diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 68a404e678..461bb5691c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -17,11 +17,6 @@ jobs:
         os:
           - ubuntu-latest
         ocaml-compiler:
-          - "4.08"
-          - "4.09"
-          - "4.10"
-          - "4.11"
-          - "4.12"
           - "4.13"
           - "5.0"
           - "5.1"
diff --git a/CHANGES.md b/CHANGES.md
index 755cfcfa3d..92af6ef0bc 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # dev
 
 ## Features/Changes
+* Misc: drop support for OCaml 4.12 and bellow
 * Compiler: use a Wasm text files preprocessor (#1822)
 * Compiler: support for OCaml 4.14.3+trunk (#1844)
 * Runtime: use es6 class (#1840)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 971cc9f30b..59afb8fbe3 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -28,8 +28,4 @@ opam install odoc lwt_log yojson ocp-indent graphics higlo
 
 ### Running the tests
 
-Tests are maintained for a single version of the OCaml compiler (currently 4.13).
-
-Make sure to use the correct opam switch (e.g. `opam switch 4.13.1`).
-
 Run `make tests`.
diff --git a/compiler/bin-js_of_ocaml/check_runtime.ml b/compiler/bin-js_of_ocaml/check_runtime.ml
index e5d6fa4447..0f688963ce 100644
--- a/compiler/bin-js_of_ocaml/check_runtime.ml
+++ b/compiler/bin-js_of_ocaml/check_runtime.ml
@@ -22,7 +22,7 @@ open Js_of_ocaml_compiler
 
