diff --git a/CHANGELOG.md b/CHANGELOG.md
index 034f1b6daf..872a72e3f0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,7 @@
 - Add [`Deno`](https://deno.land/api?s=Deno) to reserved names, so that modules named `Deno` don't clash with the globally exposed `Deno` object. https://github.com/rescript-lang/rescript-compiler/pull/6428
 - Disable ESLint/TSLint on gentype outputs properly. https://github.com/rescript-lang/rescript-compiler/pull/6442
 - Improve `rescript` CLI to use `stdout`/`stderr` appropriately for help command's message. https://github.com/rescript-lang/rescript-compiler/pull/6439
+- Generate `f()` instead of `f(undefined)` for `f()` https://github.com/rescript-lang/rescript-compiler/pull/6459
 
 # 11.0.0-rc.4
 
diff --git a/jscomp/core/j.ml b/jscomp/core/j.ml
index 099cb24901..2d3e9329bc 100644
--- a/jscomp/core/j.ml
+++ b/jscomp/core/j.ml
@@ -159,7 +159,7 @@ and expression_desc =
   *)
   | Number of number
   | Object of property_map
-  | Undefined
+  | Undefined of {isUnit: bool}
   | Null
   | Await of expression
 
diff --git a/jscomp/core/js_analyzer.ml b/jscomp/core/js_analyzer.ml
index a4ca28e105..d2de7d61bc 100644
--- a/jscomp/core/js_analyzer.ml
+++ b/jscomp/core/js_analyzer.ml
@@ -82,7 +82,7 @@ let free_variables_of_expression st =
 
 let rec no_side_effect_expression_desc (x : J.expression_desc) =
   match x with
-  | Undefined | Null | Bool _ | Var _ -> true
+  | Undefined _ | Null | Bool _ | Var _ -> true
   | Fun _ -> true
   | Number _ -> true (* Can be refined later *)
   | Static_index (obj, (_name : string), (_pos : int32 option)) ->
@@ -153,7 +153,7 @@ let rec eq_expression ({ expression_desc = x0 } : J.expression)
     ({ expression_desc = y0 } : J.expression) =
   match x0 with
   | Null -> y0 = Null
-  | Undefined -> y0 = Undefined
+  | Undefined x -> y0 = Undefined x
   | Number (Int { i }) -> (
       match y0 with Number (Int { i = j }) -> i = j | _ -> false)
   | Number (Float _) -> false
diff --git a/jscomp/core/js_cmj_format.ml b/jscomp/core/js_cmj_format.ml
index 7356a1a5ba..9f33ebc7e5 100644
--- a/jscomp/core/js_cmj_format.ml
+++ b/jscomp/core/js_cmj_format.ml
@@ -113,7 +113,7 @@ let get_result midVal =
   match midVal.persistent_closed_lambda with
   | Some
       (Lconst
-        (Const_js_null | Const_js_undefined | Const_js_true | Const_js_false))
+        (Const_js_null | Const_js_undefined _ | Const_js_true | Const_js_false))
   | None ->
       midVal
   | Some _ ->
diff --git a/jscomp/core/js_dump.ml b/jscomp/core/js_dump.ml
index 48e7a6c4dd..79a8ff13a4 100644
--- a/jscomp/core/js_dump.ml
+++ b/jscomp/core/js_dump.ml
@@ -160,7 +160,7 @@ let exp_need_paren (e : J.expression) =
   | Raw_js_code { code_info = Stmt _ }
   | Length _ | Call _ | Caml_block_tag _ | Seq _ | Static_index _ | Cond _
   | Bin _ | Is_null_or_undefined _ | String_index _ | Array_index _
-  | String_append _ | Var _ | Undefined | Null | Str _ | Array _
+  | String_append _ | Var _ | Undefined _ | Null | Str _ | Array _
   | Optional_block _ | Caml_block _ | FlatCall _ | Typeof _ | Number _
   | Js_not _ | Bool _ | New _ ->
       false
@@ -519,7 +519,7 @@ and expression_desc cxt ~(level : int) f x : cxt =
   | Null ->
       P.string f L.null;
       cxt
-  | Undefined ->
+  | Undefined _ ->
       P.string f L.undefined;
       cxt
   | Var v -> vident cxt f v
@@ -569,7 +569,14 @@ and expression_desc cxt ~(level : int) f x : cxt =
                           pp_function ~is_method ~return_unit ~async cxt f
                             ~fn_state:(No_name { single_arg = true })
                             params body env
-                      | _ -> arguments cxt f el)
+                      | _ ->
+                        let el = match el with
+                        | [e] when e.expression_desc = Undefined {isUnit = true} ->
+                          (* omit passing undefined when the call is f() *)
+                          []
+                        | _ ->
+                          el in
+                         arguments cxt f el)
               | _, _ ->
                   let len = List.length el in
                   if 1 <= len && len <= 8 then (
@@ -736,7 +743,7 @@ and expression_desc cxt ~(level : int) f x : cxt =
             let fields =
               Ext_list.array_list_filter_map fields el (fun f x ->
                   match x.expression_desc with
-                  | Undefined -> None
+                  | Undefined _ -> None
                   | _ -> Some (Js_op.Lit f, x))
             in
             expression_desc cxt ~level f (Object fields))
@@ -774,7 +781,7 @@ and expression_desc cxt ~(level : int) f x : cxt =
           | _ ->
             Ext_list.filter_map tails (fun (f, x) ->
               match x.expression_desc with
-              | Undefined when is_optional f -> None
+              | Undefined _ when is_optional f -> None
               | _ -> Some (f, x))
           in
         if untagged then
@@ -1176,7 +1183,7 @@ and statement_desc top cxt f (s : J.statement_desc) : cxt =
           in
           semi f;
           cxt
-      | Undefined ->
+      | Undefined _ ->
           return_sp f;
           semi f;
           cxt
@@ -1270,16 +1277,16 @@ and function_body (cxt : cxt) f ~return_unit (b : J.block) : unit =
       | If
           ( bool,
             then_,
-            [ { statement_desc = Return { expression_desc = Undefined } } ] ) ->
+            [ { statement_desc = Return { expression_desc = Undefined _ } } ] ) ->
           ignore
             (statement false cxt f
                { s with statement_desc = If (bool, then_, []) }
               : cxt)
-      | Return { expression_desc = Undefined } -> ()
+      | Return { expression_desc = Undefined _ } -> ()
       | Return exp when return_unit ->
           ignore (statement false cxt f (S.exp exp) : cxt)
       | _ -> ignore (statement false cxt f s : cxt))
-  | [ s; { statement_desc = Return { expression_desc = Undefined } } ] ->
+  | [ s; { statement_desc = Return { expression_desc = Undefined _ } } ] ->
       ignore (statement false cxt f s : cxt)
   | s :: r ->
       let cxt = statement false cxt f s in
diff --git a/jscomp/core/js_exp_make.ml b/jscomp/core/js_exp_make.ml
index a807fa6531..10e9e93929 100644
--- a/jscomp/core/js_exp_make.ml
+++ b/jscomp/core/js_exp_make.ml
@@ -60,7 +60,7 @@ let var ?comment id : t = { expression_desc = Var (Id id); comment }
     Invariant: it should not call an external module .. *)
 
 let js_global ?comment (v : string) = var ?comment (Ext_ident.create_js v)
-let undefined : t = { expression_desc = Undefined; comment = None }
+let undefined : t = { expression_desc = Undefined {isUnit = false}; comment = None }
 let nil : t = { expression_desc = Null; comment = None }
 
 let call ?comment ~info e0 args : t =
@@ -183,7 +183,7 @@ let is_array  (e0 : t) : t =
 let new_ ?comment e0 args : t =
   { expression_desc = New (e0, Some args); comment }
 
-let unit : t = { expression_desc = Undefined; comment = None }
+let unit : t = { expression_desc = Undefined {isUnit = true}; comment = None }
 
 (* let math ?comment v args  : t =
    {comment ; expression_desc = Math(v,args)} *)
@@ -256,17 +256,17 @@ let dummy_obj ?comment (info : Lam_tag_info.t) : t =
 *)
 let rec seq ?comment (e0 : t) (e1 : t) : t =
   match (e0.expression_desc, e1.expression_desc) with
-  | ( ( Seq (a, { expression_desc = Number _ | Undefined })
-      | Seq ({ expression_desc = Number _ | Undefined }, a) ),
+  | ( ( Seq (a, { expression_desc = Number _ | Undefined _ })
+      | Seq ({ expression_desc = Number _ | Undefined _ }, a) ),
       _ ) ->
       seq ?comment a e1
-  | _, Seq ({ expression_desc = Number _ | Undefined }, a) ->
+  | _, Seq ({ expression_desc = Number _ | Undefined _ }, a) ->
       (* Return value could not be changed*)
       seq ?comment e0 a
-  | _, Seq (a, ({ expression_desc = Number _ | Undefined } as v)) ->
+  | _, Seq (a, ({ expression_desc = Number _ | Undefined _ } as v)) ->
       (* Return value could not be changed*)
       seq ?comment (seq e0 a) v
-  | (Number _ | Var _ | Undefined), _ -> e1
+  | (Number _ | Var _ | Undefined _), _ -> e1
   | _ -> { expression_desc = Seq (e0, e1); comment }
 
 let fuse_to_seq x xs = if xs = [] then x else Ext_list.fold_left xs x seq
@@ -567,22 +567,22 @@ let str_equal (txt0:string) (delim0:External_arg_spec.delim) txt1 delim1 =
 
 let rec triple_equal ?comment (e0 : t) (e1 : t) : t =
   match (e0.expression_desc, e1.expression_desc) with
-  | ( (Null | Undefined),
+  | ( (Null | Undefined _),
       (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _) )
     when no_side_effect e1 ->
       false_
   | ( (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _),
-      (Null | Undefined) )
+      (Null | Undefined _) )
     when no_side_effect e0 ->
       false_
   | Number (Int { i = i0; _ }), Number (Int { i = i1; _ }) -> bool (i0 = i1)
   | Optional_block (a, _), Optional_block (b, _) -> triple_equal ?comment a b
-  | Undefined, Optional_block _
-  | Optional_block _, Undefined
-  | Null, Undefined
-  | Undefined, Null ->
+  | Undefined _, Optional_block _
+  | Optional_block _, Undefined _
+  | Null, Undefined _
+  | Undefined _, Null ->
       false_
-  | Null, Null | Undefined, Undefined -> true_
+  | Null, Null | Undefined _, Undefined _ -> true_
   | _ -> { expression_desc = Bin (EqEqEq, e0, e1); comment }
 
 let bin ?comment (op : J.binop) (e0 : t) (e1 : t) : t =
@@ -654,7 +654,7 @@ let and_ ?comment (e1 : t) (e2 : t) : t =
   | Var i, Bin (And, l, ({ expression_desc = Var j; _ } as r))
     when Js_op_util.same_vident i j ->
       { e2 with expression_desc = Bin (And, r, l) }
-  | ( Bin (NotEqEq, { expression_desc = Var i }, { expression_desc = Undefined }),
+  | ( Bin (NotEqEq, { expression_desc = Var i }, { expression_desc = Undefined _ }),
       Bin
         ( EqEqEq,
           { expression_desc = Var j },
@@ -702,7 +702,7 @@ let not (e : t) : t =
 
 let not_empty_branch (x : t) =
   match x.expression_desc with
-  | Number (Int { i = 0l }) | Undefined -> false
+  | Number (Int { i = 0l }) | Undefined _ -> false
   | _ -> true
 
 let rec econd ?comment (pred : t) (ifso : t) (ifnot : t) : t =
@@ -735,8 +735,8 @@ let rec econd ?comment (pred : t) (ifso : t) (ifnot : t) : t =
       econd (or_ pred (not pred1)) ifso ifso1
   | Js_not e, _, _ when not_empty_branch ifnot -> econd ?comment e ifnot ifso
   | ( _,
-      Seq (a, { expression_desc = Undefined }),
-      Seq (b, { expression_desc = Undefined }) ) ->
+      Seq (a, { expression_desc = Undefined _ }),
+      Seq (b, { expression_desc = Undefined _ }) ) ->
       seq (econd ?comment pred a b) undefined
   | _ ->
       if Js_analyzer.eq_expression ifso ifnot then
@@ -746,7 +746,7 @@ let rec econd ?comment (pred : t) (ifso : t) (ifnot : t) : t =
 let rec float_equal ?comment (e0 : t) (e1 : t) : t =
   match (e0.expression_desc, e1.expression_desc) with
   | Number (Int { i = i0; _ }), Number (Int { i = i1 }) -> bool (i0 = i1)
-  | Undefined, Undefined -> true_
+  | Undefined _, Undefined _ -> true_
   (* | (Bin(Bor,
          {expression_desc = Number(Int {i = 0l; _})},
          ({expression_desc = Caml_block_tag _; _} as a ))
@@ -983,11 +983,11 @@ let rec int_comp (cmp : Lam_compat.comparison) ?comment (e0 : t) (e1 : t) =
               args,
               call_info );
       }
-  | Ceq, Optional_block _, Undefined | Ceq, Undefined, Optional_block _ ->
+  | Ceq, Optional_block _, Undefined _ | Ceq, Undefined _, Optional_block _ ->
       false_
   | Ceq, _, _ -> int_equal e0 e1
-  | Cneq, Optional_block _, Undefined
-  | Cneq, Undefined, Optional_block _
+  | Cneq, Optional_block _, Undefined _
+  | Cneq, Undefined _, Optional_block _
   | Cneq, Caml_block _, Number _
   | Cneq, Number _, Caml_block _ ->
       true_
@@ -1281,36 +1281,36 @@ let is_null ?comment (x : t) = triple_equal ?comment x nil
 let is_undef ?comment x = triple_equal ?comment x undefined
 
