Scala.js (& 0.6.22, react v16.2+)

A react library for scala written in the spirit of ReasonReact, a react library for an OCaml dialect known as reason (sponsored by facebook). ReasonReact documentation is located here and provides a good description of how this library works since the API is very similar. While more advanced scala functionality could have been used, the scala.js implemetation was kept intentionally similar to ReasonReact so that the documentation would apply.

This facade library is small, does not use advanced scala features and provides a different component API based on reason-react vs reactjs. It explicitly supports integrating into an existing javascript UI project or an entirely new one.

You can always code a scala.js based react interface without using any facade library since scala.js supports non-native JS class definitions. scala.js's JS class support is extensive and well thought out. One of the reasons I liked the ReasonReact model is that it rethought the abstractions and API. Instead of trying to copy the reactjs API, it provides something smaller and emphasizes changes in a few dimensions of the reactive UI API problem--almost the very definition of "disruptive."

scalajs-react emphasizes integration into an existing project by making it easy to import/export components and fit into existing application including those using global state-managed solutions such as redux. At the same time, it allows you to build your entire interface in scalajs-react. As long as your front-end solution can manage the model of scala.js's output (one large module for all scala.js code, not file-by-file/module-by-module), you should consider scalajs-react for your solution.


scalajs-react supports fragments and the new context provider in react v16.3.


Include the library in your build:

resolvers += Resolver.bintrayRepo("aappddeevv", "maven")
val scalaJsReactVersion = "latest.version"
//  or
//val scalaJsReactVersion = "0.1.0-M2"

// grab the the latest version or use a specific version
libraryDependencies ++= Seq(
    "ttg" %%% "scalajs-react-core" % scalaJsReactVersion,
    "ttg" %%% "scalajs-react-vdom" % scalaJsReactVersion,
    // optionals
    "ttg" %%% "scalajs-react-fabric" % scalaJsReactVersion,
    // if you integrate with redux
    "ttg" %%% "scalajs-react-redux" % scalaJsReactVersion
    // if you need react-dom
    "ttg" %%% "scalajs-react-react-dom" % scalaJsReactVersion
    // if you need prop-types
    "ttg" %%% "scalajs-react-prop-types" % scalaJsReactVersion)


  • core: react, create-react-class
  • react-dom: react-dom
npm i --save react create-react-class
npm i --save react-dom

The latest react is recommended, v16.2+. If you wish to use the latest features such as the new context, use v16.3+.

Create a Component

You can quickly create a component and render it:

object HelloWorldC {
  val HelloWorld = statelessComponent("HelloWorld")
  import HelloWorld.ops._
  def make(name: Option[String]) =
    render{ self =>
        div("hello world" +" and welcome " + _).getOrElse(""))

object MyWebApp {
   def app() = {

You can add a className to the div using: div(new DivProps { className = "divClassName" })(...children...). DivProps is a non-native JS trait that ensure you only enter valid div props.

The "render" method copies the internal proxy component and adds a callback from reactjs to scala. If you need to use other methods, use the full syntax as the render syntax is just a shortcut to:

    def make(name: Option[String]) =
      HelloWorld.copy(new methods  {
        val render = self => { ... }
        didMount = js.defined{ oldNewSelf => 

The val is required on required methods, such as render, you must define it otherwise you will get a syntax error. Optional methods are defined as shown immediately above.

Exporting a Component to Javascript

If you want to use it in javascript or have a different "make" interface, define the export and map the properties:

object HelloWorld {
  // optionally define a trait to make it easier to typecheck component parameters
  trait HelloWorldProps extends js.Object {
    var name: js.UndefOr[String] = js.undefined
  // Exported to javascript world: <HelloWorld name={"John"} />
  private val exported =
    HelloWorld.wrapScalaForJs((jsProps: HelloWorldProps) => make(

You can define your own "make" API to be whatever suits you:

  // Alternative scala `make` definition, render must convert to scala objects if needed
  // and internal scala code would need to create a HelloWorldProps object.
  def make2(props: HelloWorldProps = noProps()) =
      .render{ self =>

Attributes are type checked properly and restricted to what you specified. Here's how you would use it:

  HelloWorldC.make2(new HelloWorldProps { name = "John" })

Most times the javascript interop has to deal with optional parameters and the conversion from jsProps to make parameters is much more messy. If you do not need to use the class in javascript, you can use standard scala parameters conventions as in make.

Importing a Component from Javascript

If you need to import a javascript class, you can use the standard scala.js machinery to import it:

@JSImport("some-package", JSImport.Namespace)
object SomePackageNS extends js.Object {
  val label: ReactJsComponent = js.native
object SomePackage {
  import ttg.react.elements._
  def Label(props: Attr*)(children: ReactNode*) = wrapJsForScala(SomePackageNS, new Attrs(props).toJs, children:_*)

As with any scala side component, you can adapt the scala side API to be pretty much anything:

object SomePackage {
  // Use a non-native JS trait
  def Label(props: js.UndefOr[LabelProps])(children: ReactNode*) = 
     wrapJsForScala(SomePackageNS.Label, props.getOrElse(noProps), children:_*)

  trait LabelProps extends HTMLAttrbutes[dom.html.Label] { // optionally extend HTMLAttributes[]
   // ...
  // or
  def Label(someProp: String)(children: ReactNode*) = 
     // createProps a function you write to convert the single scala someProp parameter to a js.Object.
     wrapJsForScala(SomePackageNS, createProps(someProp), children:_*)

For importing, you need to call wrapJsForScala at some point. You are free to use the model above for attributes and children or you can use a dedicated non-native JS trait or use whatever suits your application for scala-side API. You could also define the component to take Attr* props, a non-native JS trait or even js.Dynamic.