 let group_by_snd l =
   l
-  |> List.sort_uniq ~compare:(fun (n1, l1) (n2, l2) ->
+  |> List.sort_uniq ~cmp:(fun (n1, l1) (n2, l2) ->
          match Poly.compare l1 l2 with
          | 0 -> String.compare n1 n2
          | c -> c)
@@ -49,8 +49,8 @@ let f (runtime_files, bytecode, target_env) =
   let runtime_files, builtin =
     List.partition_map runtime_files ~f:(fun name ->
         match Builtins.find name with
-        | Some t -> `Snd t
-        | None -> `Fst name)
+        | Some t -> Right t
+        | None -> Left name)
   in
   let builtin =
     if false then builtin else Js_of_ocaml_compiler_runtime_files.runtime @ builtin
diff --git a/compiler/bin-js_of_ocaml/compile.ml b/compiler/bin-js_of_ocaml/compile.ml
index 6c19facb9b..53e82f7a97 100644
--- a/compiler/bin-js_of_ocaml/compile.ml
+++ b/compiler/bin-js_of_ocaml/compile.ml
@@ -102,7 +102,8 @@ let sourcemap_section_of_info
         | Some _ -> Filename.concat "/builtin" filename)
   in
   let ignore_list =
-    List.filter sources ~f:(fun filename -> String.is_prefix ~prefix:"/builtin/" filename)
+    List.filter sources ~f:(fun filename ->
+        String.starts_with ~prefix:"/builtin/" filename)
   in
   let offset, mappings = Source_map.Mappings.encode_with_offset mappings in
   let map =
@@ -207,8 +208,8 @@ let run
   let runtime_files, builtin =
     List.partition_map runtime_files ~f:(fun name ->
         match Builtins.find name with
-        | Some t -> `Snd t
-        | None -> `Fst name)
+        | Some t -> Right t
+        | None -> Left name)
   in
   let t1 = Timer.make () in
   let builtin =
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index cd91ca450a..c86546a4b8 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -313,8 +313,8 @@ let run
   let runtime_js_files, builtin =
     List.partition_map runtime_js_files ~f:(fun name ->
         match Builtins.find name with
-        | Some t -> `Snd t
-        | None -> `Fst name)
+        | Some t -> Right t
+        | None -> Left name)
   in
   let t1 = Timer.make () in
   let builtin = Js_of_ocaml_compiler_runtime_files.runtime @ builtin in
diff --git a/compiler/lib-runtime-files/gen/gen.ml b/compiler/lib-runtime-files/gen/gen.ml
index 147f2453e6..a13ad8d38f 100644
--- a/compiler/lib-runtime-files/gen/gen.ml
+++ b/compiler/lib-runtime-files/gen/gen.ml
@@ -58,7 +58,7 @@ let () =
   match Array.to_list Sys.argv with
   | [] -> assert false
   | _ :: rest ->
-      let rest = List.sort_uniq ~compare:String.compare rest in
+      let rest = List.sort_uniq ~cmp:String.compare rest in
       let fragments =
         List.map rest ~f:(fun f -> f, Js_of_ocaml_compiler.Linker.Fragment.parse_file f)
       in
diff --git a/compiler/lib-runtime-files/js_of_ocaml_compiler_runtime_files.ml b/compiler/lib-runtime-files/js_of_ocaml_compiler_runtime_files.ml
index 22ebd08bbf..41466846a2 100644
--- a/compiler/lib-runtime-files/js_of_ocaml_compiler_runtime_files.ml
+++ b/compiler/lib-runtime-files/js_of_ocaml_compiler_runtime_files.ml
@@ -34,7 +34,6 @@ let runtime =
     ; hash
     ; ieee_754
     ; int64
-    ; internalMod
     ; ints
     ; io
     ; jslib
diff --git a/compiler/lib-runtime-files/tests/all.ml b/compiler/lib-runtime-files/tests/all.ml
index bd76636f7d..0896252226 100644
--- a/compiler/lib-runtime-files/tests/all.ml
+++ b/compiler/lib-runtime-files/tests/all.ml
@@ -30,7 +30,6 @@ let%expect_test _ =
     +hash.js
     +ieee_754.js
     +int64.js
-    +internalMod.js
     +ints.js
     +io.js
     +jslib.js
@@ -51,7 +50,8 @@ let%expect_test _ =
     +toplevel.js
     +unix.js
     +weak.js
-    +zstd.js |}];
+    +zstd.js
+    |}];
   printl runtime;
   [%expect
     {|
@@ -73,7 +73,6 @@ let%expect_test _ =
     +hash.js
     +ieee_754.js
     +int64.js
-    +internalMod.js
     +ints.js
     +io.js
     +jslib.js
@@ -93,7 +92,8 @@ let%expect_test _ =
     +sys.js
     +unix.js
     +weak.js
-    +zstd.js |}];
+    +zstd.js
+    |}];
   printl extra;
   [%expect {|
     +dynlink.js
diff --git a/compiler/lib/build_path_prefix_map.ml b/compiler/lib/build_path_prefix_map.ml
index 75af3b2000..7fb9a469f4 100644
--- a/compiler/lib/build_path_prefix_map.ml
+++ b/compiler/lib/build_path_prefix_map.ml
@@ -100,7 +100,7 @@ let decode_map str =
         | Ok str -> Some str
         | Error err -> raise (Shortcut err))
   in
-  let pairs = String.split_char ~sep:':' str in
+  let pairs = String.split_on_char ~sep:':' str in
   match List.map ~f:decode_or_empty pairs with
   | exception Shortcut err -> Error err
   | map -> Ok map
@@ -108,7 +108,7 @@ let decode_map str =
 let rewrite_opt prefix_map path =
   let is_prefix = function
     | None -> false
-    | Some { target = _; source } -> String.is_prefix path ~prefix:source
+    | Some { target = _; source } -> String.starts_with path ~prefix:source
   in
   match
     List.find
diff --git a/compiler/lib/generate.ml b/compiler/lib/generate.ml
index 0ebfe8d891..85bf9137d5 100644
--- a/compiler/lib/generate.ml
+++ b/compiler/lib/generate.ml
@@ -1464,7 +1464,7 @@ let rec translate_expr ctx loc x e level : (_ * J.statement_list) Expr_builder.t
             match internal_prim name with
             | Some f -> f l ctx loc
             | None ->
-                if String.is_prefix name ~prefix:"%"
+                if String.starts_with name ~prefix:"%"
                 then failwith (Printf.sprintf "Unresolved internal primitive: %s" name);
                 let prim = Share.get_prim (runtime_fun ctx) name ctx.Ctx.share in
                 let* () = info ~need_loc:true (kind (Primitive.kind name)) in
@@ -1522,7 +1522,7 @@ and translate_instr ctx expr_queue loc instr =
             (* "switcher" is emitted by the OCaml compiler when compiling
                pattern matching, it does not help much to keep it in the
                generated js, let's drop it *)
-            (not (generated_name s)) && not (String.is_prefix s ~prefix:"jsoo_")
+            (not (generated_name s)) && not (String.starts_with s ~prefix:"jsoo_")
       in
       match ctx.Ctx.live.(Var.idx x), e with
       | 0, _ ->
diff --git a/compiler/lib/javascript.ml b/compiler/lib/javascript.ml
index 998341bf72..88d52dca25 100644
--- a/compiler/lib/javascript.ml
+++ b/compiler/lib/javascript.ml
@@ -56,7 +56,7 @@ end = struct
 