 let for_sure_js_null_undefined (x : t) =
-  match x.expression_desc with Null | Undefined -> true | _ -> false
+  match x.expression_desc with Null | Undefined _ -> true | _ -> false
 
 let is_null_undefined ?comment (x : t) : t =
   match x.expression_desc with
-  | Null | Undefined -> true_
+  | Null | Undefined _ -> true_
   | Number _ | Array _ | Caml_block _ -> false_
   | _ -> { comment; expression_desc = Is_null_or_undefined x }
 
 let eq_null_undefined_boolean ?comment (a : t) (b : t) =
   match (a.expression_desc, b.expression_desc) with
-  | ( (Null | Undefined),
+  | ( (Null | Undefined _),
       (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _) ) ->
       false_
   | ( (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _),
-      (Null | Undefined) ) ->
+      (Null | Undefined _) ) ->
       false_
-  | Null, Undefined | Undefined, Null -> false_
-  | Null, Null | Undefined, Undefined -> true_
+  | Null, Undefined _ | Undefined _, Null -> false_
+  | Null, Null | Undefined _, Undefined _ -> true_
   | _ -> { expression_desc = Bin (EqEqEq, a, b); comment }
 
 let neq_null_undefined_boolean ?comment (a : t) (b : t) =
   match (a.expression_desc, b.expression_desc) with
-  | ( (Null | Undefined),
+  | ( (Null | Undefined _),
       (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _) ) ->
       true_
   | ( (Bool _ | Number _ | Typeof _ | Fun _ | Array _ | Caml_block _),
-      (Null | Undefined) ) ->
+      (Null | Undefined _) ) ->
       true_
-  | Null, Null | Undefined, Undefined -> false_
-  | Null, Undefined | Undefined, Null -> true_
+  | Null, Null | Undefined _, Undefined _ -> false_
+  | Null, Undefined _ | Undefined _, Null -> true_
   | _ -> { expression_desc = Bin (NotEqEq, a, b); comment }
 
 (** TODO: in the future add a flag
diff --git a/jscomp/core/js_fold.ml b/jscomp/core/js_fold.ml
index 42d8410ba6..fd4eba1356 100644
--- a/jscomp/core/js_fold.ml
+++ b/jscomp/core/js_fold.ml
@@ -169,7 +169,7 @@ class fold =
       | Object _x0 ->
           let _self = _self#property_map _x0 in
           _self
-      | Undefined -> _self
+      | Undefined _ -> _self
       | Null -> _self
       | Await _x0 ->
         let _self = _self#expression _x0 in
diff --git a/jscomp/core/js_of_lam_option.ml b/jscomp/core/js_of_lam_option.ml
index 7d13ac5d13..7c2edf133b 100644
--- a/jscomp/core/js_of_lam_option.ml
+++ b/jscomp/core/js_of_lam_option.ml
@@ -36,7 +36,9 @@ type option_unwrap_time = Static_unwrapped | Runtime_maybe_unwrapped
 *)
 let none : J.expression = E.undefined
 
-let is_none_static (arg : J.expression_desc) = arg = Undefined
+let is_none_static (arg : J.expression_desc) = match arg with
+  | Undefined _ -> true
+  | _ -> false
 
 let is_not_none (e : J.expression) : J.expression =
   let desc = e.expression_desc in
diff --git a/jscomp/core/js_pass_flatten_and_mark_dead.ml b/jscomp/core/js_pass_flatten_and_mark_dead.ml
index d89d9cd513..6ff6066283 100644
--- a/jscomp/core/js_pass_flatten_and_mark_dead.ml
+++ b/jscomp/core/js_pass_flatten_and_mark_dead.ml
@@ -192,7 +192,7 @@ let subst_map (substitution : J.expression Hash_ident.t) =
             let _, e, bindings =
               Ext_list.fold_left ls (0, [], []) (fun (i, e, acc) x ->
                   match x.expression_desc with
-                  | Var _ | Number _ | Str _ | J.Bool _ | Undefined ->
+                  | Var _ | Number _ | Str _ | J.Bool _ | Undefined _ ->
                       (* TODO: check the optimization *)
                       (i + 1, x :: e, acc)
                   | _ ->
@@ -257,7 +257,7 @@ let subst_map (substitution : J.expression Hash_ident.t) =
                 match Ext_list.nth_opt ls (Int32.to_int i) with
                 | Some
                     ({
-                       expression_desc = J.Var _ | Number _ | Str _ | Undefined;
+                       expression_desc = J.Var _ | Number _ | Str _ | Undefined _;
                      } as x) ->
                     x
                 | None | Some _ -> super.expression self x)
diff --git a/jscomp/core/js_record_fold.ml b/jscomp/core/js_record_fold.ml
index 04b4f546ba..86117cbcf7 100644
--- a/jscomp/core/js_record_fold.ml
+++ b/jscomp/core/js_record_fold.ml
@@ -175,7 +175,7 @@ let expression_desc : 'a. ('a, expression_desc) fn =
   | Object _x0 ->
       let st = property_map _self st _x0 in
       st
-  | Undefined -> st
+  | Undefined _ -> st
   | Null -> st
   | Await _x0 ->
     let st = _self.expression _self st _x0 in
diff --git a/jscomp/core/js_record_iter.ml b/jscomp/core/js_record_iter.ml
index e7a0cdfddd..d0f523ed7e 100644
--- a/jscomp/core/js_record_iter.ml
+++ b/jscomp/core/js_record_iter.ml
@@ -131,7 +131,7 @@ let expression_desc : expression_desc fn =
   | Caml_block_tag (_x0, _tag) -> _self.expression _self _x0
   | Number _ -> ()
   | Object _x0 -> property_map _self _x0
-  | Undefined -> ()
+  | Undefined _ -> ()
   | Null -> ()
   | Await _x0 -> _self.expression _self _x0
 
diff --git a/jscomp/core/js_record_map.ml b/jscomp/core/js_record_map.ml
index 742d9e3641..371d54eb6a 100644
--- a/jscomp/core/js_record_map.ml
+++ b/jscomp/core/js_record_map.ml
@@ -173,7 +173,7 @@ let expression_desc : expression_desc fn =
   | Object _x0 ->
       let _x0 = property_map _self _x0 in
       Object _x0
-  | Undefined as v -> v
+  | Undefined _ as v -> v
   | Null as v -> v
   | Await _x0 ->
     let _x0 = _self.expression _self _x0 in
diff --git a/jscomp/core/js_stmt_make.ml b/jscomp/core/js_stmt_make.ml
index 80136396f9..82773ae80e 100644
--- a/jscomp/core/js_stmt_make.ml
+++ b/jscomp/core/js_stmt_make.ml
@@ -44,10 +44,10 @@ let rec block ?comment (b : J.block) : t =
 (* It's a statement, we can discard some values *)
 let rec exp ?comment (e : E.t) : t =
   match e.expression_desc with
-  | Seq ({ expression_desc = Number _ | Undefined }, b)
-  | Seq (b, { expression_desc = Number _ | Undefined }) ->
+  | Seq ({ expression_desc = Number _ | Undefined _ }, b)
+  | Seq (b, { expression_desc = Number _ | Undefined _ }) ->
       exp ?comment b
-  | Number _ | Undefined -> block []
+  | Number _ | Undefined _ -> block []
   (* TODO: we can do more *)
   (* | _ when is_pure e ->  block [] *)
   | _ -> { statement_desc = Exp e; comment }
@@ -63,10 +63,10 @@ let declare_variable ?comment ?ident_info ~kind (ident : Ident.t) : t =
   }
 
 let define_variable ?comment ?ident_info ~kind (v : Ident.t)
-    (exp : J.expression) : t =
-  if exp.expression_desc = Undefined then
+    (exp : J.expression) : t = match exp.expression_desc with
+  | Undefined _ ->
     declare_variable ?comment ?ident_info ~kind v
-  else
+  | _ ->
     let property : J.property = kind in
     let ident_info : J.ident_info =
       match ident_info with None -> { used_stats = NA } | Some x -> x
diff --git a/jscomp/core/lam.ml b/jscomp/core/lam.ml
index c41a70a993..c1686e2047 100644
--- a/jscomp/core/lam.ml
+++ b/jscomp/core/lam.ml
@@ -419,7 +419,7 @@ let stringswitch (lam : t) cases default : t =
 
 let true_ : t = Lconst Const_js_true
 let false_ : t = Lconst Const_js_false
-let unit : t = Lconst Const_js_undefined
+let unit : t = Lconst (Const_js_undefined {isUnit = true})
 
 let rec seq (a : t) b : t =
   match a with
@@ -635,7 +635,7 @@ let rec eval_const_as_bool (v : Lam_constant.t) : bool =
   | Const_int { i = x } -> x <> 0l
   | Const_char x -> x <> 0
   | Const_int64 x -> x <> 0L
-  | Const_js_false | Const_js_null | Const_module_alias | Const_js_undefined ->
+  | Const_js_false | Const_js_null | Const_module_alias | Const_js_undefined _ ->
       false
   | Const_js_true | Const_string _ | Const_pointer _ | Const_float _
   | Const_block _ | Const_float_array _ ->
diff --git a/jscomp/core/lam_analysis.ml b/jscomp/core/lam_analysis.ml
index 66edeb0678..eacfe0a872 100644
--- a/jscomp/core/lam_analysis.ml
+++ b/jscomp/core/lam_analysis.ml
@@ -194,7 +194,7 @@ let rec size (lam : Lam.t) =
 and size_constant x =
   match x with
   | Const_int _ | Const_char _ | Const_float _ | Const_int64 _ | Const_pointer _
-  | Const_js_null | Const_js_undefined | Const_module_alias | Const_js_true
+  | Const_js_null | Const_js_undefined _ | Const_module_alias | Const_js_true
   | Const_js_false ->
       1
   | Const_string _ ->
@@ -270,6 +270,6 @@ let safe_to_inline (lam : Lam.t) =
   | Lconst
       ( Const_pointer _
       | Const_int { comment = Pt_constructor _ }
-      | Const_js_true | Const_js_false | Const_js_undefined ) ->
+      | Const_js_true | Const_js_false | Const_js_undefined _ ) ->
       true
   | _ -> false
diff --git a/jscomp/core/lam_compile.ml b/jscomp/core/lam_compile.ml
index d3366c8023..7361a1e22d 100644
--- a/jscomp/core/lam_compile.ml
+++ b/jscomp/core/lam_compile.ml
@@ -30,6 +30,7 @@ let args_either_function_or_const (args : Lam.t list) =
       match x with Lfunction _ | Lconst _ -> true | _ -> false)
 
 let call_info_of_ap_status (ap_status : Lam.apply_status) : Js_call_info.t =
+  (* XXX *)
   match ap_status with
   | App_infer_full -> { arity = Full; call_info = Call_ml }
   | App_uncurry -> { arity = Full; call_info = Call_na }
diff --git a/jscomp/core/lam_compile_const.ml b/jscomp/core/lam_compile_const.ml
index 0a09b6b5eb..98acf07cb8 100644
--- a/jscomp/core/lam_compile_const.ml
+++ b/jscomp/core/lam_compile_const.ml
@@ -28,7 +28,7 @@ module E = Js_exp_make
 let rec is_some_none_aux (x : Lam_constant.t) acc =
   match x with
   | Const_some v -> is_some_none_aux v (acc + 1)
-  | Const_module_alias | Const_js_undefined -> acc
+  | Const_module_alias | Const_js_undefined _ -> acc
   | _ -> -1
 
