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OutWatch - Functional and reactive Web-Frontend Library with Reactive Programming, VirtualDom and Scala Typelevel incubator Build Status Scala.js Gitter chat

Syntax is almost exactly as in ScalaTags. The UI can be made reactive and allows for easy integration of third-party FRP libraries (for example Monix, scala.rx or airstream). We integrate tightly with cats and cats-effect to build safe web applications. In OutWatch, you can describe your whole web application without doing any side effect - you only run your application when rendering it.

import outwatch._
import outwatch.dsl._

val hello = h1("Hello World")

val app = OutWatch.renderInto[IO]("#app", hello)

// Nothing happend yet, you just described your web application.
// Now it's time to push the start button:

You can find more examples and features at the end of this readme.

Getting started

Start with a template

For a quick start, install java, sbt, nodejs and yarn and use the following g8 template:

sbt new outwatch/seed.g8

In your newly created project folder, run:

sbt dev

and point your browser to http://localhost:8080.

Changes to the code will trigger a recompile and automatically refresh the page in the browser.

Use in an already existing project

Install java, sbt and nodejs, if you haven't already. Create a new SBT project and add the ScalaJS and Scala-js-bundler plugin to your plugins.sbt:

addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.1.1")
addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "0.18.0")

Then add the outwatch dependency to your build.sbt, you can depend on the current master branch:

resolvers += "jitpack" at ""
libraryDependencies += "com.github.outwatch.outwatch" %%% "outwatch" % "master-SNAPSHOT"

If you want to use the latest stable release, you can use:

libraryDependencies += "io.github.outwatch" %%% "outwatch" % "1.0.0-RC2"

And enable the scalajs-bundler plugin:


If you want to use utilities for Store, WebSocket or Http, add the following:

libraryDependencies += "com.github.outwatch.outwatch" %%% "outwatch-util" % "master-SNAPSHOT"

If you want support for Monix in OutWatch, you need to add the following dependency as well:

libraryDependencies += "com.github.cornerman.colibri" %%% "colibri-monix" % "master-SNAPSHOT"
libraryDependencies += "com.github.outwatch.outwatch" %%% "outwatch-monix" % "master-SNAPSHOT" // for handler factories

If you want support for scala.rx in OutWatch, you need to add the following dependency as well:

libraryDependencies += "com.github.cornerman.colibri" %%% "colibri-rx" % "master-SNAPSHOT"

When using JitPack, it is often more useful to point to a specific commit, to make your builds reproducible:

libraryDependencies += "com.github.outwatch.outwatch" %%% "outwatch" % "f07849c81"

Like that you can try the latest features from specific commits on master, other branches or PRs.

To configure hot reloading with webpack devserver, check out build.sbt and from the g8 template.

If anything is not working, cross-check how things are done in the template.

Use common javascript libraries with OutWatch

We have prepared helpers for some javascript libraries. You can find them in the OutWatch-libs Repository.

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.

Documentation and Examples

Outwatch is a web frontend UI framework written in ScalaJS.

If you find any error in the examples, please open an issue on GitHub.

There is a changelog which contains examples of the latest changes:

There is also the outdated but conceptually still correct documentation - contributions welcome.

Hello World

In your html file, create an element, which you want to replace by dynamic content:

    <div id="app"></div>
    <!-- your compiled javascript should be imported here -->

To render html content with outwatch, create a component and render it into the given element:

import outwatch._
import outwatch.dsl._

object Main {
  def main(args: Array[String]): Unit = {
    val myComponent = div("Hello World")

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

Running Main will replace <div id="app"></div> with the content defined in myComponent:

    <div id="app">Hello World</div>

Important: In your application, OutWatch.renderReplace should be called only once at the end of the main method. To create dynamic content, you will design your data-flow with Obvervable, Subject and/or Handler and then instantiate it only once with this method call. Before that, no Observable subscription will happen.

Static Content

First, we will focus on creating immutable/static content that will not change over time. The following examples illustrate to construct and transform HTML/SVG tags, attributes and inline stylesheets.

