let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          el
          |> Canvas.setGlobalCompositeOperation(CanvasGlobalCompositeOperation::Xor)
          el
          |> Canvas.setFillStyle(CanvasFillStyle::String("blue"))
          |> Canvas.fillRect(10, 10, 100, 100)
          el
          |> Canvas.setFillStyle(CanvasFillStyle::String("red"))
          |> Canvas.fillRect(50, 50, 100, 100)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>