 let rec nested_some_none n none =
@@ -37,7 +37,7 @@ let rec nested_some_none n none =
 let rec translate_some (x : Lam_constant.t) : J.expression =
   let depth = is_some_none_aux x 0 in
   if depth < 0 then E.optional_not_nest_block (translate x)
-  else nested_some_none depth (E.optional_block (translate Const_js_undefined))
+  else nested_some_none depth (E.optional_block (translate (Const_js_undefined {isUnit = false})))
 
 and translate (x : Lam_constant.t) : J.expression =
   match x with
@@ -46,7 +46,8 @@ and translate (x : Lam_constant.t) : J.expression =
   | Const_js_true -> E.bool true
   | Const_js_false -> E.bool false
   | Const_js_null -> E.nil
-  | Const_js_undefined -> E.undefined
+  | Const_js_undefined {isUnit = true} -> E.unit
+  | Const_js_undefined {isUnit = false} -> E.undefined
   | Const_int { i; comment = Pt_constructor {cstr_name={name; tag_type=None}}} when name <> "[]" ->
       E.str name
   | Const_int { i; comment = Pt_constructor {cstr_name={tag_type = Some t}}}  ->
diff --git a/jscomp/core/lam_compile_primitive.ml b/jscomp/core/lam_compile_primitive.ml
index 99d092d702..b25269aaec 100644
--- a/jscomp/core/lam_compile_primitive.ml
+++ b/jscomp/core/lam_compile_primitive.ml
@@ -95,14 +95,14 @@ let translate output_prefix loc (cxt : Lam_compile_context.t)
       match args with
       | [ e ] -> (
           match e.expression_desc with
-          | Var _ | Undefined | Null -> Js_of_lam_option.null_to_opt e
+          | Var _ | Undefined _ | Null -> Js_of_lam_option.null_to_opt e
           | _ -> E.runtime_call Js_runtime_modules.option "null_to_opt" args)
       | _ -> assert false)
   | Pundefined_to_opt -> (
       match args with
       | [ e ] -> (
           match e.expression_desc with
-          | Var _ | Undefined | Null -> Js_of_lam_option.undef_to_opt e
+          | Var _ | Undefined _ | Null -> Js_of_lam_option.undef_to_opt e
           | _ ->
               E.runtime_call Js_runtime_modules.option "undefined_to_opt" args)
       | _ -> assert false)
@@ -110,7 +110,7 @@ let translate output_prefix loc (cxt : Lam_compile_context.t)
       match args with
       | [ e ] -> (
           match e.expression_desc with
-          | Var _ | Undefined | Null -> Js_of_lam_option.null_undef_to_opt e
+          | Var _ | Undefined _ | Null -> Js_of_lam_option.null_undef_to_opt e
           | _ -> E.runtime_call Js_runtime_modules.option "nullable_to_opt" args
           )
       | _ -> assert false)
diff --git a/jscomp/core/lam_constant_convert.ml b/jscomp/core/lam_constant_convert.ml
index fa7f51e504..5e3fa49537 100644
--- a/jscomp/core/lam_constant_convert.ml
+++ b/jscomp/core/lam_constant_convert.ml
@@ -39,7 +39,7 @@ let rec convert_constant (const : Lambda.structured_constant) : Lam_constant.t =
   | Const_base (Const_nativeint _) -> assert false
   | Const_pointer (0, Pt_constructor { name = "()"; const = 1; non_const = 0 })
     ->
-      Const_js_undefined
+      Const_js_undefined {isUnit = true}
   | Const_false -> Const_js_false
   | Const_true -> Const_js_true
   | Const_pointer (i, p) -> (
diff --git a/jscomp/core/lam_convert.ml b/jscomp/core/lam_convert.ml
index a26dad1e07..922184e3a1 100644
--- a/jscomp/core/lam_convert.ml
+++ b/jscomp/core/lam_convert.ml
@@ -442,7 +442,7 @@ let convert (exports : Set_ident.t) (lam : Lambda.lambda) :
     | _ when s = "#null" -> Lam.const Const_js_null
     | _ when s = "#os_type" ->
         prim ~primitive:(Pctconst Ostype) ~args:[ unit ] loc
-    | _ when s = "#undefined" -> Lam.const Const_js_undefined
+    | _ when s = "#undefined" -> Lam.const (Const_js_undefined {isUnit = false})
     | _ when s = "#init_mod" -> (
         let args = Ext_list.map args convert_aux in
         match args with
diff --git a/jscomp/core/lam_eta_conversion.ml b/jscomp/core/lam_eta_conversion.ml
index 25f5bd1957..a92639f1f2 100644
--- a/jscomp/core/lam_eta_conversion.ml
+++ b/jscomp/core/lam_eta_conversion.ml
@@ -44,7 +44,7 @@ let transform_under_supply n ap_info fn args =
         | Lconst
             ( Const_int _ | Const_char _ | Const_string _ | Const_float _
             | Const_int64 _ | Const_pointer _ | Const_js_true | Const_js_false
-            | Const_js_undefined )
+            | Const_js_undefined _ )
         | Lprim { primitive = Pfield (_, Fld_module _); _ }
         | Lfunction _ ->
             (lam :: acc, bind)
diff --git a/jscomp/core/lam_pass_lets_dce.ml b/jscomp/core/lam_pass_lets_dce.ml
index 618526706a..b9459eb168 100644
--- a/jscomp/core/lam_pass_lets_dce.ml
+++ b/jscomp/core/lam_pass_lets_dce.ml
@@ -53,7 +53,7 @@ let lets_helper (count_var : Ident.t -> Lam_pass_count.used_info) lam : Lam.t =
                  ((
                    Const_int _ | Const_char _ | Const_float _ 
                  )
-                 | Const_pointer _ |Const_js_true | Const_js_false | Const_js_undefined) (* could be poly-variant [`A] -> [65a]*)
+                 | Const_pointer _ |Const_js_true | Const_js_false | Const_js_undefined _) (* could be poly-variant [`A] -> [65a]*)
               | Lprim {primitive = Pfield (_);
                        args = [ 
                          Lglobal_module _
diff --git a/jscomp/core/lam_pass_remove_alias.ml b/jscomp/core/lam_pass_remove_alias.ml
index 9ad2ef7f9e..4889d1c268 100644
--- a/jscomp/core/lam_pass_remove_alias.ml
+++ b/jscomp/core/lam_pass_remove_alias.ml
@@ -33,7 +33,7 @@ let id_is_for_sure_true_in_boolean (tbl : Lam_stats.ident_tbl) id =
       Eval_true
   | Some (Constant (Const_int { i })) ->
       if i = 0l then Eval_false else Eval_true
-  | Some (Constant (Const_js_false | Const_js_null | Const_js_undefined)) ->
+  | Some (Constant (Const_js_false | Const_js_null | Const_js_undefined _)) ->
       Eval_false
   | Some
       ( Constant _ | Module _ | FunctionId _ | Exception | Parameter | NA
diff --git a/jscomp/core/lam_print.ml b/jscomp/core/lam_print.ml
index 82bbf31b48..f7390234c5 100644
--- a/jscomp/core/lam_print.ml
+++ b/jscomp/core/lam_print.ml
@@ -19,7 +19,7 @@ let rec struct_const ppf (cst : Lam_constant.t) =
   | Const_js_false -> fprintf ppf "#false"
   | Const_js_null -> fprintf ppf "#null"
   | Const_module_alias -> fprintf ppf "#alias"
-  | Const_js_undefined -> fprintf ppf "#undefined"
+  | Const_js_undefined _ -> fprintf ppf "#undefined"
   | Const_int { i } -> fprintf ppf "%ld" i
   | Const_char i -> fprintf ppf "%s" (Ext_util.string_of_int_as_char i)
   | Const_string { s } -> fprintf ppf "%S" s
diff --git a/jscomp/core/lam_stats_export.ml b/jscomp/core/lam_stats_export.ml
index d7fcc61f0f..c4b98bbb87 100644
--- a/jscomp/core/lam_stats_export.ml
+++ b/jscomp/core/lam_stats_export.ml
@@ -55,7 +55,7 @@ let values_of_export (meta : Lam_stats.t) (export_map : Lam.t Map_ident.t) :
         match optlam with
         | Some
             (Lconst
-              ( Const_js_null | Const_js_undefined | Const_js_true
+              ( Const_js_null | Const_js_undefined _ | Const_js_true
               | Const_js_false ))
         | None ->
             optlam
diff --git a/jscomp/frontend/lam_constant.ml b/jscomp/frontend/lam_constant.ml
index 26cbb04b66..346dc02368 100644
--- a/jscomp/frontend/lam_constant.ml
+++ b/jscomp/frontend/lam_constant.ml
@@ -42,7 +42,7 @@ let string_of_pointer_info (x : pointer_info) : string option =
 
 type t =
   | Const_js_null
-  | Const_js_undefined
+  | Const_js_undefined of {isUnit: bool}
   | Const_js_true
   | Const_js_false
   | Const_int of {i: int32; comment: pointer_info}
@@ -63,7 +63,7 @@ let rec eq_approx (x : t) (y : t) =
   match x with
   | Const_module_alias -> y = Const_module_alias
   | Const_js_null -> y = Const_js_null
-  | Const_js_undefined -> y = Const_js_undefined
+  | Const_js_undefined b -> y = Const_js_undefined b
   | Const_js_true -> y = Const_js_true
   | Const_js_false -> y = Const_js_false
   | Const_int ix -> (
@@ -104,4 +104,4 @@ let rec eq_approx (x : t) (y : t) =
     | Const_some iy -> eq_approx ix iy
     | _ -> false)
 
-let lam_none : t = Const_js_undefined
+let lam_none : t = Const_js_undefined {isUnit = false}
diff --git a/jscomp/frontend/lam_constant.mli b/jscomp/frontend/lam_constant.mli
index e4fd1c5a5f..3426b9a466 100644
--- a/jscomp/frontend/lam_constant.mli
+++ b/jscomp/frontend/lam_constant.mli
@@ -38,7 +38,7 @@ val string_of_pointer_info : pointer_info -> string option
 
 type t =
   | Const_js_null
-  | Const_js_undefined
+  | Const_js_undefined of {isUnit: bool}
   | Const_js_true
   | Const_js_false
   | Const_int of {i: int32; comment: pointer_info}
diff --git a/jscomp/test/UntaggedVariants.js b/jscomp/test/UntaggedVariants.js
index e950e7f1d5..0ee038afc0 100644
--- a/jscomp/test/UntaggedVariants.js
+++ b/jscomp/test/UntaggedVariants.js
@@ -574,7 +574,7 @@ function test(t) {
     case "string" :
         return t;
     case "function" :
-        return t(undefined);
+        return t();
     
   }
 }
diff --git a/jscomp/test/alias_test.js b/jscomp/test/alias_test.js
index 924eb499f8..0ea3140892 100644
--- a/jscomp/test/alias_test.js
+++ b/jscomp/test/alias_test.js
@@ -17,7 +17,7 @@ function ff(param) {
   return "cool " + a22;
 }
 
-var a23 = ff(undefined);
+var a23 = ff();
 
 var a15 = a10;
 
diff --git a/jscomp/test/async_await.js b/jscomp/test/async_await.js
index 6b77ece736..0c674d3b90 100644
--- a/jscomp/test/async_await.js
+++ b/jscomp/test/async_await.js
@@ -30,9 +30,9 @@ var arr = [
   3
 ];
 
-var toplevelAwait = await topFoo(undefined);
+var toplevelAwait = await topFoo();
 
-var toplevelAwait2 = Caml_array.get(arr, await topFoo(undefined));
+var toplevelAwait2 = Caml_array.get(arr, await topFoo());
 
 exports.next = next;
 exports.useNext = useNext;
diff --git a/jscomp/test/async_inline.js b/jscomp/test/async_inline.js
index 0e2191d0fa..1c891d0c22 100644
--- a/jscomp/test/async_inline.js
+++ b/jscomp/test/async_inline.js
@@ -8,7 +8,7 @@ async function willBeInlined(param) {
   return 3;
 }
 
-var inlined = willBeInlined(undefined);
+var inlined = willBeInlined();
 
 function wrapSomethingAsync(param) {
   ((async function (param) {
@@ -21,7 +21,7 @@ function wrapSomethingAsync2(param) {
   ((async function (param) {
           var test = await Promise.resolve("Test");
           console.log(test);
-        })(undefined));
+        })());
 }
 
 async function doSomethingAsync(someAsyncFunction) {
diff --git a/jscomp/test/bdd.js b/jscomp/test/bdd.js
index 92e4100b44..a2314104bf 100644
--- a/jscomp/test/bdd.js
+++ b/jscomp/test/bdd.js
@@ -361,7 +361,7 @@ function random(param) {
 function random_vars(n) {
   var vars = Caml_array.make(n, false);
   for(var i = 0; i < n; ++i){
-    Caml_array.set(vars, i, random(undefined));
+    Caml_array.set(vars, i, random());
   }
   return vars;
 }
@@ -411,7 +411,7 @@ function main(param) {
       };
 }
 