   let to_targetint s =
     if
-      String.is_prefix s ~prefix:"0"
+      String.starts_with s ~prefix:"0"
       && String.length s > 1
       && String.for_all s ~f:(function
            | '0' .. '7' -> true
diff --git a/compiler/lib/js_output.ml b/compiler/lib/js_output.ml
index f2cf8c79ed..f547dc62d2 100644
--- a/compiler/lib/js_output.ml
+++ b/compiler/lib/js_output.ml
@@ -542,7 +542,8 @@ struct
     for i = 0 to l - 1 do
       let c = s.[i] in
       match c with
-      | '\000' when i = l - 1 || not (Char.is_num s.[i + 1]) -> Buffer.add_string b "\\0"
+      | '\000' when i = l - 1 || not (Char.is_digit s.[i + 1]) ->
+          Buffer.add_string b "\\0"
       | '\b' (* 008 *) -> Buffer.add_string b "\\b"
       | '\t' (* 009 *) -> Buffer.add_string b "\\t"
       | '\n' (* 010 *) -> Buffer.add_string b "\\n"
diff --git a/compiler/lib/link_js.ml b/compiler/lib/link_js.ml
index 71bd724a50..a95c83136f 100644
--- a/compiler/lib/link_js.ml
+++ b/compiler/lib/link_js.ml
@@ -130,10 +130,10 @@ type action =
   | Source_map of Source_map.t
 
 let prefix_kind line =
-  match String.is_prefix ~prefix:info_prefix line with
+  match String.starts_with ~prefix:info_prefix line with
   | false -> `Other
   | true -> (
-      match String.is_prefix ~prefix:sourceMappingURL line with
+      match String.starts_with ~prefix:sourceMappingURL line with
       | false -> (
           match Build_info.parse line with
           | Some bi -> `Build_info bi
@@ -142,7 +142,7 @@ let prefix_kind line =
               | Some _ -> `Unit
               | None -> `Other))
       | true -> (
-          match String.is_prefix ~prefix:sourceMappingURL_base64 line with
+          match String.starts_with ~prefix:sourceMappingURL_base64 line with
           | true -> `Json_base64 (String.length sourceMappingURL_base64)
           | false -> `Url (String.length sourceMappingURL)))
 
