You might want to use js_of_ocaml-compiler.runtime which expose Jsoo_runtime.

Thanks for the tip @hhugo. I tried to use that approach, but then the type of Js.t changes, from having one argument, to zero arguments. So it becomes impossible to express types like Js.js_string Js.t.

Should I just replace Js.js_string Js.t with Js.t? If yes, what are the consequences? I am not sure I understand.

These are the two definitions:

https://github.com/ocsigen/js_of_ocaml/blob/5.1.1/lib/runtime/jsoo_runtime.ml#L2

vs

https://github.com/ocsigen/js_of_ocaml/blob/5.1.1/lib/js_of_ocaml/js.mli#L94-L96

Js.t? If yes, what are the consequences? I am not sure I understand.

It depends on what you want to do. Jsoo_runtime was created so that gen_js_api and brr can use it without the "jsoo typing" layer.
If you want to rely on the jsoo (object based) typing, keep the current approach. Jsoo_runtime is merely a collection of external to interact with the jsoo runtime.

Hm, I see, thx. We rely on values like undefined as well as some of the functions in Js.Unsafe. For example, Js.Unsafe provides functions to create JS objects with the values having multiple types, e.g.:

let t =
  Js.Unsafe.(
    obj [| ("js_error_msg", inject (Js.string "foo")); ("row", inject 2) |])

But this fails to build when using Jsoo_runtime:

5 | let t = Js.(obj [| ("js_error_msg", Js.string "foo"); ("row", 2) |])
                                                                  ^
Error: This expression has type int but an expression was expected of type
         Js.t

let t = Js.(obj [| ("js_error_msg", Js.string "foo"); ("row", Obj.magic 2) |])

or

let t = Js.(obj [| ("js_error_msg", Js.string "foo"); ("row", Js.number_of_float 2.) |])

undefined would be Js.pure_js_expr "undefined"

Thanks, I applied the suggested changes. I am not fond of the 5 appearances of Obj.magic but I guess it's the same as using Unsafe.inject, essentially.