-main(undefined);
+main();
 
 var initSize_1 = 8191;
 
diff --git a/jscomp/test/bench.js b/jscomp/test/bench.js
index bd6f9c2804..38ca9b8632 100644
--- a/jscomp/test/bench.js
+++ b/jscomp/test/bench.js
@@ -59,7 +59,7 @@ function f2(param) {
   console.log(Pervasives.string_of_float(v));
 }
 
-f2(undefined);
+f2();
 
 exports.map = map;
 exports.init = init;
diff --git a/jscomp/test/bs_MapInt_test.js b/jscomp/test/bs_MapInt_test.js
index affc4e0246..7dea07a710 100644
--- a/jscomp/test/bs_MapInt_test.js
+++ b/jscomp/test/bs_MapInt_test.js
@@ -25,7 +25,7 @@ function test(param) {
   should(Belt_MapInt.isEmpty(m));
 }
 
-test(undefined);
+test();
 
 var M;
 
diff --git a/jscomp/test/bs_auto_uncurry.js b/jscomp/test/bs_auto_uncurry.js
index 7713d21050..31f7a653f2 100644
--- a/jscomp/test/bs_auto_uncurry.js
+++ b/jscomp/test/bs_auto_uncurry.js
@@ -122,7 +122,7 @@ function unit_magic(param) {
   return 3;
 }
 
-var f_unit_magic = unit_magic(undefined);
+var f_unit_magic = unit_magic();
 
 function hh(xs) {
   return function (param) {
diff --git a/jscomp/test/bs_hashtbl_string_test.js b/jscomp/test/bs_hashtbl_string_test.js
index 66a8c76044..f747ef5b4e 100644
--- a/jscomp/test/bs_hashtbl_string_test.js
+++ b/jscomp/test/bs_hashtbl_string_test.js
@@ -294,7 +294,7 @@ function bench7(param) {
 
 console.time("bs_hashtbl_string_test.res 181");
 
-bench7(undefined);
+bench7();
 
 console.timeEnd("bs_hashtbl_string_test.res 181");
 
diff --git a/jscomp/test/bs_rbset_int_bench.js b/jscomp/test/bs_rbset_int_bench.js
index 84b9732348..c26328a964 100644
--- a/jscomp/test/bs_rbset_int_bench.js
+++ b/jscomp/test/bs_rbset_int_bench.js
@@ -47,7 +47,7 @@ function bench(param) {
 
 console.time("bs_rbset_int_bench.res 24");
 
-bench(undefined);
+bench();
 
 console.timeEnd("bs_rbset_int_bench.res 24");
 
diff --git a/jscomp/test/bs_set_bench.js b/jscomp/test/bs_set_bench.js
index f0583c36bd..c522372a1a 100644
--- a/jscomp/test/bs_set_bench.js
+++ b/jscomp/test/bs_set_bench.js
@@ -47,7 +47,7 @@ function bench(param) {
 
 console.time("bs_set_bench.res 24");
 
-bench(undefined);
+bench();
 
 console.timeEnd("bs_set_bench.res 24");
 
diff --git a/jscomp/test/bs_unwrap_test.js b/jscomp/test/bs_unwrap_test.js
index 215b5d5d5e..b020cf41f4 100644
--- a/jscomp/test/bs_unwrap_test.js
+++ b/jscomp/test/bs_unwrap_test.js
@@ -30,7 +30,7 @@ var arg_pair = {
 
 console.log(arg_pair.VAL);
 
-console.log(undefined);
+console.log();
 
 console.log(1, undefined);
 
diff --git a/jscomp/test/const_block_test.js b/jscomp/test/const_block_test.js
index 6d60450f1f..0d6830a09f 100644
--- a/jscomp/test/const_block_test.js
+++ b/jscomp/test/const_block_test.js
@@ -35,7 +35,7 @@ function h(param) {
 }
 
 function g(param) {
-  f(undefined);
+  f();
   return {
           TAG: "Eq",
           _0: [
diff --git a/jscomp/test/cps_test.js b/jscomp/test/cps_test.js
index 3fc32e65a8..96d22f2c4a 100644
--- a/jscomp/test/cps_test.js
+++ b/jscomp/test/cps_test.js
@@ -81,7 +81,7 @@ Mt.from_pair_suites("Cps_test", {
             return {
                     TAG: "Eq",
                     _0: 55,
-                    _1: test(undefined)
+                    _1: test()
                   };
           })
       ],
@@ -92,7 +92,7 @@ Mt.from_pair_suites("Cps_test", {
               return {
                       TAG: "Eq",
                       _0: 15,
-                      _1: test_closure(undefined)
+                      _1: test_closure()
                     };
             })
         ],
@@ -103,7 +103,7 @@ Mt.from_pair_suites("Cps_test", {
                 return {
                         TAG: "Eq",
                         _0: 30,
-                        _1: test_closure2(undefined)
+                        _1: test_closure2()
                       };
               })
           ],
diff --git a/jscomp/test/demo_int_map.js b/jscomp/test/demo_int_map.js
index 9b9d5aedc6..8e4a314c90 100644
--- a/jscomp/test/demo_int_map.js
+++ b/jscomp/test/demo_int_map.js
@@ -160,6 +160,6 @@ function test(param) {
   }
 }
 
-test(undefined);
+test();
 
 /*  Not a pure module */
diff --git a/jscomp/test/earger_curry_test.js b/jscomp/test/earger_curry_test.js
index 3db1c75c36..123a360b94 100644
--- a/jscomp/test/earger_curry_test.js
+++ b/jscomp/test/earger_curry_test.js
@@ -60,7 +60,7 @@ function f2(param) {
   console.log(Pervasives.string_of_float(v));
 }
 
-f2(undefined);
+f2();
 
 var suites = {
   contents: /* [] */0
diff --git a/jscomp/test/event_ffi.js b/jscomp/test/event_ffi.js
index f68c99ff36..e422d719e3 100644
--- a/jscomp/test/event_ffi.js
+++ b/jscomp/test/event_ffi.js
@@ -5,11 +5,11 @@ var List = require("../../lib/js/list.js");
 var Curry = require("../../lib/js/curry.js");
 
 function h0(x) {
-  return x(undefined);
+  return x();
 }
 
 function h00(x) {
-  return x(undefined);
+  return x();
 }
 
 function h1(x, y) {
diff --git a/jscomp/test/exception_raise_test.js b/jscomp/test/exception_raise_test.js
index 48138dac16..6ea49475de 100644
--- a/jscomp/test/exception_raise_test.js
+++ b/jscomp/test/exception_raise_test.js
@@ -188,7 +188,7 @@ function eq(loc, x, y) {
 }
 
 try {
-  ((()=>{throw 2})(undefined));
+  ((()=>{throw 2})());
 }
 catch (raw_e$2){
   var e = Caml_js_exceptions.internalToOCamlException(raw_e$2);
diff --git a/jscomp/test/exception_rebound_err_test.js b/jscomp/test/exception_rebound_err_test.js
index 92e5dc1952..40ffd42b90 100644
--- a/jscomp/test/exception_rebound_err_test.js
+++ b/jscomp/test/exception_rebound_err_test.js
@@ -79,7 +79,7 @@ function f(g) {
   }
 }
 
-eq("File \"exception_rebound_err_test.res\", line 31, characters 3-10", test_js_error4(undefined), 7);
+eq("File \"exception_rebound_err_test.res\", line 31, characters 3-10", test_js_error4(), 7);
 
 Mt.from_pair_suites("Exception_rebound_err_test", suites.contents);
 
diff --git a/jscomp/test/ext_filename_test.js b/jscomp/test/ext_filename_test.js
index 65c301b886..229a506941 100644
--- a/jscomp/test/ext_filename_test.js
+++ b/jscomp/test/ext_filename_test.js
@@ -23,7 +23,7 @@ var node_current = ".";
 var cwd = {
   LAZY_DONE: false,
   VAL: (function () {
-      return Caml_sys.sys_getcwd(undefined);
+      return Caml_sys.sys_getcwd();
     })
 };
 
diff --git a/jscomp/test/ffi_arity_test.js b/jscomp/test/ffi_arity_test.js
index 410aaca3b1..500d287b13 100644
--- a/jscomp/test/ffi_arity_test.js
+++ b/jscomp/test/ffi_arity_test.js
@@ -55,7 +55,7 @@ function fff(param) {
 }
 
 function g() {
-  fff(undefined);
+  fff();
 }
 
 function abc(x, y, z) {
@@ -66,7 +66,7 @@ function abc(x, y, z) {
 
 var abc_u = abc;
 
-fff(undefined);
+fff();
 
 Mt.from_pair_suites("Ffi_arity_test", {
       hd: [
diff --git a/jscomp/test/for_loop_test.js b/jscomp/test/for_loop_test.js
index 7571c529bb..9b6a0b1e22 100644
--- a/jscomp/test/for_loop_test.js
+++ b/jscomp/test/for_loop_test.js
@@ -276,7 +276,7 @@ var suites_1 = {
               return {
                       TAG: "Eq",
                       _0: 84,
-                      _1: for_7(undefined)
+                      _1: for_7()
                     };
             })
         ],
@@ -287,7 +287,7 @@ var suites_1 = {
                 return {
                         TAG: "Eq",
                         _0: 294,
-                        _1: for_8(undefined)
+                        _1: for_8()
                       };
               })
           ],
@@ -307,7 +307,7 @@ var suites_1 = {
                               ],
                               5
                             ]],
-                          _1: for_9(undefined)
+                          _1: for_9()
                         };
                 })
             ],
diff --git a/jscomp/test/for_side_effect_test.js b/jscomp/test/for_side_effect_test.js
index c23e93c0cb..e770b02941 100644
--- a/jscomp/test/for_side_effect_test.js
+++ b/jscomp/test/for_side_effect_test.js
@@ -25,7 +25,7 @@ var suites_0 = [
       return {
               TAG: "Eq",
               _0: 10,
-              _1: test2(undefined)
+              _1: test2()
             };
     })
 ];
diff --git a/jscomp/test/fun_pattern_match.js b/jscomp/test/fun_pattern_match.js
index ee24bdd38e..25975a53c3 100644
--- a/jscomp/test/fun_pattern_match.js
+++ b/jscomp/test/fun_pattern_match.js
@@ -69,7 +69,7 @@ function r(param) {
   return x;
 }
 
-var match = r(undefined);
+var match = r();
 
 var v = Curry._1(match.VAL, undefined);
 
diff --git a/jscomp/test/functor_app_test.js b/jscomp/test/functor_app_test.js
index b3cb51dc08..635ae811b6 100644
--- a/jscomp/test/functor_app_test.js
+++ b/jscomp/test/functor_app_test.js
@@ -39,7 +39,7 @@ eq("File \"functor_app_test.res\", line 15, characters 3-10", Curry._2(Y0.h, 1,
 
 eq("File \"functor_app_test.res\", line 16, characters 3-10", Curry._2(Y1.h, 2, 3), 6);
 
-var v = Functor_def.$$return(undefined);
+var v = Functor_def.$$return();
 
 eq("File \"functor_app_test.res\", line 20, characters 3-10", v, 2);
 
diff --git a/jscomp/test/global_module_alias_test.js b/jscomp/test/global_module_alias_test.js
index 9595de9a20..991a40febb 100644
--- a/jscomp/test/global_module_alias_test.js
+++ b/jscomp/test/global_module_alias_test.js
@@ -110,9 +110,9 @@ function xx(param) {
   return List;
 }
 
-eq("File \"global_module_alias_test.res\", line 80, characters 12-19", g(undefined), 4);
+eq("File \"global_module_alias_test.res\", line 80, characters 12-19", g(), 4);
 
-var V = xx(undefined);
+var V = xx();
 
 eq("File \"global_module_alias_test.res\", line 84, characters 5-12", Curry._1(V.length, {
           hd: 1,
@@ -127,7 +127,7 @@ eq("File \"global_module_alias_test.res\", line 84, characters 5-12", Curry._1(V
 
 eq("File \"global_module_alias_test.res\", line 85, characters 5-12", v.contents, 15);
 
-var H$1 = f(undefined);
+var H$1 = f();
 
 eq("File \"global_module_alias_test.res\", line 87, characters 5-12", Curry._1(H$1.length, {
           hd: 1,
diff --git a/jscomp/test/gpr_1409_test.js b/jscomp/test/gpr_1409_test.js
index 9d9dc6ae81..80826b9193 100644
--- a/jscomp/test/gpr_1409_test.js
+++ b/jscomp/test/gpr_1409_test.js
@@ -58,9 +58,9 @@ function make(foo) {
   };
 }
 