Concatenating Strings

div("Hello ", "World")
// <div>Hello World</div>


div(span("Hey ", b("you"), "!"))
// <div><span>Hey <b>you</b>!</span></div>


div(true, 0, 1000L, 3.0)
// <div>true010003.0</div>


Attributes are put inside the tag.

div(idAttr := "test")
// <div id="test"></div>

The order of content and attributes does not matter.

div("How ", idAttr := "test", "are", title := "cool", " you?")
// <div id="test" title="cool">How are you?</div>


Styles are also written into the tag. All style properties have to be written in camelCase.

div(color := "tomato", "Hello")
// <div style="color: tomato">Hello</div>

Multiple styles will me merged to one style attribute:

div(backgroundColor := "powderblue", border := "2px solid #222", "Hello")
// <div style="background-color: powderblue; border: 2px solid #222">Hello</div>

Again, the order of styles, attributes and inner tags does not matter:

div(h1("Welcome to my website"), backgroundColor := "powderblue", idAttr := "header")
// <div style="background-color: powderblue" id="header">Welcome to my website</div>

Some styles have type safe values:

div(cursor.pointer, fontWeight.bold, display.flex)
// <div style="cursor: pointer; font-weight: bold; display: flex;"></div>

If you are missing more type safe values, please contribute to Scala Dom Types. Example implementation: fontWeight

Attributes, which are scala keywords

There are some attributes and styles which are reserved scala keywords. You can use them with backticks:

div(`class` := "item", "My Item")
// <div class="item">My Item</div>

label(`for` := "inputid")
// <label for="inputid" />

There is also a shortcut for the class atrribute:

div(cls := "myclass")
// <div class="myclass"></div>

Overriding attributes

Attributes and styles with the same name will be overwritten. Last wins.

div(color := "blue", color := "green")
// <div style="color: green"></div>

Class accumulation

Classes are not overwritten, they accumulate.

div(cls := "tiny", cls := "button")
// <div class="tiny button"></div>

Custom attributes, styles and tags

All the tags, attributes and styles available in outwatch come from Scala Dom Types. If you want to use something not available in Scala Dom Types, you can use custom builders:

htmlTag("app")(style("user-select") := "none", attr("everything") := "possible")
// <app style="user-select: none" everything="possible"></div>

You can also define the accumulation behavior of custom attributes:

  attr("everything").accum("-") := "is",
  attr("everything").accum("-") := "possible",
// <div everything="is-possible"></div>

If you think there is something missing in Scala Dom Types, please open a PR or Issue. Usually it's just one line of code.

Source Code: DomTypes.scala

Data attributes

Data attributes make use of scala.Dynamic, so you can write things like:

div(data.payload := "17")
// <div data-payload="17"></div>

Source Code: OutwatchAttributes.scala, Builder.scala


SVG tags and attributes are available via an extra import. Namespacing is automatically handled for you.

val graphic = {
 import svg._
   viewBox := "0 0 10 10",
     transform := "matrix(.096584 0 0 .096584 -.0071925 -18.66)",
     path(d := "M10 10 C 20 20, 40 20, 50 10", fill := "mistyrose")

// <svg viewBox="0 0 10 10"><g transform="matrix(.096584 0 0 .096584 -.0071925 -18.66)"><path d="M10 10 C 20 20, 40 20, 50 10" fill="mistyrose"></path></g></svg>

Option and Seq

Outwatch tries hard to render everything you throw at it. Combine Option and Seq to fit your needs. Note, that outwatch does not accept Set, since the order is undefined.

  Some(color := "steelblue"),
  fontSize :=? Some("70px"),
  Seq("Hey", "How are you?"),
  List("a", "b", "c").map(span(_)),
// <div style="color: steelblue; font-size: 70px;">
//   thing
//   Hey
//   How are you?
//   <span>a</span>
//   <span>b</span>
//   <span>c</span>
//   x
// </div>


The important types we were using in the examples above are VNode and VDomModifier:

val vnode: VNode = div()
val modifiers: List[VDomModifier] = List("Hello", idAttr := "main", color := "tomato", vnode)

Every VNode contains a sequence of VDomModifier. A VNode is a VDomModifier itself.

There are implicits for converting primitives, Option[VDomModifier], Seq[VDomModifier] to VDomModifier.

Source Code: Render.scala

Grouping Modifiers

To make a set of modifiers reusable you can group them to become one VDomModifier.

val bigFont = VDomModifier(fontSize := "40px", fontWeight.bold)
div("Argh!", bigFont)
// <div style="font-size: 40px; font-weight: bold;">Argh!</div>

If you want to reuse the bigFont, but want to overwrite one of its properties, you can combine two VDomModifier. Here the latter fontSize will overwrite the one from bigFont:

val bigFont2 = VDomModifier(bigFont, fontSize := "99px")

You can also use a Seq[VDomModifier] directly instead of using apply defined in the VDomModifier object.


