First you will need to install Java and SBT if you haven't already. Create a new SBT project and add the ScalaJS plugin to your plugins.sbt. Then add the following line to your build.sbt.

Programming language: Scala
License: Apache License 2.0
Latest version: v1.0.0-RC2

OutWatch alternatives and similar packages

Based on the "Reactive Web Frameworks" category.
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The Functional and Reactive Web-Frontend Library for Scala.js

Typelevel incubator Discord outwatch Scala version support

import outwatch._
import outwatch.dsl._
import colibri._
import cats.effect.{IO, IOApp}

object Main extends IOApp.Simple {
  override def run = {
    val counter = Subject.behavior(0)
    val myComponent = div(
      button("+", onClick(counter.map(_ + 1)) --> counter),

    Outwatch.renderReplace[IO]("#app", myComponent)

In Outwatch, you can describe your whole web application without doing any side effect - you only run your application when rendering it.

  • Write UI-components using pure functions
  • Manage state in a referentially transparent way using cats-effect
  • Built-in lightweight Observable and Subject types from colibri
  • Seamlessly works with existing reactive programming libraries: ZIO, fs2, Airstream, scala.rx
  • Low-boilerplate, many convenient helper functions
  • Built on top of snabbdom, a virtual dom library

You will find interactive examples and explanations in our documentation.




The quickest way to play with outwatch is to use our template.

Make sure that java, sbt, nodejs and yarn are installed.

sbt new outwatch/seed.g8

In your newly created project directory, run:

sbt dev

and point your browser to http://localhost:12345.

Manual setup

Add the scalajs and scalajs-bundler plugins to your project/plugins.sbt:

addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.x.x")
addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "x.x.x")

Add the outwatch dependencies to your build.sbt:


resolvers += "jitpack" at "https://jitpack.io"
val outwatchVersion = "<latest outwatch version>"
libraryDependencies ++= Seq(
  "io.github.outwatch"   %%% "outwatch"          % outwatchVersion,
  // optional dependencies:
  "com.github.cornerman" %%% "colibri-zio"       % "0.7.1", // zio support
  "com.github.cornerman" %%% "colibri-fs2"       % "0.7.1", // fs2 support
  "com.github.cornerman" %%% "colibri-airstream" % "0.7.1", // sirstream support
  "com.github.cornerman" %%% "colibri-rx"        % "0.7.1", // scala.rx support
  "com.github.cornerman" %%% "colibri-router"    % "0.7.1", // Url Router support

Bugs and Feedback

For bugs, questions and discussions please use GitHub Issues.


We adopted the Scala Code of Conduct. People are expected to follow it when discussing Outwatch on the Github page, Gitter channel, or other venues.


Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

*Note that all licence references and agreements mentioned in the OutWatch README section above are relevant to that project's source code only.