New bindings in js_of_ocaml

Learning
1

Consider the following code:

open Js_of_ocaml

class type message_port =
  object ('self)
    (* these types are wrong; fix them later *)
    method postMessage : unit Js.meth
    method start : unit Js.meth
    method close : unit Js.meth
  end

class type message_channel =
  object ('self)
    method port1 : message_port Js.t Js.readonly_prop
    method port2 : message_port Js.t Js.readonly_prop
  end

Ignore the incorrect message bindings on message_port; I will fix those later.

Here is my question: given MessageChannel - Web APIs | MDN , should the type signatures on port1/port2 be:

message_port Js.t Js.readonly_prop or message_port Js.readonly_prop – and why ?

Note the lack of Js.t in the latter.