diff --git a/compiler/lib/linker.ml b/compiler/lib/linker.ml
index f3ce13b689..a94b29e969 100644
--- a/compiler/lib/linker.ml
+++ b/compiler/lib/linker.ml
@@ -101,7 +101,7 @@ module Check = struct
             (fun x acc ->
               match x with
               | S { name = Utf8_string.Utf8 s; _ } ->
-                  if String.is_prefix s ~prefix:"_" || String.equal s name
+                  if String.starts_with s ~prefix:"_" || String.equal s name
                   then acc
                   else s :: acc
               | V _ -> acc)
diff --git a/compiler/lib/magic_number.ml b/compiler/lib/magic_number.ml
index e476410004..a5e11d1f76 100644
--- a/compiler/lib/magic_number.ml
+++ b/compiler/lib/magic_number.ml
@@ -67,11 +67,6 @@ let equal a b = compare a b = 0
 let v =
   let current = Ocaml_version.current in
   match current with
-  | 4 :: 08 :: _ -> 25
-  | 4 :: 09 :: _ -> 26
-  | 4 :: 10 :: _ -> 27
-  | 4 :: 11 :: _ -> 28
-  | 4 :: 12 :: _ -> 29
   | 4 :: 13 :: _ -> 30
   | 4 :: 14 :: _ -> 31
   | 5 :: 00 :: _ -> 32
@@ -79,8 +74,8 @@ let v =
   | 5 :: 02 :: _ -> 34
   | 5 :: 03 :: _ -> 35
   | _ ->
-      if Ocaml_version.compare current [ 4; 8 ] < 0
-      then failwith "OCaml version unsupported. Upgrade to OCaml 4.08 or newer."
+      if Ocaml_version.compare current [ 4; 13 ] < 0
+      then failwith "OCaml version unsupported. Upgrade to OCaml 4.13 or newer."
       else (
         assert (Ocaml_version.compare current [ 5; 4 ] >= 0);
         failwith "OCaml version unsupported. Upgrade js_of_ocaml.")
diff --git a/compiler/lib/ocaml_compiler.ml b/compiler/lib/ocaml_compiler.ml
index f8cd33453e..135bef8fd1 100644
--- a/compiler/lib/ocaml_compiler.ml
+++ b/compiler/lib/ocaml_compiler.ml
@@ -24,8 +24,7 @@ let rec constant_of_const c : Code.constant =
   match c with
   | Const_base (Const_int i) -> Int (Targetint.of_int_warning_on_overflow i)
   | Const_base (Const_char c) -> Int (Targetint.of_int_exn (Char.code c))
-  | ((Const_base (Const_string (s, _))) [@if ocaml_version < (4, 11, 0)])
-  | ((Const_base (Const_string (s, _, _))) [@if ocaml_version >= (4, 11, 0)]) -> String s
+  | Const_base (Const_string (s, _, _)) -> String s
   | Const_base (Const_float s) -> Float (float_of_string s)
   | Const_base (Const_int32 i) -> (
       match Config.target () with
@@ -40,8 +39,6 @@ let rec constant_of_const c : Code.constant =
   | Const_float_array sl ->
       let l = List.map ~f:(fun f -> float_of_string f) sl in
       Float_array (Array.of_list l)
-  | ((Const_pointer i) [@if ocaml_version < (4, 12, 0)]) ->
-      Int (Targetint.of_int_warning_on_overflow i)
   | Const_block (tag, l) ->
       let l = Array.of_list (List.map l ~f:constant_of_const) in
       Tuple (tag, l, Unknown)
diff --git a/compiler/lib/parse_bytecode.ml b/compiler/lib/parse_bytecode.ml
index edb9cd7cc1..ce62238525 100644
--- a/compiler/lib/parse_bytecode.ml
+++ b/compiler/lib/parse_bytecode.ml
@@ -33,8 +33,6 @@ type bytecode = string
 let predefined_exceptions =
   Runtimedef.builtin_exceptions |> Array.to_list |> List.mapi ~f:(fun i name -> i, name)
 
