reactivecore / kreuzberg   0.7.6

Apache License 2.0 GitHub

ScalaJS Component System

Scala versions: 3.x
Scala.js versions: 1.x
Scala Native versions: 0.4


Kreuzberg is a minimalistic library designed to evaluate the use of Scala JS in web applications through a simple component architecture.

It is built upon:

Please note, this is beta software and some parts may have been hastily written.

Design Goals and Features

The aim of Kreuzberg is to simplify the use of Scala in web applications. Its key features include:

  • Scala 3

  • Full type safety on both the client and server sides

  • Declarative components

  • Separation of the runtime engine from the component implementation. Each component declares how it functions, while the act of combining and running components is performed by an engine.

  • Automatically generated API calls between client and server. One trait is defined and implemented on the server. Stubs (on the client side) and dispatchers (on the server side) are macro-generated.

    Note: This depends on experimental Scala features.

  • Deriving HTML Forms from Case-Classes, including Validation.

  • HTML technique agnosticism. There is support for Scalatags and Scala XML, and more implementations can be added.

  • Theoretically, it should be possible to implement a server-side engine, but this has not yet been done.

Kreuzberg prioritizes ease-of-use over optimal performance or full functional programming.

Building Blocks

  • The most important element is a Component. Each component has an ID and is integrated into the HTML DOM.
  • Each component features an assemble method, which is called when it is about to be rendered or when its associated model changes. The assemble method returns the HTML code, event bindings, and subscribed models.
  • A Model is a mutable element. It has an ID and a default value (if it does not exist yet). Models can be subscribed to by components, meaning they will be re-rendered if the model changes. Models can only change on Events.
  • An EventBinding Components can declare Event Bindings. They are the only way to change model, which triggers component re-rendering.
  • Channels enable n:m communication. Any event can trigger a channel, and any component can register events on them. Unlike Model, Channels do not carry a current value, but each invocation carries a value.
  • A State encapsulates a component's viewing state (e.g., the text entered into a form field). States can be accessed during event handling.

Components, except for the ID mechanism, are functional. They return data structures, and the interpretation is handled by an engine.

For simplification, there is a SimpleComponentBase, which makes it easier to implement a Component. Note, however, that it maintains an inner state to facilitate the implementation of its assemble method.


  • lib: The main Kreuzberg library, required for implementing components.
  • scalatags: Support for ScalaTags.
  • xml: Support for Scala XML. Adds at least 50kb (13kb compressed).
  • extras: Contains various components, including a simple router.
  • rpc: An experimental RPC library for making calls between JavaScript and JVM.
  • examples: Sample applications.
  • miniserver: A simple ZIO-ZHTTP-based server for starting the example application.
  • engine-common: Contains common engine code.
  • engine-naive: Contains the naive rendering engine.
  • engine-zio: A ZIO-based rendering engine.
  • runner: Wraps the naive engine with examples.
  • runner-zio: Wraps the ZIO engine with examples.

How to

  • Compile JavaScript code, in SBT

  • Start the Examples (naive engine)

    runner/run serve
  • Start the Examples (ZIO Engine)

    runnerZio/run serve
  • Watching for changes

    • ScalaJS Only: ~examplesJS/fastOptJS
    • Full: ~runner/reStart


  • Scala Native is not tested