
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* [1.17.0 scalajs-linker-interface]({{ site.production_url }}/api/scalajs-linker-interface/1.17.0/org/scalajs/linker/interface/index.html) ([Scala.js version]({{ site.production_url }}/api/scalajs-linker-interface-js/1.17.0/org/scalajs/linker/interface/index.html))
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* [1.17.0 scalajs-linker]({{ site.production_url }}/api/scalajs-linker/1.17.0/org/scalajs/linker/index.html) ([Scala.js version]({{ site.production_url }}/api/scalajs-linker-js/1.17.0/org/scalajs/linker/index.html))
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* [1.17.0 scalajs-test-adapter]({{ site.production_url }}/api/scalajs-sbt-test-adapter/1.17.0/org/scalajs/testing/adapter/index.html)
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* [1.17.0 sbt-scalajs]({{ site.production_url }}/api/sbt-scalajs/1.17.0/#org.scalajs.sbtplugin.package)
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### Scala.js 1.16.0
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* [1.16.0 scalajs-library]({{ site.production_url }}/api/scalajs-library/1.16.0/scala/scalajs/js/index.html)
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* [1.16.0 scalajs-test-interface]({{ site.production_url }}/api/scalajs-test-interface/1.16.0/)

‎doc/internals/version-history.md

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## Version history of Scala.js
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- [1.17.0](/news/2024/09/28/announcing-scalajs-1.17.0/)
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- [1.16.0](/news/2024/03/19/announcing-scalajs-1.16.0/)
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- [1.15.0](/news/2023/12/29/announcing-scalajs-1.15.0/)
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- [1.14.0](/news/2023/09/25/announcing-scalajs-1.14.0/)

‎doc/project/webassembly.md

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---
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layout: doc
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title: Emitting JavaScript modules
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---
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# Experimental WebAssembly backend
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Since Scala.js 1.17.0, there is an *experimental* WebAssembly backend (Wasm for short).
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Under some conditions, it is designed to be a *drop-in* replacement for the usual JavaScript backend.
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## Experimental status
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Being experimental means that:
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* The Wasm backend may be removed in a future *minor* version of Scala.js (or moved to a separate plugin).
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* Future versions of Scala.js may emit Wasm that requires *newer* versions of Wasm engines, dropping support for older engines.
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However, we do *not* expect the the Wasm backend to be any less *correct* than the JS backend, modulo the limitations listed below.
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Feel free to report any issue you may experience with the same expectations as for the JS backend.
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Non-functional aspects, notably performance and size of the generated code, may not be as good as the JS backend for now.
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The backend is also not incremental yet, which means a slower `fastLinkJS` in the development cycle.
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## Requirements
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The Wasm backend emits code with the following requirements:
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* A JavaScript host (i.e., we do not currently generate standalone Wasm)
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* A Wasm engine with support for:
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* Wasm 3.0
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* Wasm GC
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* Exception handling, including the latest `exnref`-based variant
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* The `ESModule` module kind (see [emitting modules](./module.html))
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* Strict floats (which is the default since Scala.js 1.9.0; non-strict floats are deprecated)
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Supported engines include Node.js 22, Chrome and Firefox, all using some experimental flags (see below).
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## Language semantics
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The Wasm backend is nothing but an alternative backend for the Scala.js language.
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Its semantics are the same as Scala.js-on-JS, including JavaScript interoperability features, with one big limitation.
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### Limitation: no `@JSExport` support
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Due to the current feature set of Wasm, it is not possible to implement the semantics of `@JSExport`.
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Therefore, the Wasm backend currently *silently ignores* all `@JSExport` and `@JSExportAll` annotations (the latter being sugar for many `@JSExport`s).
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This limitation has the following consequences:
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* JavaScript code cannot call `@JSExport`ed methods of Scala classes.
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* Since that includes `toString()`, instances of Scala classes cannot be converted to string from JavaScript, including as part of string concatenation.
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(String concatenation *in Scala.js code* is supported.)
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### Limitation: no support for emitting multiple modules
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The WebAssembly backend does not yet support emitting multiple modules.
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The module split style must be set to `ModuleSplitStyle.FewestModules` (which is the default).
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Moreover, the codebase must not contain any feature that require emitting multiple modules: `@JSExportToplevel` annotations with several module names, or `js.dynamicImport`.
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We expect to lift that limitation in the future.
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## Minimal setup
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The following sbt setup enables the Wasm backend and configures flags for Node.js 22.
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{% highlight scala %}
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// Emit ES modules with the Wasm backend
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scalaJSLinkerConfig := {
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scalaJSLinkerConfig.value
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.withExperimentalUseWebAssembly(true) // use the Wasm backend
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.withModuleKind(ModuleKind.ESModule) // required by the Wasm backend
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},
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// Configure Node.js (at least v22) to support the required Wasm features
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jsEnv := {
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val config = NodeJSEnv.Config()
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.withArgs(List(
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"--experimental-wasm-exnref", // required
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"--experimental-wasm-imported-strings", // optional (good for performance)
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"--turboshaft-wasm", // optional, but significantly increases stability
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))
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new NodeJSEnv(config)
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},
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{% endhighlight %}
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Compared to a setup with ES modules with the JS backend, the above setup should be a drop-in replacement.
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The backend emits ES modules with the same layout and interface as those produced by the JS backend.
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## Supported engines
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Here are some engines known to support enough Wasm features.
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### Node.js 22
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As mentioned above, Node.js 22 and above requires the following flags:
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* `--experimental-wasm-exnref`: required
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* `--experimental-wasm-imported-strings`: optional (good for performance)
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* `--turboshaft-wasm`: optional, bug significantly increases stability
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### Chrome
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In `chrome://flags/`, enable ["Experimental WebAssembly"](chrome://flags/#enable-experimental-webassembly-features).
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### Firefox
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In `about:config`, enable `javascript.options.wasm_exnref`.
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Make sure to *disable* `javascript.options.wasm_js_string_builtins`.
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Firefox has two issues with it that break Scala.js ([1919901](https://bugzilla.mozilla.org/show_bug.cgi?id=1919901) and [1920337](https://bugzilla.mozilla.org/show_bug.cgi?id=1920337)).
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## Performance
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Performance of the generated code is currently a hit-or-miss.
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Depending on the codebase, it may be several times faster or slower than the JS backend.
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Further work on improving performance is ongoing.
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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.
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## Code size
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The generated code size of the Wasm backend is currently about twice as big as the JS backend in `fullLink` mode.
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We hope to significantly improve code size in the future by using [`wasm-opt`](https://github.com/WebAssembly/binaryen), a Wasm-to-Wasm optimizer.
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## Implementation details
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Looking for some implementation details of how we compile Scala.js to WebAssembly?
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Start with [the technical readme of the Wasm backend](https://github.com/scala-js/scala-js/tree/main/linker/shared/src/main/scala/org/scalajs/linker/backend/wasmemitter#readme).

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