Outwatch does not have the concept of a component itself. You can just pass the VNodes and VDomModifiers around and build your own abstractions using functions and classes. When we are talking about components in this documentation, we are usually referring to a VNode or a function returning a VNode.

def fancyHeadLine(content: String) = h1(borderBottom := "1px dashed tomato", content)
fancyHeadLine("I like tomatoes.")
// <h1 style="border-bottom: 1px dashed tomato;">I like tomatoes.</h1>

Transforming Components

Components are immutable, we can only modify them by creating a changed copy. Like you may know from Scalatags, you can call .apply(...) on any VNode, append more modifiers and get a new VNode with the applied changes back.

val a = div("dog")
a(title := "the dog")
// <div title="the dog">dog</div>

This can be useful for reusing html snippets.

val box = div(width := "100px", height := "100px")

  box(backgroundColor := "powderblue"),
  box(backgroundColor := "mediumseagreen"),

// <div>
//  <div style="width: 100px; height: 100px; background-color: powderblue;"> </div>
//  <div style="width: 100px; height: 100px; background-color: mediumseagreen;"></div>
// </div>

Since modifiers are appended, they can overwrite existing ones. This is useful to adjust existing components to your needs.

val box = div(width := "100px", height := "100px")
box(backgroundColor := "mediumseagreen", width := "200px")
// <div style="width: 200px; height: 100px; background-color: mediumseagreen;"></div>

You can also prepend modifiers. This can be useful to provide defaults retroactively.

def withBorderIfNotProvided(vnode: VNode) = vnode.prepend(border := "3px solid coral")
  withBorderIfNotProvided(div("hello", border := "7px solid moccasin")),
// <div>
//   <div style="border: 7px solid moccasin;">hello</div>
//   <div style="border: 3px solid coral;">hello</div>
// </div>

Source Code: VDomModifier.scala

Use-Case: Flexbox

When working with Flexbox, you can set styles for the container and children. With VNode.apply() you can have all flexbox-related styles in one place. The child-components don't have to know anything about flexbox, even though they get specific styles assigned.

val itemA = div("A", backgroundColor := "mediumseagreen")
val itemB = div("B", backgroundColor := "tomato")

  height := "100px",
  border := "1px solid black",


  itemA(flexBasis := "50px"),
// <div style="height: 100px; border: 1px solid black; display: flex;">
//   <div style="background-color: mediumseagreen; flex-basis: 50px;">A</div>
//   <div style="background-color: tomato; align-self: center;">B</div>
// </div>

Rendering Async Effects

You can render any cats.effect.Effect type like cats.effect.IO or monix.eval.Task. The effect will be run async whenever an element is rendered with this modifier. So you can do:

  IO {

If you have an effect that is synchronous in nature, you should consider using a sync effect type instead for performance reasons. A sync effect can be rendered in one go, whereas an async effect might need to patch the dom after it is finished.

Rendering Sync Effects

You can render synchronous effects like cats.effect.SyncIO or monix.eval.Coeval as well (we currently use our own typeclass colibri.effect.RunSyncEffect and will port to cats.effect.SyncEffect as soon as it is available). The effect will be run sync whenever an element is rendered with this modifier. Example:

  SyncIO {

Alternatively you can do the following to achieve the same effect:

  VDomModifier.delay {

Rendering Futures

You can render Futures as well:

  Future {

Rendering Custom Types

You can render any custom type! Say you have the following type:

case class Person(name: String, age: Int)

And now you want to be able to render a Person just like a normal modifier. This can be done by providing an instance of the Render typeclass:

object Person {

  implicit object PersonRender extends Render[Person] {
    def render(person: Person): VDomModifier = div(
        b(, span(person.age, marginLeft := "5px")


Thats it, now you can just use Person in your dom definitions:

val person = Person("Hans", age = 48)


Source Code: Render.scala

Dynamic Content

Dynamic content can be displayed as well. OutWatch comes with its own minimal reactive library that you can just start using. Additionally we have first-class support for third-party streaming libraries.