-var a_ = make(undefined)(undefined);
+var a_ = make(undefined)();
 
-var b_ = make(42)(undefined);
+var b_ = make(42)();
 
 eq("File \"gpr_1409_test.res\", line 26, characters 3-10", b_.foo, "42");
 
diff --git a/jscomp/test/gpr_1817_test.js b/jscomp/test/gpr_1817_test.js
index 13a831129e..16a623257e 100644
--- a/jscomp/test/gpr_1817_test.js
+++ b/jscomp/test/gpr_1817_test.js
@@ -39,7 +39,7 @@ function f(param) {
         ];
 }
 
-var match = f(undefined);
+var match = f();
 
 var a2 = match[2];
 
diff --git a/jscomp/test/gpr_2682_test.js b/jscomp/test/gpr_2682_test.js
index 2696a65731..a169156607 100644
--- a/jscomp/test/gpr_2682_test.js
+++ b/jscomp/test/gpr_2682_test.js
@@ -43,7 +43,7 @@ forIn({
 
 var f3 = (()=>true);
 
-var bbbb = f3(undefined);
+var bbbb = f3();
 
 if (!bbbb) {
   throw {
diff --git a/jscomp/test/gpr_2731_test.js b/jscomp/test/gpr_2731_test.js
index 9dcd6a23ee..a12d56ab70 100644
--- a/jscomp/test/gpr_2731_test.js
+++ b/jscomp/test/gpr_2731_test.js
@@ -14,9 +14,9 @@ function g(param) {
   return 1;
 }
 
-var c = g(undefined);
+var c = g();
 
-var d = g(undefined);
+var d = g();
 
 exports.f = f;
 exports.a = a;
diff --git a/jscomp/test/gpr_3697_test.js b/jscomp/test/gpr_3697_test.js
index e5990aecb7..8f637814e5 100644
--- a/jscomp/test/gpr_3697_test.js
+++ b/jscomp/test/gpr_3697_test.js
@@ -9,7 +9,7 @@ function fix(param) {
           _0: {
             LAZY_DONE: false,
             VAL: (function () {
-                return fix(undefined);
+                return fix();
               })
           }
         };
diff --git a/jscomp/test/hashtbl_test.js b/jscomp/test/hashtbl_test.js
index dd2a67101a..afb8268a04 100644
--- a/jscomp/test/hashtbl_test.js
+++ b/jscomp/test/hashtbl_test.js
@@ -61,7 +61,7 @@ var suites_0 = [
                   tl: /* [] */0
                 }
               },
-              _1: f(undefined)
+              _1: f()
             };
     })
 ];
diff --git a/jscomp/test/imm_map_bench.js b/jscomp/test/imm_map_bench.js
index a54dbd0ca9..63b569c7c7 100644
--- a/jscomp/test/imm_map_bench.js
+++ b/jscomp/test/imm_map_bench.js
@@ -46,13 +46,13 @@ function test2(param) {
 
 console.time("imm_map_bench.res 43");
 
-test(undefined);
+test();
 
 console.timeEnd("imm_map_bench.res 43");
 
 console.time("imm_map_bench.res 44");
 
-test2(undefined);
+test2();
 
 console.timeEnd("imm_map_bench.res 44");
 
diff --git a/jscomp/test/incomplete_toplevel_test.js b/jscomp/test/incomplete_toplevel_test.js
index c8a4a699e9..701ec21cf2 100644
--- a/jscomp/test/incomplete_toplevel_test.js
+++ b/jscomp/test/incomplete_toplevel_test.js
@@ -11,7 +11,7 @@ function f(param) {
         ];
 }
 
-var match = f(undefined);
+var match = f();
 
 var a = match[0];
 
diff --git a/jscomp/test/io_test.js b/jscomp/test/io_test.js
index 58e4915e0a..61e043cbb0 100644
--- a/jscomp/test/io_test.js
+++ b/jscomp/test/io_test.js
@@ -4,9 +4,9 @@
 
 function f(param) {
   console.error("x");
-  console.log(undefined);
+  console.log();
   console.log("hi");
-  console.log(undefined);
+  console.log();
 }
 
 exports.f = f;
diff --git a/jscomp/test/js_null_test.js b/jscomp/test/js_null_test.js
index 7cf7afa641..2143ef0a8e 100644
--- a/jscomp/test/js_null_test.js
+++ b/jscomp/test/js_null_test.js
@@ -23,7 +23,7 @@ var suites_1 = {
         return {
                 TAG: "Eq",
                 _0: Caml_option.some(undefined),
-                _1: Caml_option.some(undefined)
+                _1: Caml_option.some()
               };
       })
   ],
diff --git a/jscomp/test/jsoo_400_test.js b/jscomp/test/jsoo_400_test.js
index 24e1328720..fa8e5fdc63 100644
--- a/jscomp/test/jsoo_400_test.js
+++ b/jscomp/test/jsoo_400_test.js
@@ -22,7 +22,7 @@ Mt.from_pair_suites("Jsoo_400_test", {
             return {
                     TAG: "ThrowAny",
                     _0: (function (param) {
-                        u(undefined);
+                        u();
                       })
                   };
           })
diff --git a/jscomp/test/lazy_test.js b/jscomp/test/lazy_test.js
index 02ff1e0d02..b973dbdc5e 100644
--- a/jscomp/test/lazy_test.js
+++ b/jscomp/test/lazy_test.js
@@ -188,7 +188,7 @@ Mt.from_pair_suites("Lazy_test", {
         (function (param) {
             return {
                     TAG: "Eq",
-                    _0: lazy_test(undefined),
+                    _0: lazy_test(),
                     _1: [
                       3,
                       32
diff --git a/jscomp/test/loop_regression_test.js b/jscomp/test/loop_regression_test.js
index 6bbb9e3a86..4cee928188 100644
--- a/jscomp/test/loop_regression_test.js
+++ b/jscomp/test/loop_regression_test.js
@@ -27,7 +27,7 @@ var suites_0 = [
       return {
               TAG: "Eq",
               _0: 55,
-              _1: f(undefined)
+              _1: f()
             };
     })
 ];
diff --git a/jscomp/test/mario_game.js b/jscomp/test/mario_game.js
index 31ae3eb138..95150eadbc 100644
--- a/jscomp/test/mario_game.js
+++ b/jscomp/test/mario_game.js
@@ -847,7 +847,7 @@ function make$2($staropt$star, $staropt$star$1, spawnable, context, param) {
   var dir = $staropt$star$1 !== undefined ? $staropt$star$1 : "Left";
   var spr = make(spawnable, dir, context);
   var params = make_type$2(spawnable);
-  var id$1 = id !== undefined ? id : new_id(undefined);
+  var id$1 = id !== undefined ? id : new_id();
   var obj = {
     params: params,
     pos: {
@@ -2253,7 +2253,7 @@ function translate_keys(param) {
 function run_update_collid(state, collid, all_collids) {
   if (collid.TAG === "Player") {
     var o = collid._2;
-    var keys = translate_keys(undefined);
+    var keys = translate_keys();
     o.crouch = false;
     var match = update_player(o, keys, state.ctx);
     var player;
@@ -3313,7 +3313,7 @@ function generate(w, h, context) {
 }
 
 function init(param) {
-  Random.self_init(undefined);
+  Random.self_init();
 }
 
 var Procedural_generator = {
@@ -3326,7 +3326,7 @@ var loadCount = {
 };
 
 function load(param) {
-  Random.self_init(undefined);
+  Random.self_init();
   var canvas_id = "canvas";
   var el = document.getElementById(canvas_id);
   var canvas;
@@ -3343,7 +3343,7 @@ function load(param) {
   var context = Curry._1(canvas.getContext, "2d");
   document.addEventListener("keydown", keydown, true);
   document.addEventListener("keyup", keyup, true);
-  Random.self_init(undefined);
+  Random.self_init();
   update_loop(canvas, generate(2400, 256, context), [
         2400,
         256
@@ -3354,7 +3354,7 @@ function load(param) {
 function inc_counter(param) {
   loadCount.contents = loadCount.contents + 1 | 0;
   if (loadCount.contents === 4) {
-    return load(undefined);
+    return load();
   }
   
 }
@@ -3365,7 +3365,7 @@ function preload(param) {
                 var img = document.createElement("img");
                 img.src = img_src$1;
                 img.addEventListener("load", (function (ev) {
-                        inc_counter(undefined);
+                        inc_counter();
                         return true;
                       }), true);
               }), {
@@ -3384,7 +3384,7 @@ function preload(param) {
 }
 
 window.onload = (function (param) {
-    preload(undefined);
+    preload();
     return true;
   });
 
diff --git a/jscomp/test/mt.js b/jscomp/test/mt.js
index c71b21abe4..ba3c501e1a 100644
--- a/jscomp/test/mt.js
+++ b/jscomp/test/mt.js
@@ -201,7 +201,7 @@ var from_pair_suites = (function from_pair_suites(name, suites) {
   
 });
 
-var val_unit = Promise.resolve(undefined);
+var val_unit = Promise.resolve();
 
 var from_promise_suites = (function from_promise_suites(name, suites) {
   var match = $$Array.to_list(Process.argv);
@@ -230,7 +230,7 @@ var from_promise_suites = (function from_promise_suites(name, suites) {
 function old_from_promise_suites_donotuse(name, suites) {
   var match = $$Array.to_list(Process.argv);
   if (match) {
-    if (is_mocha(undefined)) {
+    if (is_mocha()) {
       describe(name, (function () {
               List.iter((function (param) {
                       var code = param[1];
diff --git a/jscomp/test/ocaml_re_test.js b/jscomp/test/ocaml_re_test.js
index d3ca5ba7f6..a93a34a8c1 100644
--- a/jscomp/test/ocaml_re_test.js
+++ b/jscomp/test/ocaml_re_test.js
@@ -3490,7 +3490,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
               Error: new Error()
             };
       }
-      var c = get(undefined);
+      var c = get();
       switch (c) {
         case 48 :
         case 49 :
@@ -3646,7 +3646,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
               Error: new Error()
             };
       }
-      var c$1 = get(undefined);
+      var c$1 = get();
       if (c$1 >= 64) {
         if (c$1 !== 92) {
           if (c$1 !== 123) {
@@ -3693,7 +3693,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
     if (i.contents === l) {
       return ;
     }
-    var d = get(undefined);
+    var d = get();
     if (d > 57 || d < 48) {
       i.contents = i.contents - 1 | 0;
       return ;
@@ -3704,7 +3704,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
         if (i.contents === l) {
           return i$1;
         }
-        var d$1 = get(undefined);
+        var d$1 = get();
         if (d$1 > 57 || d$1 < 48) {
           i.contents = i.contents - 1 | 0;
           return i$1;
@@ -3728,7 +3728,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
             Error: new Error()
           };
     }
-    var c = get(undefined);
+    var c = get();
     if (c === /* '[' */91) {
       if (accept(/* '=' */61)) {
         throw {
@@ -3819,7 +3819,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
               Error: new Error()
             };
       }
-      var c$1 = get(undefined);
+      var c$1 = get();
       if (!accept(/* '.' */46)) {
         throw {
               RE_EXN_ID: Not_supported,
@@ -3843,7 +3843,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
               VAL: c
             };
     }
-    var c$2 = get(undefined);
+    var c$2 = get();
     if (c$2 >= 58) {
       if (c$2 >= 123) {
         return {
@@ -4022,7 +4022,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
       if (s !== /* [] */0 && accept(/* ']' */93)) {
         return s;
       }
-      var match = $$char(undefined);
+      var match = $$char();
       if (match.NAME === "Char") {
         var c = match.VAL;
         if (accept(/* '-' */45)) {
@@ -4047,7 +4047,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
                     }
                   };
           }
-          var match$1 = $$char(undefined);
+          var match$1 = $$char();
           if (match$1.NAME !== "Char") {
             return {
                     hd: {
@@ -4098,7 +4098,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
     };
   };
   var piece = function (param) {
-    var r = atom(undefined);
+    var r = atom();
     if (accept(/* '*' */42)) {
       return greedy_mod(repn(r, 0, undefined));
     }
@@ -4111,9 +4111,9 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
     if (!accept(/* '{' */123)) {
       return r;
     }
-    var i$1 = integer(undefined);
+    var i$1 = integer();
     if (i$1 !== undefined) {
-      var j = accept(/* ',' */44) ? integer(undefined) : i$1;
+      var j = accept(/* ',' */44) ? integer() : i$1;
       if (!accept(/* '}' */125)) {
         throw {
               RE_EXN_ID: Parse_error,
@@ -4138,7 +4138,7 @@ function parse(multiline, dollar_endonly, dotall, ungreedy, s) {
         return seq$2(List.rev(left));
       }
       _left = {
-        hd: piece(undefined),
+        hd: piece(),
         tl: left
       };
       continue ;
diff --git a/jscomp/test/random_test.js b/jscomp/test/random_test.js
index d0f146b030..e32d5eadf9 100644
--- a/jscomp/test/random_test.js
+++ b/jscomp/test/random_test.js
@@ -33,14 +33,14 @@ function approx(f) {
   };
 }
 
