open Js_of_ocaml
(* open Js_of_ocaml.Dom_html *)

let document = Dom_html.document

let by_id id =
  Js.Opt.get
    (document##getElementById (Js.string id))
    (fun () -> failwith ("Missing element: " ^ id))

let () =
  let button = by_id "btn" in
  let output = by_id "output" in

  let clicks = ref 0 in

  button##.onclick :=
    Dom_html.handler (fun _ ->
      incr clicks;
      output##.textContent :=
        Js.some (Js.string ("Clicked " ^ string_of_int !clicks ^ " times"));
      Js._false
    )