You can use observables, streams and reactive variables as if they were a VDomModifier.

import outwatch._
import outwatch.dsl._

import colibri.Observable

import concurrent.duration._

object Main {
  def main(args: Array[String]): Unit = {

    val counter = Observable.interval(1 second)
    val counterComponent = div("count: ", counter)

    OutWatch.renderReplace[IO]("#app", counterComponent).unsafeRunSync()

You can write the same application with monix:

import outwatch._
import outwatch.dsl._

import monix.reactive.Observable
import colibri.ext.monix._

import concurrent.duration._

object Main {
  def main(args: Array[String]): Unit = {

    val counter = Observable.interval(1 second) // using monix observable
    val counterComponent = div("count: ", counter)

    OutWatch.renderReplace[IO]("#app", counterComponent).unsafeRunSync()

Important: In your application, OutWatch.renderReplace should be called only once at the end of the main method. To create dynamic content, you will design your data-flow with Obvervable, Subject and/or Handler and then instantiate it only once with this method call. Before that, no Observable subscription will happen.

Dynamic attributes

Attributes can also take dynamic values.

val color = Observable.interval(1 second).map(i => if(i % 2 == 0) "deepskyblue" else "gold")
div(width := "100px", height := "100px", backgroundColor <-- color)

Streaming Modifiers and VNodes

You can stream arbitrary VDomModifiers.

val dynamicSize: Observable[VDomModifier] = Observable.interval(1 second).map(i => fontSize := s"${i}px")
div("Hello!", dynamicSize)
val nodeStream: Observable[VNode] = Observable(div("I am delayed!")).delay(5.seconds)
div("Hello ", nodeStream)

Events and EmitterBuilders

We are working with dom elements and want to react to events of these dom elements. For this, there is EmitterBuilder. For example, take the click event for which onClick is an EmitterBuilder:

button(onClick.foreach { println("Element was clicked!") })

You can further combine, map, collect, filter and transform EmitterBuilder:

button(EmitterBuilder.combine(onMouseUp.use(false), onMouseDown.use(true)).foreach { isDown => println("Button is down? " + isDown })
// this is the same as: button(EmitterBuilder.combine( => false), => true)).foreach { isDown => println("Button is down? " + isDown })

Furthermore, you can create EmitterBuilders from streams with emitter(stream) (EmitterBuilder.ofSource) or create custom EmitterBuilders with EmitterBuilder.ofModifier, EmitterBuilder.ofNode or EmitterBuilder.apply.

Global events

There are helpers for getting a stream of global document or window events, where you get a colibri.Observable for these events. For example:

import outwatch.dsl._

events.document.onClick.foreach { event => console.log("Click Event", event) }

events.window.onStorage.foreach { event => console.log("Storage Event", event) }

div(emitter(events.document.onClick) --> sink)

Managing dynamic state

We have seen how to render dynamic content. But how to manage dynamic state? You want to have a reactive variable to hold a variable for you, so you can write into it and stream values from it.

Example: Counter

Let's implement a counter in a button. OutWatch provides Handler, which is a factory to create a reactive variable (more details later):

import outwatch.reactive.handler._

// Handler.create returns `SyncIO`
val counter: SyncIO[VNode] = Handler.create[Int](0).map { handler =>
    button(onClick( + 1)) --> handler, "Counter: ", handler)

Alternative version of a counter:

val counter = button(
    onClick.useScan0(0)(_ + 1).handled { source =>
        VDomModifier("Counter: ", source)
Example: Input Field

A more involved example is an input field where you want to capture the current value:

  def render: SyncIO[VNode] = for {
    handler <- Handler.create[String]("Initial")
  } yield {
      label( "Example!" ),
        value <-- handler, // by writing into the `handler`, you could change the input value
        onChange.value --> handler
      div("Current Value:", handler)
Handler factories

Methods like Handler.create are available for different streaming libarries (e.g. our own colibri and monix):

import outwatch.reactive.handler._ // or outwatch.reactive.handlers.monix._

val handler: SyncIO[Handler[Int]] = Handler.create[Int](1)
val source: HandlerSource[Int] = HandlerSource[Int](3)
val sink: HandlerSink[Int] = HandlerSink.create(onNext, onError)

You typically just want one of these Environments in scope, the types would name-clash overwise. And the handler environment is totally optional, you can create your Handlers of the library of your choice yourself and everything will work out of the box without the environment. The environment is just to have a vocabulary of how to create sources, sinks, handlers without using concrete types names from third-party libraries. This way you can switch a whole file to another streaming library by merely changing an import from outwatch.reactive.handler._ to outwatch.reactive.handlers.monix._. Of course, this will never cover all complex details that might be needed, but is target for basic use-cases and should be enough for most things. You have the streaming typeclasses as well to abstract further over your types.