-Mt_global.collect_neq(id, suites, "File \"random_test.res\", line 9, characters 2-9", (Random.self_init(undefined), Random.$$int(10000)), (Random.self_init(undefined), Random.$$int(1000)));
+Mt_global.collect_neq(id, suites, "File \"random_test.res\", line 9, characters 2-9", (Random.self_init(), Random.$$int(10000)), (Random.self_init(), Random.$$int(1000)));
 
 Random.init(0);
 
 var v = Caml_array.make(10, false);
 
 for(var i = 0; i <= 9; ++i){
-  Caml_array.set(v, i, Random.bool(undefined));
+  Caml_array.set(v, i, Random.bool());
 }
 
 Mt_global.collect_eq(id, suites, "File \"random_test.res\", line 28, characters 12-19", v, [
@@ -63,7 +63,7 @@ var h = Random.int64([
       3
     ]);
 
-var vv = Random.bits(undefined);
+var vv = Random.bits();
 
 var xx = Random.$$float(3.0);
 
diff --git a/jscomp/test/raw_hash_tbl_bench.js b/jscomp/test/raw_hash_tbl_bench.js
index b3872389b4..6252b105ba 100644
--- a/jscomp/test/raw_hash_tbl_bench.js
+++ b/jscomp/test/raw_hash_tbl_bench.js
@@ -27,7 +27,7 @@ function bench(param) {
   }
 }
 
-bench(undefined);
+bench();
 
 var count = 1000000;
 
diff --git a/jscomp/test/raw_output_test.js b/jscomp/test/raw_output_test.js
index 3facdda2aa..515fb69a3f 100644
--- a/jscomp/test/raw_output_test.js
+++ b/jscomp/test/raw_output_test.js
@@ -7,11 +7,11 @@ function mk(fn) {
   return Curry._1(fn, undefined);
 }
 
-(((_)=> console.log('should works'))(undefined));
+(((_)=> console.log('should works'))());
 
 console.log((function () {
           return 1;
-        })(undefined));
+        })());
 
 exports.mk = mk;
 /*  Not a pure module */
diff --git a/jscomp/test/reasonReactRouter.js b/jscomp/test/reasonReactRouter.js
index c8f193aa8f..708d608960 100644
--- a/jscomp/test/reasonReactRouter.js
+++ b/jscomp/test/reasonReactRouter.js
@@ -129,9 +129,9 @@ function urlNotEqual(a, b) {
 
 function url(param) {
   return {
-          path: path(undefined),
-          hash: hash(undefined),
-          search: search(undefined)
+          path: path(),
+          hash: hash(),
+          search: search()
         };
 }
 
@@ -143,7 +143,7 @@ function watchUrl(callback) {
     };
   }
   var watcherID = function (param) {
-    Curry._1(callback, url(undefined));
+    Curry._1(callback, url());
   };
   $$window.addEventListener("popstate", watcherID);
   return watcherID;
@@ -163,7 +163,7 @@ function useUrl(serverUrl, param) {
         if (serverUrl !== undefined) {
           return serverUrl;
         } else {
-          return url(undefined);
+          return url();
         }
       });
   var setUrl = match[1];
@@ -174,7 +174,7 @@ function useUrl(serverUrl, param) {
                         return url;
                       }));
               });
-          var newUrl = url(undefined);
+          var newUrl = url();
           if (urlNotEqual(newUrl, url$1)) {
             Curry._1(setUrl, (function (param) {
                     return newUrl;
diff --git a/jscomp/test/rec_fun_test.js b/jscomp/test/rec_fun_test.js
index 5333ec8b7e..34a5cbd997 100644
--- a/jscomp/test/rec_fun_test.js
+++ b/jscomp/test/rec_fun_test.js
@@ -49,7 +49,7 @@ function g(param) {
   console.log(String(next(0, true)));
 }
 
-g(undefined);
+g();
 
 var x = {};
 
diff --git a/jscomp/test/stack_test.js b/jscomp/test/stack_test.js
index 9c3e68f93d..dc7dc1d548 100644
--- a/jscomp/test/stack_test.js
+++ b/jscomp/test/stack_test.js
@@ -42,7 +42,7 @@ var suites_0 = [
                   }
                 }
               },
-              _1: v(undefined)
+              _1: v()
             };
     })
 ];
diff --git a/jscomp/test/stream_parser_test.js b/jscomp/test/stream_parser_test.js
index 5069483368..d6b499381b 100644
--- a/jscomp/test/stream_parser_test.js
+++ b/jscomp/test/stream_parser_test.js
@@ -32,12 +32,12 @@ function parse(token) {
     }
   };
   var parse_atom = function (param) {
-    var n = token$1(undefined);
+    var n = token$1();
     switch (n.TAG) {
       case "Kwd" :
           if (n._0 === "(") {
-            var v = parse_expr_aux(parse_term_aux(parse_atom(undefined)));
-            var match = token$1(undefined);
+            var v = parse_expr_aux(parse_term_aux(parse_atom()));
+            var match = token$1();
             if (match.TAG === "Kwd") {
               if (match._0 === ")") {
                 return v;
@@ -72,13 +72,13 @@ function parse(token) {
     }
   };
   var parse_term_aux = function (e1) {
-    var e = token$1(undefined);
+    var e = token$1();
     if (e.TAG === "Kwd") {
       switch (e._0) {
         case "*" :
-            return Math.imul(e1, parse_term_aux(parse_atom(undefined)));
+            return Math.imul(e1, parse_term_aux(parse_atom()));
         case "/" :
-            return Caml_int32.div(e1, parse_term_aux(parse_atom(undefined)));
+            return Caml_int32.div(e1, parse_term_aux(parse_atom()));
         default:
           Queue.push(e, look_ahead);
           return e1;
@@ -89,13 +89,13 @@ function parse(token) {
     }
   };
   var parse_expr_aux = function (e1) {
-    var e = token$1(undefined);
+    var e = token$1();
     if (e.TAG === "Kwd") {
       switch (e._0) {
         case "+" :
-            return e1 + parse_expr_aux(parse_term_aux(parse_atom(undefined))) | 0;
+            return e1 + parse_expr_aux(parse_term_aux(parse_atom())) | 0;
         case "-" :
-            return e1 - parse_expr_aux(parse_term_aux(parse_atom(undefined))) | 0;
+            return e1 - parse_expr_aux(parse_term_aux(parse_atom())) | 0;
         default:
           Queue.push(e, look_ahead);
           return e1;
@@ -105,7 +105,7 @@ function parse(token) {
       return e1;
     }
   };
-  var r = parse_expr_aux(parse_term_aux(parse_atom(undefined)));
+  var r = parse_expr_aux(parse_term_aux(parse_atom()));
   return [
           r,
           Queue.fold((function (acc, x) {
@@ -167,14 +167,14 @@ function l_parse(token) {
   var parse_f_aux = function (_a) {
     while(true) {
       var a = _a;
-      var t = token$1(undefined);
+      var t = token$1();
       if (t.TAG === "Kwd") {
         switch (t._0) {
           case "*" :
-              _a = Math.imul(a, parse_f(undefined));
+              _a = Math.imul(a, parse_f());
               continue ;
           case "/" :
-              _a = Caml_int32.div(a, parse_f(undefined));
+              _a = Caml_int32.div(a, parse_f());
               continue ;
           default:
             Queue.push(t, look_ahead);
@@ -187,12 +187,12 @@ function l_parse(token) {
     };
   };
   var parse_f = function (param) {
-    var i = token$1(undefined);
+    var i = token$1();
     switch (i.TAG) {
       case "Kwd" :
           if (i._0 === "(") {
-            var v = parse_t_aux(parse_f_aux(parse_f(undefined)));
-            var t = token$1(undefined);
+            var v = parse_t_aux(parse_f_aux(parse_f()));
+            var t = token$1();
             if (t.TAG === "Kwd") {
               if (t._0 === ")") {
                 return v;
@@ -227,14 +227,14 @@ function l_parse(token) {
   var parse_t_aux = function (_a) {
     while(true) {
       var a = _a;
-      var t = token$1(undefined);
+      var t = token$1();
       if (t.TAG === "Kwd") {
         switch (t._0) {
           case "+" :
-              _a = a + parse_f_aux(parse_f(undefined)) | 0;
+              _a = a + parse_f_aux(parse_f()) | 0;
               continue ;
           case "-" :
-              _a = a - parse_f_aux(parse_f(undefined)) | 0;
+              _a = a - parse_f_aux(parse_f()) | 0;
               continue ;
           default:
             Queue.push(t, look_ahead);
@@ -246,7 +246,7 @@ function l_parse(token) {
       }
     };
   };
-  var r = parse_t_aux(parse_f_aux(parse_f(undefined)));
+  var r = parse_t_aux(parse_f_aux(parse_f()));
   return [
           r,
           Queue.fold((function (acc, x) {
diff --git a/jscomp/test/tailcall_inline_test.js b/jscomp/test/tailcall_inline_test.js
index f1d7923e50..491916a1ab 100644
--- a/jscomp/test/tailcall_inline_test.js
+++ b/jscomp/test/tailcall_inline_test.js
@@ -30,7 +30,7 @@ var suites_0 = [
   (function (param) {
       return {
               TAG: "Eq",
-              _0: f(undefined),
+              _0: f(),
               _1: [
                 0,
                 1,
diff --git a/jscomp/test/test_closure.js b/jscomp/test/test_closure.js
index 0665a00d5c..a9d1b9d69f 100644
--- a/jscomp/test/test_closure.js
+++ b/jscomp/test/test_closure.js
@@ -23,7 +23,7 @@ function f(param) {
   return arr;
 }
 
-var u = f(undefined);
+var u = f();
 
 $$Array.iter((function (x) {
         Curry._1(x, undefined);
diff --git a/jscomp/test/test_per.js b/jscomp/test/test_per.js
index 4bffdb0292..27edfb87f7 100644
--- a/jscomp/test/test_per.js
+++ b/jscomp/test/test_per.js
@@ -217,7 +217,7 @@ function open_out_bin(name) {
 }
 
 function flush_all(param) {
-  var _x = Caml_external_polyfill.resolve("ml_out_channels_list")(undefined);
+  var _x = Caml_external_polyfill.resolve("ml_out_channels_list")();
   while(true) {
     var x = _x;
     if (!x) {
diff --git a/jscomp/test/test_while_closure.js b/jscomp/test/test_while_closure.js
index 6a3ee2d576..430c53070d 100644
--- a/jscomp/test/test_while_closure.js
+++ b/jscomp/test/test_while_closure.js
@@ -26,7 +26,7 @@ function f(param) {
   };
 }
 
-f(undefined);
+f();
 
 $$Array.iter((function (x) {
         Curry._1(x, undefined);
diff --git a/jscomp/test/tramp_fib.js b/jscomp/test/tramp_fib.js
index c1ac8e7852..11f7db3f2e 100644
--- a/jscomp/test/tramp_fib.js
+++ b/jscomp/test/tramp_fib.js
@@ -45,7 +45,7 @@ function iter(_bounce) {
     if (bounce.TAG === "Continue") {
       return bounce._0;
     }
-    _bounce = bounce._0(undefined);
+    _bounce = bounce._0();
     continue ;
   };
 }
diff --git a/jscomp/test/uncurried_default.args.js b/jscomp/test/uncurried_default.args.js
index 7ece8cb8ec..1c24e3b6dc 100644
--- a/jscomp/test/uncurried_default.args.js
+++ b/jscomp/test/uncurried_default.args.js
@@ -103,7 +103,7 @@ function foo3$1(xOpt, yOpt) {
 var r3$1 = foo3$1(undefined, undefined);
 
 function foo(func) {
-  return func(undefined) + 1 | 0;
+  return func() + 1 | 0;
 }
 
 var M = {
diff --git a/jscomp/test/unit_undefined_test.js b/jscomp/test/unit_undefined_test.js
index 2d336c22a1..4a43c971bb 100644
--- a/jscomp/test/unit_undefined_test.js
+++ b/jscomp/test/unit_undefined_test.js
@@ -54,11 +54,11 @@ function u2(x) {
 
 var u3 = Caml_option.some(undefined);
 
