Omit trailing undefined in external function calls

CHANGELOG.md

@@ -12,6 +12,9 @@
 
 # 11.1.0-rc.4 (Unreleased)
 
+#### :nail-care: Polish
+- Omit `undefined` in external function calls for optional arguments when not supplied. https://github.com/rescript-lang/rescript-compiler/pull/6653
+
 # 11.1.0-rc.3
 
 #### :nail_care: Polish

jscomp/core/lam_compile_external_call.ml

@@ -270,6 +270,16 @@ let translate_ffi (cxt : Lam_compile_context.t) arg_types
           else E.call ~info:{ arity = Full; call_info = Call_na } fn args)
       else
         let args, eff = assemble_args_no_splice arg_types args in
+        let rec keepNonUndefinedArgs argsList (argTypes : specs) =
+          match (argsList, argTypes) with
+          | ( {J.expression_desc = Undefined {isUnit = false}; _} :: rest,
+              {External_arg_spec.arg_label = Arg_optional; _} :: argTypes ) ->
+            keepNonUndefinedArgs rest argTypes
+          | _ -> argsList
+        in
+        let args =
+          keepNonUndefinedArgs (List.rev args) (List.rev arg_types) |> List.rev
+        in
         add_eff eff
         @@ E.call ~info:{ arity = Full; call_info = Call_na } fn args
   | Js_module_as_fn { external_module_name; splice } ->

jscomp/test/omit_trailing_undefined_in_external_calls.res

@@ -0,0 +1,11 @@
+@@uncurried
+
+type dateFormatOptions = {someOption?: bool}
+
+@module("SomeModule")
+external formatDate: (Js.Date.t, ~options: dateFormatOptions=?, ~done: bool=?) => string =
+  "formatDate"
+
+let x = formatDate(Js.Date.make())
+let x = formatDate(Js.Date.make(), ~options={someOption: true})
+let x = formatDate(Js.Date.make(), ~done=true)