-let new_closure_repr = Ocaml_version.compare Ocaml_version.current [ 4; 12 ] >= 0
-
 (* Read and manipulate debug section *)
 module Debug : sig
   type t
@@ -249,10 +247,7 @@ end = struct
       let { event; _ } = Int_table.find events_by_pc pc in
       let env = event.ev_compenv in
       let names =
-        Ident.fold_name
-          (fun ident i acc -> ((if new_closure_repr then i / 3 else i / 2), ident) :: acc)
-          env.ce_rec
-          []
+        Ident.fold_name (fun ident i acc -> (i / 3, ident) :: acc) env.ce_rec []
       in
       List.sort names ~cmp:(fun (i, _) (j, _) -> compare i j)
     with Not_found -> []
@@ -498,7 +493,6 @@ type globals =
   ; mutable is_const : bool array
   ; mutable is_exported : bool array
   ; mutable named_value : string option array
-  ; mutable override : (Var.t -> Code.instr list -> Var.t * Code.instr list) option array
   ; constants : Code.constant array
   ; primitives : string array
   }
@@ -508,7 +502,6 @@ let make_globals size constants primitives =
   ; is_const = Array.make size false
   ; is_exported = Array.make size false
   ; named_value = Array.make size None
-  ; override = Array.make size None
   ; constants
   ; primitives
   }
@@ -522,8 +515,7 @@ let resize_globals g size =
   g.vars <- resize_array g.vars size None;
   g.is_const <- resize_array g.is_const size false;
   g.is_exported <- resize_array g.is_exported size true;
-  g.named_value <- resize_array g.named_value size None;
-  g.override <- resize_array g.override size None
+  g.named_value <- resize_array g.named_value size None
 
 (* State of the VM *)
 module State = struct
@@ -788,7 +780,7 @@ let compiled_blocks : (_ * instr list * last) Addr.Map.t ref = ref Addr.Map.empt
 
 let method_cache_id = ref 1
 
-let clo_offset_3 = if new_closure_repr then 3 else 2
+let clo_offset_3 = 3
 
 type compile_info =
   { blocks : Blocks.t
@@ -1194,7 +1186,7 @@ and compile infos pc state (instrs : instr list) =
         let env =
           let code = State.Dummy "closure(code)" in
           let closure_info = State.Dummy "closure(info)" in
-          if new_closure_repr then code :: closure_info :: env else code :: env
+          code :: closure_info :: env
         in
         let env = Array.of_list env in
         if debug_parser () then Format.printf "fun %a (" Var.print x;
@@ -1241,9 +1233,7 @@ and compile infos pc state (instrs : instr list) =
         List.iter !vars ~f:(fun (i, x) ->
             let code = State.Var x in
             let closure_info = State.Dummy "closurerec(info)" in
-            if new_closure_repr
-            then env := code :: closure_info :: !env
-            else env := code :: !env;
+            env := code :: closure_info :: !env;
             if i > 0
             then
               let infix_tag = State.Dummy "closurerec(infix_tag)" in
@@ -1325,16 +1315,7 @@ and compile infos pc state (instrs : instr list) =
 
         assert (Option.is_none g.vars.(i));
         if debug_parser () then Format.printf "(global %d) = %a@." i Var.print y;
-        let instrs =
-          match g.override.(i) with
-          | Some f ->
-              let v, instrs = f y instrs in
-              g.vars.(i) <- Some v;
-              instrs
-          | None ->
-              g.vars.(i) <- Some y;
-              instrs
-        in
+        g.vars.(i) <- Some y;
         let x, state = State.fresh_var state in
         if debug_parser () then Format.printf "%a = 0@." Var.print x;
         let instrs = register_global g i instrs in
@@ -2477,26 +2458,6 @@ let parse_bytecode code globals debug_data =
   tagged_blocks := Addr.Map.empty;
   p
 
-(* HACK - override module *)
-
-let override_global =
-  match Ocaml_version.compare Ocaml_version.current [ 4; 13 ] >= 0 with
-  | true -> []
-  | false ->
-      [ ( "CamlinternalMod"
-        , fun _orig instrs ->
-            let x = Var.fresh_n "internalMod" in
-            let init_mod = Var.fresh_n "init_mod" in
-            let update_mod = Var.fresh_n "update_mod" in
-            ( x
-            , Let (x, Block (0, [| init_mod; update_mod |], NotArray, Immutable))
-              :: Let (init_mod, Special (Alias_prim "caml_CamlinternalMod_init_mod"))
-              :: Let (update_mod, Special (Alias_prim "caml_CamlinternalMod_update_mod"))
-              :: instrs ) )
-      ]
-
-(* HACK END *)
-
 module Toc : sig
   type t
 