-eq("File \"unit_undefined_test.res\", line 41, characters 3-10", Caml_option.some(undefined), Caml_option.some(undefined));
+eq("File \"unit_undefined_test.res\", line 41, characters 3-10", Caml_option.some(), Caml_option.some(undefined));
 
 eq("File \"unit_undefined_test.res\", line 42, characters 3-10", u1, Caml_option.some(undefined));
 
-eq("File \"unit_undefined_test.res\", line 43, characters 3-10", Caml_option.some(undefined), Caml_option.some(undefined));
+eq("File \"unit_undefined_test.res\", line 43, characters 3-10", Caml_option.some(), Caml_option.some(undefined));
 
 eq("File \"unit_undefined_test.res\", line 44, characters 3-10", u3, Caml_option.some(undefined));
 
diff --git a/lib/es6/arg.js b/lib/es6/arg.js
index 65d286c7d5..082ea58bec 100644
--- a/lib/es6/arg.js
+++ b/lib/es6/arg.js
@@ -327,7 +327,7 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
             case "Unit" :
                 return Curry._1(f._0, undefined);
             case "Bool" :
-                var arg = get_arg(undefined);
+                var arg = get_arg();
                 var s$1 = bool_of_string_opt(arg);
                 if (s$1 !== undefined) {
                   Curry._1(f._0, s$1);
@@ -343,24 +343,24 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set" :
-                no_arg(undefined);
+                no_arg();
                 f._0.contents = true;
                 return ;
             case "Clear" :
-                no_arg(undefined);
+                no_arg();
                 f._0.contents = false;
                 return ;
             case "String" :
-                var arg$1 = get_arg(undefined);
+                var arg$1 = get_arg();
                 Curry._1(f._0, arg$1);
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_string" :
-                f._0.contents = get_arg(undefined);
-                return consume_arg(undefined);
+                f._0.contents = get_arg();
+                return consume_arg();
             case "Int" :
-                var arg$2 = get_arg(undefined);
+                var arg$2 = get_arg();
                 var x = int_of_string_opt(arg$2);
                 if (x !== undefined) {
                   Curry._1(f._0, x);
@@ -376,9 +376,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_int" :
-                var arg$3 = get_arg(undefined);
+                var arg$3 = get_arg();
                 var x$1 = int_of_string_opt(arg$3);
                 if (x$1 !== undefined) {
                   f._0.contents = x$1;
@@ -394,9 +394,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Float" :
-                var arg$4 = get_arg(undefined);
+                var arg$4 = get_arg();
                 var x$2 = float_of_string_opt(arg$4);
                 if (x$2 !== undefined) {
                   Curry._1(f._0, x$2);
@@ -412,9 +412,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_float" :
-                var arg$5 = get_arg(undefined);
+                var arg$5 = get_arg();
                 var x$3 = float_of_string_opt(arg$5);
                 if (x$3 !== undefined) {
                   f._0.contents = x$3;
@@ -430,15 +430,15 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Tuple" :
                 return List.iter(treat_action, f._0);
             case "Symbol" :
                 var symb = f._0;
-                var arg$6 = get_arg(undefined);
+                var arg$6 = get_arg();
                 if (List.mem(arg$6, symb)) {
                   Curry._1(f._1, arg$6);
-                  return consume_arg(undefined);
+                  return consume_arg();
                 }
                 throw {
                       RE_EXN_ID: Stop,
@@ -454,7 +454,7 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                 var f$1 = f._0;
                 while(current.contents < (argv.contents.length - 1 | 0)) {
                   Curry._1(f$1, Caml_array.get(argv.contents, current.contents + 1 | 0));
-                  consume_arg(undefined);
+                  consume_arg();
                 };
                 return ;
             case "Expand" :
@@ -465,9 +465,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                var arg$7 = get_arg(undefined);
+                var arg$7 = get_arg();
                 var newarg = Curry._1(f._0, arg$7);
-                consume_arg(undefined);
+                consume_arg();
                 var before = $$Array.sub(argv.contents, 0, current.contents + 1 | 0);
                 var after = $$Array.sub(argv.contents, current.contents + 1 | 0, (argv.contents.length - current.contents | 0) - 1 | 0);
                 argv.contents = Caml_array.concat({
diff --git a/lib/es6/camlinternalLazy.js b/lib/es6/camlinternalLazy.js
index 7b80d775fb..697cd9e5de 100644
--- a/lib/es6/camlinternalLazy.js
+++ b/lib/es6/camlinternalLazy.js
@@ -9,7 +9,7 @@ function is_val(l) {
 var Undefined = /* @__PURE__ */Caml_exceptions.create("CamlinternalLazy.Undefined");
 
 function forward_with_closure(blk, closure) {
-  var result = closure(undefined);
+  var result = closure();
   blk.VAL = result;
   blk.LAZY_DONE = true;
   return result;
diff --git a/lib/es6/genlex.js b/lib/es6/genlex.js
index 18dcf24292..09a9b041b3 100644
--- a/lib/es6/genlex.js
+++ b/lib/es6/genlex.js
@@ -104,7 +104,7 @@ function make_lexer(keywords) {
                   continue ;
               case 34 :
                   Stream.junk(strm__);
-                  reset_buffer(undefined);
+                  reset_buffer();
                   return {
                           TAG: "String",
                           _0: string(strm__)
@@ -161,12 +161,12 @@ function make_lexer(keywords) {
                   var c$2 = Stream.peek(strm__);
                   if (c$2 !== undefined && !(c$2 > 57 || c$2 < 48)) {
                     Stream.junk(strm__);
-                    reset_buffer(undefined);
+                    reset_buffer();
                     store(/* '-' */45);
                     store(c$2);
                     return number(strm__);
                   } else {
-                    reset_buffer(undefined);
+                    reset_buffer();
                     store(/* '-' */45);
                     return ident2(strm__);
                   }
@@ -265,30 +265,30 @@ function make_lexer(keywords) {
             return keyword_or_error(c);
         case 2 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             while(true) {
               var c$3 = Stream.peek(strm__);
               if (c$3 === undefined) {
-                return ident_or_keyword(get_string(undefined));
+                return ident_or_keyword(get_string());
               }
               if (c$3 >= 91) {
                 if (c$3 > 122 || c$3 < 95) {
                   if (c$3 > 255 || c$3 < 192) {
-                    return ident_or_keyword(get_string(undefined));
+                    return ident_or_keyword(get_string());
                   }
                   
                 } else if (c$3 === 96) {
-                  return ident_or_keyword(get_string(undefined));
+                  return ident_or_keyword(get_string());
                 }
                 
               } else if (c$3 >= 48) {
                 if (!(c$3 > 64 || c$3 < 58)) {
-                  return ident_or_keyword(get_string(undefined));
+                  return ident_or_keyword(get_string());
                 }
                 
               } else if (c$3 !== 39) {
-                return ident_or_keyword(get_string(undefined));
+                return ident_or_keyword(get_string());
               }
               Stream.junk(strm__);
               store(c$3);
@@ -296,12 +296,12 @@ function make_lexer(keywords) {
             };
         case 3 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             return ident2(strm__);
         case 4 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             return number(strm__);
         
@@ -312,26 +312,26 @@ function make_lexer(keywords) {
     while(true) {
       var c = Stream.peek(strm__);
       if (c === undefined) {
-        return ident_or_keyword(get_string(undefined));
+        return ident_or_keyword(get_string());
       }
       if (c >= 94) {
         if (c > 125 || c < 95) {
           if (c >= 127) {
-            return ident_or_keyword(get_string(undefined));
+            return ident_or_keyword(get_string());
           }
           
         } else if (c !== 124) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         
       } else if (c >= 65) {
         if (c !== 92) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         
       } else {
         if (c < 33) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         switch (c) {
           case 34 :
@@ -351,7 +351,7 @@ function make_lexer(keywords) {
           case 56 :
           case 57 :
           case 59 :
-              return ident_or_keyword(get_string(undefined));
+              return ident_or_keyword(get_string());
           case 33 :
           case 35 :
           case 36 :
@@ -416,14 +416,14 @@ function make_lexer(keywords) {
             }
             return {
                     TAG: "Float",
-                    _0: Caml_format.float_of_string(get_string(undefined))
+                    _0: Caml_format.float_of_string(get_string())
                   };
           };
         }
       }
       return {
               TAG: "Int",
-              _0: Caml_format.int_of_string(get_string(undefined))
+              _0: Caml_format.int_of_string(get_string())
             };
     };
   };
@@ -443,13 +443,13 @@ function make_lexer(keywords) {
       if (c === undefined) {
         return {
                 TAG: "Float",
-                _0: Caml_format.float_of_string(get_string(undefined))
+                _0: Caml_format.float_of_string(get_string())
               };
       }
       if (c > 57 || c < 48) {
         return {
                 TAG: "Float",
-                _0: Caml_format.float_of_string(get_string(undefined))
+                _0: Caml_format.float_of_string(get_string())
               };
       }
       Stream.junk(strm__);
@@ -487,7 +487,7 @@ function make_lexer(keywords) {
           continue ;
         }
         Stream.junk(strm__);
-        return get_string(undefined);
+        return get_string();
       }
       throw {
             RE_EXN_ID: Stream.Failure,
diff --git a/lib/es6/hashtbl.js b/lib/es6/hashtbl.js
index 71dec6eff7..621cc7b164 100644
--- a/lib/es6/hashtbl.js
+++ b/lib/es6/hashtbl.js
@@ -42,7 +42,7 @@ function is_randomized(param) {
 var prng = {
   LAZY_DONE: false,
   VAL: (function () {
-      return Random.State.make_self_init(undefined);
+      return Random.State.make_self_init();
     })
 };
 
diff --git a/lib/es6/parsing.js b/lib/es6/parsing.js
index f192934e19..ff983c6466 100644
--- a/lib/es6/parsing.js
+++ b/lib/es6/parsing.js
@@ -93,12 +93,12 @@ function yyparse(tables, start, lexer, lexbuf) {
                   Error: new Error()
                 };
         case "Grow_stacks_1" :
-            grow_stacks(undefined);
+            grow_stacks();
             _arg = undefined;
             _cmd = "Stacks_grown_1";
             continue ;
         case "Grow_stacks_2" :
-            grow_stacks(undefined);
+            grow_stacks();
             _arg = undefined;
             _cmd = "Stacks_grown_2";
             continue ;
@@ -191,11 +191,11 @@ function rhs_end_pos(n) {
 }
 
 function symbol_start(param) {
-  return symbol_start_pos(undefined).pos_cnum;
+  return symbol_start_pos().pos_cnum;
 }
 
 function symbol_end(param) {
-  return symbol_end_pos(undefined).pos_cnum;
+  return symbol_end_pos().pos_cnum;
 }
 
 function rhs_start(n) {
diff --git a/lib/es6/pervasivesU.js b/lib/es6/pervasivesU.js
index 98659ee37b..66770b5ddb 100644
--- a/lib/es6/pervasivesU.js
+++ b/lib/es6/pervasivesU.js
@@ -199,13 +199,13 @@ var exit_function = {
 function at_exit(f) {
   var g = exit_function.contents;
   exit_function.contents = (function () {
-      f(undefined);
-      g(undefined);
+      f();
+      g();
     });
 }
 
 function exit(retcode) {
-  exit_function.contents(undefined);
+  exit_function.contents();
   return Caml_sys.sys_exit(retcode);
 }
 
diff --git a/lib/es6/random.js b/lib/es6/random.js
index 2dad856f57..89cc63f188 100644
--- a/lib/es6/random.js
+++ b/lib/es6/random.js
@@ -52,7 +52,7 @@ function make(seed) {
 }
 
 function make_self_init(param) {
-  return make(random_seed(undefined));
+  return make(random_seed());
 }
 
 function copy(s) {
@@ -240,7 +240,7 @@ function init(seed) {
 }
 
 function self_init(param) {
-  full_init$1(random_seed(undefined));
+  full_init$1(random_seed());
 }
 
 function get_state(param) {
diff --git a/lib/es6/sys.js b/lib/es6/sys.js
index 2031b91a13..c40e28b6df 100644
--- a/lib/es6/sys.js
+++ b/lib/es6/sys.js
@@ -3,9 +3,9 @@
 import * as Caml_sys from "./caml_sys.js";
 import * as Caml_exceptions from "./caml_exceptions.js";
 
-var match = Caml_sys.sys_get_argv(undefined);
+var match = Caml_sys.sys_get_argv();
 
-var os_type = Caml_sys.os_type(undefined);
+var os_type = Caml_sys.os_type();
 
 var backend_type = {
   TAG: "Other",
@@ -14,9 +14,9 @@ var backend_type = {
 
 var big_endian = false;
 
-var unix = Caml_sys.os_type(undefined) === "Unix";
+var unix = Caml_sys.os_type() === "Unix";
 