Since functions are used, there are many advanced scala and scala.js patterns you can use to alter the way that components are created and the API to create elements.For example, you could also use type classes to change the attributes to a js.Object since all you need is some type of "writer."

There is no JSX support.


Styling can be performed different ways. For a quick summary of different aspects of styling designs see this blog.

fabric's merge-styles is provided in the fabric package and you can use css-in-scala concepts to create your styles easily. See the ToDo example source code for a detailed example. It works nicely and is independent of fabric. Tree shaking should remove any fabric UI code if you only want to use merge-styles but not the UI part, and sill want a lean bundle. merge-styles creates stylesheets from scala code. Here's an example showing the use of CSS variables. merge-styles is a little like glamor (which we recommend as well) but is also a bit different in that it is explicit about "selectors".

  // Each key indicates an area of our component.
  trait ToDoClassNames extends js.Object {
    var root: String      = js.native
    var todo: String      = js.native
    var title: String     = js.native
    var dataEntry: String = js.native

  def getClassNames(width: Int = 400) =
        "root" -> new IRawStyle {
          selectors = selectorset(
            ":global(:root)" -> lit(
              "--label-width" -> s"${width}px",
         "todo" -> new IRawStyle {
          displayName = "machina"
          display = "flex"
          marginBottom = "10px"
          selectors = selectorset("& $title" -> new IRawStyle {})
        "title" -> new IRawStyle {
          width = "var(--label-width)"
          marginRight = "10px"
        "dataEntry" -> new IRawStyle {
          display = "flex"
          selectors = selectorset("& .ms-Textfield" -> new IRawStyle {
            width = "var(--label-width)"
            marginRight = "10px"

// now use it

val cn = getClassNames()
val props = new DivProps {
  className = cn.root

If you have dynamically calculated styles, its best to memoize the result so you do not incur the processing overhead on each component creation. The above was static so we just create it once. To memoize, use yours or fabric's memoize. Fabric's memoize needs a js function, so:

// notice the fromFunction1 conversion from scala to js.Function, the "N" is for arity
val memoizedClassNames = fabric.UitilitiesNS.memoizeFunction(js.Any.fromFunction1(getClassName))
val cn = memoized(500)

If you keep your CSS external and process it at build time, you can just import your style sheets and have them picked up by your bundler such as webpack:

@JSImport("Examples/todo/todo.css", JSImport.Namespace)
object componentStyles extends js.Object

object styles {
  // or cast to js.Dictionary or anything else you want to use
  val component = componentStyles.asInstanceOf[js.Dynamic]

// use: styles.component.root

In the example above, you would need to ensure webpack has an alias to the Examples directory and that you have setup your CSS processors correctly. post-css is quite popular when using webpack. Since a Dynamics was used, this is not type safe.

A styling trait is provided to help you ensure you only provide style attributes for inline styling:

val hstyle = new StyleAttr { display = "flex" }

val props = new DivProps {
    style = hstyle

Styling can also be performed using scala specific libraries such as ScalaCSS.


Redux integration requires you to export your component then import it with redux enhanced props. Please see the extended documentation for details.


A simple 100-line router is included based on the reason-react router implementation. See the example app for an example of how to use it.



Integrated API documentation:





The ReasonReact facade, using a modified OCaml syntax, keeps it quite simple. This library is actually quite small and simple and relies on functions. Scala.js easily supports creating classes that map into javascript classes and hence into react component classes. However, with parallel fiber coming to react and other enhancements that are planned, a more functional library should withstand the change in the core react infrastructure to something that is more sustainable in the long run. The current react approach makes it easy to introduce errors both at compile time and runtime. Even with the use of typescript, it's still quite a clunky API. Of course it is clunky for alot of "reasons" which is why Reason and ReasonReact was born. Facebook took an innovative approach to structuring Reason. Instead of trying to make a js superscript, which is what typescript does well, it is trying to go orthogonal like many other javascript transpiled languages have done e.g. elm, dart and of course scala.js.


Scala.js requires a bundling model that does not allow it to be broken apart and deployed on a pure "file/module" basis. Because of this, scalajs-react is best suited for UIs where it is not required to code split the scala portion of the library unless some type of fancy splitter can split apart optimized scala.js emitted code. Scala.js makes use of optimizations, such as inlining, and integrated type infrastructure that makes it difficult to split.



There are a few scala.js react facades/implementations available:



Libraries you may be interested in:

  • diode: redux replacement
  • laminar: a reactive web framework
  • levsha: Fast pure scala.js virtual dom
  • pine: XML/HTML builder.
  • scalacss: A solid css-in-scala solution.
  • scala-dom-types: for dom attributes
  • scala-tags: XML/HTML builder.
  • outwatch: reactive UI framework


MIT license. See the LICENSE file.

Copyright 2018 The Trapelo Group LLC.