[wasm] Json.output improvements

Make it work with native ints and JavaScript values

Author: vouillon

lib/js_of_ocaml/json.ml

@@ -94,10 +94,15 @@ let rec write b v =
     then
       match custom_identifier v with
       | "_i" -> Printf.bprintf b "%ld" (Obj.obj v : int32)
+      | "_n" -> Printf.bprintf b "%nd" (Obj.obj v : nativeint)
       | "_j" ->
           let i : int64 = Obj.obj v in
           write_int64 b i
       | id -> failwith (Printf.sprintf "Json.output: unsupported custom value %s " id)
+    else if t = Obj.abstract_tag
+      then
+        (* Presumably a JavaScript value *)
+        Buffer.add_string b (Js.to_string (Unsafe.global##._JSON##stringify v))
     else failwith (Printf.sprintf "Json.output: unsupported tag %d " t)
 
 let to_json v =

lib/tests/gen-rules/gen.ml

@@ -55,7 +55,7 @@ type enabled_if =
 let enabled_if = function
   | "test_sys" -> GE5
   | "test_fun_call" -> No_effects_not_wasm
-  | "test_json" | "test_poly_compare" -> Not_wasm
+  | "test_poly_compare" -> Not_wasm
   | _ -> Any
 
 let () =

lib/tests/test_json.ml

@@ -61,3 +61,10 @@ let%expect_test _ =
   [%expect {| [254,1,2,3] |}];
   round_trip 2n;
   [%expect {| 2 |}]
+
+let%expect_test "JavaScript object" =
+  let x = object%js val x = 1 val y = Js.string "abcd" end in
+  let s = Json.output x in
+  let s1 = Js.to_string s in
+  Printf.printf "%s\n" s1;
+  [%expect {| {"x":1,"y":"abcd"} |}]