-var win32 = Caml_sys.os_type(undefined) === "Win32";
+var win32 = Caml_sys.os_type() === "Win32";
 
 function getenv_opt(s) {
   var x = typeof process === "undefined" ? undefined : process;
diff --git a/lib/js/arg.js b/lib/js/arg.js
index 998f7ec0be..caae76bace 100644
--- a/lib/js/arg.js
+++ b/lib/js/arg.js
@@ -327,7 +327,7 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
             case "Unit" :
                 return Curry._1(f._0, undefined);
             case "Bool" :
-                var arg = get_arg(undefined);
+                var arg = get_arg();
                 var s$1 = bool_of_string_opt(arg);
                 if (s$1 !== undefined) {
                   Curry._1(f._0, s$1);
@@ -343,24 +343,24 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set" :
-                no_arg(undefined);
+                no_arg();
                 f._0.contents = true;
                 return ;
             case "Clear" :
-                no_arg(undefined);
+                no_arg();
                 f._0.contents = false;
                 return ;
             case "String" :
-                var arg$1 = get_arg(undefined);
+                var arg$1 = get_arg();
                 Curry._1(f._0, arg$1);
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_string" :
-                f._0.contents = get_arg(undefined);
-                return consume_arg(undefined);
+                f._0.contents = get_arg();
+                return consume_arg();
             case "Int" :
-                var arg$2 = get_arg(undefined);
+                var arg$2 = get_arg();
                 var x = int_of_string_opt(arg$2);
                 if (x !== undefined) {
                   Curry._1(f._0, x);
@@ -376,9 +376,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_int" :
-                var arg$3 = get_arg(undefined);
+                var arg$3 = get_arg();
                 var x$1 = int_of_string_opt(arg$3);
                 if (x$1 !== undefined) {
                   f._0.contents = x$1;
@@ -394,9 +394,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Float" :
-                var arg$4 = get_arg(undefined);
+                var arg$4 = get_arg();
                 var x$2 = float_of_string_opt(arg$4);
                 if (x$2 !== undefined) {
                   Curry._1(f._0, x$2);
@@ -412,9 +412,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Set_float" :
-                var arg$5 = get_arg(undefined);
+                var arg$5 = get_arg();
                 var x$3 = float_of_string_opt(arg$5);
                 if (x$3 !== undefined) {
                   f._0.contents = x$3;
@@ -430,15 +430,15 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                return consume_arg(undefined);
+                return consume_arg();
             case "Tuple" :
                 return List.iter(treat_action, f._0);
             case "Symbol" :
                 var symb = f._0;
-                var arg$6 = get_arg(undefined);
+                var arg$6 = get_arg();
                 if (List.mem(arg$6, symb)) {
                   Curry._1(f._1, arg$6);
-                  return consume_arg(undefined);
+                  return consume_arg();
                 }
                 throw {
                       RE_EXN_ID: Stop,
@@ -454,7 +454,7 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                 var f$1 = f._0;
                 while(current.contents < (argv.contents.length - 1 | 0)) {
                   Curry._1(f$1, Caml_array.get(argv.contents, current.contents + 1 | 0));
-                  consume_arg(undefined);
+                  consume_arg();
                 };
                 return ;
             case "Expand" :
@@ -465,9 +465,9 @@ function parse_and_expand_argv_dynamic_aux(allow_expand, current, argv, speclist
                         Error: new Error()
                       };
                 }
-                var arg$7 = get_arg(undefined);
+                var arg$7 = get_arg();
                 var newarg = Curry._1(f._0, arg$7);
-                consume_arg(undefined);
+                consume_arg();
                 var before = $$Array.sub(argv.contents, 0, current.contents + 1 | 0);
                 var after = $$Array.sub(argv.contents, current.contents + 1 | 0, (argv.contents.length - current.contents | 0) - 1 | 0);
                 argv.contents = Caml_array.concat({
diff --git a/lib/js/camlinternalLazy.js b/lib/js/camlinternalLazy.js
index 867d359675..f812859e83 100644
--- a/lib/js/camlinternalLazy.js
+++ b/lib/js/camlinternalLazy.js
@@ -9,7 +9,7 @@ function is_val(l) {
 var Undefined = /* @__PURE__ */Caml_exceptions.create("CamlinternalLazy.Undefined");
 
 function forward_with_closure(blk, closure) {
-  var result = closure(undefined);
+  var result = closure();
   blk.VAL = result;
   blk.LAZY_DONE = true;
   return result;
diff --git a/lib/js/genlex.js b/lib/js/genlex.js
index 89aa30a1c1..27b3c7d43d 100644
--- a/lib/js/genlex.js
+++ b/lib/js/genlex.js
@@ -104,7 +104,7 @@ function make_lexer(keywords) {
                   continue ;
               case 34 :
                   Stream.junk(strm__);
-                  reset_buffer(undefined);
+                  reset_buffer();
                   return {
                           TAG: "String",
                           _0: string(strm__)
@@ -161,12 +161,12 @@ function make_lexer(keywords) {
                   var c$2 = Stream.peek(strm__);
                   if (c$2 !== undefined && !(c$2 > 57 || c$2 < 48)) {
                     Stream.junk(strm__);
-                    reset_buffer(undefined);
+                    reset_buffer();
                     store(/* '-' */45);
                     store(c$2);
                     return number(strm__);
                   } else {
-                    reset_buffer(undefined);
+                    reset_buffer();
                     store(/* '-' */45);
                     return ident2(strm__);
                   }
@@ -265,30 +265,30 @@ function make_lexer(keywords) {
             return keyword_or_error(c);
         case 2 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             while(true) {
               var c$3 = Stream.peek(strm__);
               if (c$3 === undefined) {
-                return ident_or_keyword(get_string(undefined));
+                return ident_or_keyword(get_string());
               }
               if (c$3 >= 91) {
                 if (c$3 > 122 || c$3 < 95) {
                   if (c$3 > 255 || c$3 < 192) {
-                    return ident_or_keyword(get_string(undefined));
+                    return ident_or_keyword(get_string());
                   }
                   
                 } else if (c$3 === 96) {
-                  return ident_or_keyword(get_string(undefined));
+                  return ident_or_keyword(get_string());
                 }
                 
               } else if (c$3 >= 48) {
                 if (!(c$3 > 64 || c$3 < 58)) {
-                  return ident_or_keyword(get_string(undefined));
+                  return ident_or_keyword(get_string());
                 }
                 
               } else if (c$3 !== 39) {
-                return ident_or_keyword(get_string(undefined));
+                return ident_or_keyword(get_string());
               }
               Stream.junk(strm__);
               store(c$3);
@@ -296,12 +296,12 @@ function make_lexer(keywords) {
             };
         case 3 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             return ident2(strm__);
         case 4 :
             Stream.junk(strm__);
-            reset_buffer(undefined);
+            reset_buffer();
             store(c);
             return number(strm__);
         
@@ -312,26 +312,26 @@ function make_lexer(keywords) {
     while(true) {
       var c = Stream.peek(strm__);
       if (c === undefined) {
-        return ident_or_keyword(get_string(undefined));
+        return ident_or_keyword(get_string());
       }
       if (c >= 94) {
         if (c > 125 || c < 95) {
           if (c >= 127) {
-            return ident_or_keyword(get_string(undefined));
+            return ident_or_keyword(get_string());
           }
           
         } else if (c !== 124) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         
       } else if (c >= 65) {
         if (c !== 92) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         
       } else {
         if (c < 33) {
-          return ident_or_keyword(get_string(undefined));
+          return ident_or_keyword(get_string());
         }
         switch (c) {
           case 34 :
@@ -351,7 +351,7 @@ function make_lexer(keywords) {
           case 56 :
           case 57 :
           case 59 :
-              return ident_or_keyword(get_string(undefined));
+              return ident_or_keyword(get_string());
           case 33 :
           case 35 :
           case 36 :
@@ -416,14 +416,14 @@ function make_lexer(keywords) {
             }
             return {
                     TAG: "Float",
-                    _0: Caml_format.float_of_string(get_string(undefined))
+                    _0: Caml_format.float_of_string(get_string())
                   };
           };
         }
       }
       return {
               TAG: "Int",
-              _0: Caml_format.int_of_string(get_string(undefined))
+              _0: Caml_format.int_of_string(get_string())
             };
     };
   };
@@ -443,13 +443,13 @@ function make_lexer(keywords) {
       if (c === undefined) {
         return {
                 TAG: "Float",
-                _0: Caml_format.float_of_string(get_string(undefined))
+                _0: Caml_format.float_of_string(get_string())
               };
       }
       if (c > 57 || c < 48) {
         return {
                 TAG: "Float",
-                _0: Caml_format.float_of_string(get_string(undefined))
+                _0: Caml_format.float_of_string(get_string())
               };
       }
       Stream.junk(strm__);
@@ -487,7 +487,7 @@ function make_lexer(keywords) {
           continue ;
         }
         Stream.junk(strm__);
-        return get_string(undefined);
+        return get_string();
       }
       throw {
             RE_EXN_ID: Stream.Failure,
diff --git a/lib/js/hashtbl.js b/lib/js/hashtbl.js
index 72aeaf29ef..83edfef324 100644
--- a/lib/js/hashtbl.js
+++ b/lib/js/hashtbl.js
@@ -42,7 +42,7 @@ function is_randomized(param) {
 var prng = {
   LAZY_DONE: false,
   VAL: (function () {
-      return Random.State.make_self_init(undefined);
+      return Random.State.make_self_init();
     })
 };
 
diff --git a/lib/js/parsing.js b/lib/js/parsing.js
index e024f371cd..a041925b2c 100644
--- a/lib/js/parsing.js
+++ b/lib/js/parsing.js
@@ -93,12 +93,12 @@ function yyparse(tables, start, lexer, lexbuf) {
                   Error: new Error()
                 };
         case "Grow_stacks_1" :
-            grow_stacks(undefined);
+            grow_stacks();
             _arg = undefined;
             _cmd = "Stacks_grown_1";
             continue ;
         case "Grow_stacks_2" :
-            grow_stacks(undefined);
+            grow_stacks();
             _arg = undefined;
             _cmd = "Stacks_grown_2";
             continue ;
@@ -191,11 +191,11 @@ function rhs_end_pos(n) {
 }
 
 function symbol_start(param) {
-  return symbol_start_pos(undefined).pos_cnum;
+  return symbol_start_pos().pos_cnum;
 }
 
 function symbol_end(param) {
-  return symbol_end_pos(undefined).pos_cnum;
+  return symbol_end_pos().pos_cnum;
 }
 
 function rhs_start(n) {
diff --git a/lib/js/pervasivesU.js b/lib/js/pervasivesU.js
index 2ebec348ac..31c6560d8a 100644
--- a/lib/js/pervasivesU.js
+++ b/lib/js/pervasivesU.js
@@ -199,13 +199,13 @@ var exit_function = {
 function at_exit(f) {
   var g = exit_function.contents;
   exit_function.contents = (function () {
-      f(undefined);
-      g(undefined);
+      f();
+      g();
     });
 }
 
 function exit(retcode) {
-  exit_function.contents(undefined);
+  exit_function.contents();
   return Caml_sys.sys_exit(retcode);
 }
 
diff --git a/lib/js/random.js b/lib/js/random.js
index 3c5436cce6..f0ceb6521d 100644
--- a/lib/js/random.js
+++ b/lib/js/random.js
@@ -52,7 +52,7 @@ function make(seed) {
 }
 
 function make_self_init(param) {
-  return make(random_seed(undefined));
+  return make(random_seed());
 }
 
 function copy(s) {
@@ -240,7 +240,7 @@ function init(seed) {
 }
 
 function self_init(param) {
-  full_init$1(random_seed(undefined));
+  full_init$1(random_seed());
 }
 
 function get_state(param) {
diff --git a/lib/js/sys.js b/lib/js/sys.js
index 34fc5edfd8..a0088d3afc 100644
--- a/lib/js/sys.js
+++ b/lib/js/sys.js
@@ -3,9 +3,9 @@
 var Caml_sys = require("./caml_sys.js");
 var Caml_exceptions = require("./caml_exceptions.js");
 
-var match = Caml_sys.sys_get_argv(undefined);
+var match = Caml_sys.sys_get_argv();
 
-var os_type = Caml_sys.os_type(undefined);
+var os_type = Caml_sys.os_type();
 
 var backend_type = {
   TAG: "Other",
@@ -14,9 +14,9 @@ var backend_type = {
 
 var big_endian = false;
 
-var unix = Caml_sys.os_type(undefined) === "Unix";
+var unix = Caml_sys.os_type() === "Unix";
 
-var win32 = Caml_sys.os_type(undefined) === "Win32";
+var win32 = Caml_sys.os_type() === "Win32";
 
 function getenv_opt(s) {
   var x = typeof process === "undefined" ? undefined : process;