@@ -2569,7 +2530,7 @@ end
 let read_primitives toc ic =
   let prim = Toc.read_prim toc ic in
   assert (Char.equal (String.get prim (String.length prim - 1)) '\000');
-  String.split_char ~sep:'\000' (String.sub prim ~pos:0 ~len:(String.length prim - 1))
+  String.split_on_char ~sep:'\000' (String.sub prim ~pos:0 ~len:(String.length prim - 1))
 
 type bytesections =
   { symb : Ocaml_compiler.Symtable.GlobalMap.t
@@ -2634,14 +2595,6 @@ let from_exe
   if times () then Format.eprintf "    read debug events: %a@." Timer.print t;
 
   let globals = make_globals (Array.length init_data) init_data primitive_table in
-  (* Initialize module override mechanism *)
-  List.iter override_global ~f:(fun (name, v) ->
-      try
-        let nn = Ocaml_compiler.Symtable.Global.Glob_compunit name in
-        let i = Ocaml_compiler.Symtable.GlobalMap.find nn orig_symbols in
-        globals.override.(i) <- Some v;
-        if debug_parser () then Format.eprintf "overriding global %s@." name
-      with Not_found -> ());
   if linkall
   then
     (* export globals *)
@@ -2900,13 +2853,6 @@ module Reloc = struct
     let globals = make_globals (Array.length constants) constants primitives in
     resize_globals globals t.pos;
     Hashtbl.iter (fun name i -> globals.named_value.(i) <- Some name) t.names;
-    (* Initialize module override mechanism *)
-    List.iter override_global ~f:(fun (name, v) ->
-        try
-          let i = Hashtbl.find t.names name in
-          globals.override.(i) <- Some v;
-          if debug_parser () then Format.eprintf "overriding global %s@." name
-        with Not_found -> ());
     globals
 end
 
diff --git a/compiler/lib/pseudo_fs.ml b/compiler/lib/pseudo_fs.ml
index b5e4a453e3..f45b0e68d4 100644
--- a/compiler/lib/pseudo_fs.ml
+++ b/compiler/lib/pseudo_fs.ml
@@ -61,7 +61,7 @@ let list_files name paths =
   in
   let name, exts (* extensions filter *) =
     match String.lsplit2 name ~on:'=' with
-    | Some (name, exts) -> name, String.split_char ~sep:',' exts
+    | Some (name, exts) -> name, String.split_on_char ~sep:',' exts
     | None -> name, []
   in
   let file =
diff --git a/compiler/lib/stdlib.ml b/compiler/lib/stdlib.ml
index e528cc8e3f..2906321bd2 100644
--- a/compiler/lib/stdlib.ml
+++ b/compiler/lib/stdlib.ml
@@ -30,6 +30,8 @@ end = struct
   let open_out = open_out
 end
 
+include Deprecated
+
 module Poly = struct
   external ( < ) : 'a -> 'a -> bool = "%lessthan"
 
@@ -99,31 +101,9 @@ let failwith_ fmt =
 let raise_ exn =
   if !fail then raise exn else Format.eprintf "%s@." (Printexc.to_string exn)
 
-let int_num_bits = Sys.int_size
-
 module List = struct
   include ListLabels
 
-  let rec equal ~eq a b =
-    match a, b with
-    | [], [] -> true
-    | x :: xs, y :: ys -> eq x y && equal ~eq xs ys
-    | [], _ :: _ | _ :: _, [] -> false
-
-  let rec find_map ~f = function
-    | [] -> None
-    | x :: l -> (
-        match f x with
-        | Some _ as result -> result
-        | None -> find_map ~f l)
-
-  let rec find_map_value ~f ~default = function
-    | [] -> default
-    | x :: l -> (
-        match f x with
-        | Some result -> result
-        | None -> find_map_value ~f ~default l)
-
   let rec rev_append_map ~f l acc =
     match l with
     | [] -> acc
@@ -196,34 +176,12 @@ module List = struct
     | [ x ] -> Some x
     | _ :: xs -> last xs
 
