DependentEventTypePoC.scala

package example

import scala.scalajs.js
import org.scalajs.dom

@js.native
trait EventName extends js.Any {
  type EventType <: dom.Event
}

object EventName {
  def make[T <: dom.Event](name: String): EventName { type EventType = T } =
    name.asInstanceOf[EventName { type EventType = T }]

  val mousedown = make[dom.MouseEvent]("mousedown")
}

@js.native
trait ElementExt extends js.Object {
  def addEventListener(name: EventName)(
      f: js.Function1[name.EventType, _]): Unit
}

object ScalaJSExample extends js.JSApp {
  def main(): Unit = {
    val paragraph = dom.document.createElement("p")
    paragraph.innerHTML = "<strong>It works! (click me)</strong>"
    dom.document.getElementById("playground").appendChild(paragraph)

    val p = paragraph.asInstanceOf[ElementExt]
    p.addEventListener(EventName.mousedown) { (e: dom.MouseEvent) =>
      println("clicked " + e.button)
    }
  }
}