Drawing on the canvas in Scala.js

package example

import scala.scalajs.js.annotation.JSExport
import org.scalajs.dom
import org.scalajs.dom.html
import scala.util.Random

case class Point(x: Int, y: Int) {
  def +(p: Point) = Point(x + p.x, y + p.y)

  def /(d: Int) = Point(x / d, y / d)
}

@JSExport
object ScalaJSExample {
  @JSExport
  def main(canvas: html.Canvas): Unit = {
    val renderer = canvas.getContext("2d")
      .asInstanceOf[dom.CanvasRenderingContext2D]

    canvas.width = canvas.parentElement.clientWidth
    canvas.height = canvas.parentElement.clientHeight

    renderer.fillStyle = "#f8f8f8"
    renderer.fillRect(0, 0, canvas.width, canvas.height)
    renderer.fillStyle = "black"
    var down = false
    canvas.onmousedown =
      (e: dom.MouseEvent) => down = true

    canvas.onmouseup =
      (e: dom.MouseEvent) => down = false

    canvas.onmousemove = {
      (e: dom.MouseEvent) =>
        val rect =
          canvas.getBoundingClientRect()
        if (down) renderer.fillRect(
          e.clientX - rect.left,
          e.clientY - rect.top,
          10, 10
        )
    }

  }
}