-  let sort_uniq ~compare l =
-    let l = List.sort compare l in
-    match l with
-    | ([] | [ _ ]) as l -> l
-    | x :: xs ->
-        let rec loop prev = function
-          | [] -> [ prev ]
-          | x :: rest when compare x prev = 0 -> loop prev rest
-          | x :: rest -> prev :: loop x rest
-        in
-        loop x xs
-
   let is_empty = function
     | [] -> true
     | _ -> false
+  [@@if ocaml_version < (5, 1, 0)]
 
-  let partition_map t ~f =
-    let rec loop t fst snd =
-      match t with
-      | [] -> rev fst, rev snd
-      | x :: t -> (
-          match f x with
-          | `Fst y -> loop t (y :: fst) snd
-          | `Snd y -> loop t fst (y :: snd))
-    in
-    loop t [] []
-
-  let tail_append l1 l2 = rev_append (rev l1) l2
+  let tail_append l1 l2 = rev_append (rev l1) l2 [@@if ocaml_version < (5, 1, 0)]
 
   let rec count_append l1 l2 count =
     match l2 with
@@ -245,8 +203,9 @@ module List = struct
             (if count > max_non_tailcall
              then tail_append tl l2
              else count_append tl l2 (count + 1)))
+  [@@if ocaml_version < (5, 1, 0)]
 
-  let append l1 l2 = count_append l1 l2 0
+  let append l1 l2 = count_append l1 l2 0 [@@if ocaml_version < (5, 1, 0)]
 
   let group l ~f =
     let rec loop (l : 'a list) (this_group : 'a list) (acc : 'a list list) : 'a list list
@@ -263,15 +222,6 @@ module List = struct
     | [] -> []
     | x :: xs -> loop xs [ x ] []
 
-  let concat_map ~f l =
-    let rec aux f acc = function
-      | [] -> rev acc
-      | x :: l ->
-          let xs = f x in
-          aux f (rev_append xs acc) l
-    in
-    aux f [] l
-
   let split_last xs =
     let rec aux acc = function
       | [] -> None
@@ -303,12 +253,6 @@ end
 
 let ( @ ) = List.append
 
-module Nativeint = struct
-  include Nativeint
-
-  external equal : nativeint -> nativeint -> bool = "%equal"
-end
-
 module Int32 = struct
   include Int32
 
@@ -324,10 +268,6 @@ module Int32 = struct
 
   external ( >= ) : int32 -> int32 -> bool = "%greaterequal"
 
-  external compare : int32 -> int32 -> int = "%compare"
-
-  external equal : int32 -> int32 -> bool = "%equal"
-
   let warn_overflow name ~to_dec ~to_hex i i32 =
     warn
       "Warning: integer overflow: %s 0x%s (%s) truncated to 0x%lx (%ld); the generated \
@@ -356,15 +296,13 @@ module Int32 = struct
 end
 
 module Option = struct
+  include Option
+
   let map ~f x =
     match x with
     | None -> None
     | Some v -> Some (f v)
 
-  let to_list = function
-    | None -> []
-    | Some x -> [ x ]
-
   let bind ~f x =
     match x with
     | None -> None
@@ -380,40 +318,13 @@ module Option = struct
     | None -> None
     | Some v -> if f v then Some v else None
 