Referential transparency

The factory Handler.create returns a SyncIO to be referential transparent. Take the counter example:

def getCounterComponent: SyncIO[VNode] = Handler.create(0).map { handler => // alternative: Handler.createF[IO]
    button(onClick( + 1)) --> handler, handler)

def getUnsafeCounterComponent: VNode = {
    val handler = Handler.unsafe(0)
    button(onClick( + 1)) --> handler, handler)

// We can test whether the above functions are referential transparent

// The first one is referential transparent, because component1 and component2 are equivalent
val counter = getCounterComponent
val component1 = div(counter, counter) // two counter buttons with separate state
val component2 = div(getCounterComponent, getCounterComponent) // two counter buttons with separate state

// The second one is not, because component1 and component2 behave differently
val counter = getUnsafeCounterComponent
val component1 = div(counter, counter) // two counter buttons which share the same state
val component2 = div(getUnsafeCounterComponent, getUnsafeCounterComponent) // two counter buttons with separate state

// For the first one, you can share the state explicitly
val component = { counter => div(counter, counter) }

Why should we care? Because referential transparent functions can easily be refactored without affecting the meaning of the program and it is easier to reason about them. If you do not care about this, you could use Handler.unsafe or create the Handler yourself with the streaming library of your choice.

Lifecycle Mangement

OutWatch automatically handles subscriptions of streams and observables in your components. You desribe your component with static and dynamic content (handlers, event emitters, --> and <--). When using these components, the needed subscriptions will be tight to the lifecycle of the respective dom elements and are managed for you. So whenever an element is mounted the subscriptions are run and whenever it is unmounted the subscriptions are killed.

If you ever need to manually subscribe to a stream, you can let OutWatch manage them for you:

  managed(SyncIO { observable.subscribe(...) }) // this subscription is now bound to the lifetime of the outer div element


Accessing the DOM Element

Sometimes you need to access the underlying DOM element of a component. But a VNode in OutWatch is just a description of a dom element and we can create multiple different elements from one VNode. Therefore, there is no static element attached to a component. Though, you can get access to the dom element via hooks (callbacks):

    onDomMount.foreach { element => // the element into which this node is mounted
    onDomUnmount.foreach { element => // the element from which this node is unmounted

We have a higher-level API to work with these kinds of callbacks, called managedElement, which can be used like this:

    managedElement { element => // the element into which this node is mounted
        ??? // do something with the dom element
        cancelable(() => ???) // cleanup when the dom element is unmounted

You can also get the current element when handling dom events, for example onClick:

    onClick.asElement.foreach { element =>
    } // this is the same as

If the emitter does not emit events or elements, but you still want to access the current element, you can combine it with another emitter. For example:

    emitter(someObservable).useLatestEmitter(onDomMount).foreach { element =>

If you need an HTML or SVG Element instead of just an Element, you can do:

onDomMount.asHtml --> ???
onDomMount.asSvg --> ???
onClick.asHtml --> ???
onClick.asSvg --> ???
managedElement.asHtml { ??? }
managedElement.asSvg { ??? }

Using other streaming libraries

We use the library colibri for a minimal reactive library and for typeclasses around streaming. These typeclasses like Source and Sink allow to integrate third-party libraries for streaming easily.

For using outwatch with monix:

import colibri.ext.monix._
import outwatch.reactive.handlers.monix._ // for handler factories

For using outwatch with scala.rx:

import colibri.ext.rx._


Source Code: OutwatchTracing.scala

Tracing snabbdom patches

Show what patches snabbdom emits:

helpers.OutwatchTracing.patch.zipWithIndex.foreach { case (proxy, index) =>
  org.scalajs.dom.console.log(s"Snabbdom patch ($index)!", JSON.parse(JSON.stringify(proxy)), proxy)

Tracing exceptions in your components

Dynamic components with Observables can have errors. This is if onError is called on the underlying Observer. You can trace them in OutWatch with:

helpers.OutwatchTracing.error.foreach { case throwable =>
  org.scalajs.dom.console.log(s"Exception while patching an Outwatch compontent: ${throwable.getMessage}")


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.

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