Scalajs

Scala.js is a compiler that targets JavaScript from Scala source, enabling developers to write client-side code in Scala and run it in web browsers or on Node.js. It translates Scala programs into highly optimised JavaScript and provides a runtime that implements a sizeable portion of the Scala standard library and collections API for the JavaScript environment.

Scala.js was created by Sébastien Doeraene and a community of contributors in the early 2010s, with the aim of bringing the expressiveness and tooling of Scala to frontend development while enabling code reuse with server-side Scala. The project is maintained as an open-source effort with ongoing evolution.

At the core, the Scala.js compiler emits JavaScript and a small runtime library. It supports interoperability with JavaScript through facade types in scala.scalajs.js, allowing Scala to call JavaScript libraries and for JavaScript to call into Scala code via annotations such as @JSExportTopLevel, @JSExportAll, and @JSExport. Scala objects, methods, and classes can be exposed to JavaScript, and Scala can manipulate JavaScript values using dynamic typing through js.Dynamic.

Development uses sbt with the Scala.js sbt plugin. For bundling with modern JavaScript tooling, projects often use scalajs-bundler to integrate with Webpack or Rollup and npm packages. The output is a JavaScript artifact (often with source maps) that runs on modern browsers and Node.js.

Ecosystem and use cases: libraries such as scalajs-dom and scalajs-react provide DOM bindings and React wrappers; Scala.js is used for frontend web apps and libraries, and can share code with backend Scala in some cross-project configurations. Limitations include bridging to JVM-only libraries, performance considerations, and occasional JavaScript environment quirks.