SPA in scala.js for a simple quizz - http://www.scala-js-fiddle.com/gist/df22a82904df7c80c523bcc7284ab9bf

ScalaJSExample.scala

import org.scalajs.dom
import dom.html

object ScalaJSExample extends js.JSApp {
  val document = js.Dynamic.global.document
  var question:Option[Question] = None

  def main() = {
    //fullScreenMode()
    println(div(
      configZone,
      questionZone,
      div(box),
      output).render)
    showNextQuestion()
  }
  
  def fullScreenMode() = {
    document.getElementById("logspam").style="display:none"
    document.getElementById("sandbox").style="bottom:0; left:0"
    document.getElementById("editor").style="display:none"
  }

  val questionZone = div("question", style:="font-size:10vw").render
  
  def configCB(label:String) = 
    div(
      input(
        `type` := "checkbox", 
        name := label, 
        id := label,
        value := label,
        checked := true),
      label).render
      
  val configZone=div(
    Seq("+","-","x","/").map(configCB),
    style:="font-size:2vw;display:flex").render

  def config={
    val Seq(add,sub,mul,div)=Seq("+","-","x","/").map(op=>document.getElementById(op).checked.asInstanceOf[Boolean])
    Config(add,sub,mul,div)
  }
  
  val box = input(
    `type` := "text",
    autofocus := true,
    style:="font-size:10vw;display:flex",
    placeholder := "?").render

  val output = div(style:="font-size:3vw").render

  box.onkeyup = (e: dom.Event) => checkAnswer()

  def checkAnswer() = {  
    if(box.value == question.get.a) {
      output.insertBefore(div(question.get.toString).render, output.firstChild)
      showNextQuestion()
      box.value=""
      box.focus()
    } 
  }
  
  def showNextQuestion() = {
    question = Some(Judge.build(config))
    questionZone.innerHTML = question.get.q
  }
}

object Judge {
  def build(config:Config)= config.next(util.Random.nextFloat())(nextInt(), nextInt())

  private def nextInt() =
    (util.Random.nextFloat()*9 + 2).toInt

  type OperationGenerator = (Int,Int) => Question

  val ADD:OperationGenerator=(a,b)=> Question(s"$a + $b", s"${a+b}")
  val MUL:OperationGenerator=(a,b)=> Question(s"$a * $b", s"${a*b}")
  val SUB:OperationGenerator=(a,b)=> Question(s"${a+b} - $b", s"${a}")
  val DIV:OperationGenerator=(a,b)=> Question(s"${a*b} / $b", s"${a}")
}


case class Question(q:String, a:String, start:Long = System.nanoTime) {
  override def toString=s"$q = $a ($durationMs)"
  def durationMs = (System.nanoTime - start) / 1000000
}

case class Config(add:Boolean, sub:Boolean, mul:Boolean, div:Boolean) {
  import Judge._
  
  def next(rng:Float) : OperationGenerator= {
    candidates((rng*1000).toInt % candidates.size)
  }
  
  val candidates =for {
    (b, op) <- Seq(add,sub,mul,div) zip Seq(ADD,SUB,MUL,DIV)
    if b
  } yield op
  

}