-  let compare compare_elt a b =
-    match a, b with
-    | None, None -> 0
-    | None, Some _ -> -1
-    | Some _, None -> 1
-    | Some a, Some b -> compare_elt a b
-
-  let equal equal_elt a b =
-    match a, b with
-    | None, None -> true
-    | Some a, Some b -> equal_elt a b
-    | Some _, None | None, Some _ -> false
-
-  let is_none = function
-    | None -> true
-    | Some _ -> false
-
-  let is_some = function
-    | None -> false
-    | Some _ -> true
-
   let value ~default = function
     | None -> default
     | Some s -> s
 end
 
-module Int64 = struct
-  include Int64
-
-  let equal (a : int64) (b : int64) = Poly.(a = b)
-end
-
 module Float = struct
-  type t = float
+  include Float
 
   let equal (_ : float) (_ : float) = `Use_ieee_equal_or_bitwise_equal
 
@@ -422,9 +333,6 @@ module Float = struct
   let bitwise_equal (a : float) (b : float) =
     Int64.equal (Int64.bits_of_float a) (Int64.bits_of_float b)
 
-  (* Re-defined here to stay compatible with OCaml 4.02 *)
-  external classify_float : float -> fpclass = "caml_classify_float"
-
   external ( < ) : t -> t -> bool = "%lessthan"
 
   external ( <= ) : t -> t -> bool = "%lessequal"
@@ -439,13 +347,13 @@ module Float = struct
 end
 
 module Bool = struct
+  include Bool
+
   external ( <> ) : bool -> bool -> bool = "%notequal"
 
   external ( = ) : bool -> bool -> bool = "%equal"
 
   external ( > ) : bool -> bool -> bool = "%greaterthan"
-
-  external equal : bool -> bool -> bool = "%equal"
 end
 
 module Char = struct
@@ -463,27 +371,13 @@ module Char = struct
 
   external ( >= ) : char -> char -> bool = "%greaterequal"
 
-  external compare : char -> char -> int = "%compare"
-
-  external equal : char -> char -> bool = "%equal"
-
-  let is_alpha = function
+  let is_letter = function
     | 'a' .. 'z' | 'A' .. 'Z' -> true
     | _ -> false
 
-  let is_num = function
+  let is_digit = function
     | '0' .. '9' -> true
     | _ -> false
-
-  let lowercase_ascii c =
-    match c with
-    | 'A' .. 'Z' as c -> Char.unsafe_chr (Char.code c + 32)
-    | _ -> c
-
-  let uppercase_ascii c =
-    match c with
-    | 'a' .. 'z' as c -> Char.unsafe_chr (Char.code c - 32)
-    | _ -> c
 end
 
 module Uchar = struct
@@ -585,73 +479,17 @@ module Buffer = struct
     Buffer.add_char b (Array.unsafe_get array_conv (c land 0xf))
 end
 
-module Bytes = struct
-  include BytesLabels
-
-  let sub_string b ~pos:ofs ~len = unsafe_to_string (Bytes.sub b ofs len)
-
-  let fold_left ~f ~init b =
-    let r = ref init in
-    for i = 0 to length b - 1 do
-      r := f !r (unsafe_get b i)
-    done;
-    !r
-
-  let fold_right ~f b ~init =
-    let r = ref init in
-    for i = length b - 1 downto 0 do
-      r := f (unsafe_get b i) !r
-    done;
-    !r
-end
+module Bytes = BytesLabels
 
 module String = struct
   include StringLabels
 
-  let equal (a : string) (b : string) = Poly.(a = b)
-
   let hash (a : string) = Hashtbl.hash a
 
   let is_empty = function
     | "" -> true
     | _ -> false
 
-  let is_prefix ~prefix s =
-    let len_a = length prefix in
-    let len_s = length s in
-    if len_a > len_s
-    then false
-    else
-      let max_idx_a = len_a - 1 in
-      let rec loop i =
-        if i > max_idx_a
-        then true
-        else if not (Char.equal (unsafe_get prefix i) (unsafe_get s i))
-        then false
-        else loop (i + 1)
-      in
-      loop 0
-
-  let is_suffix ~suffix s =
-    let len_a = length suffix in
-    let len_s = length s in
-    if len_a > len_s
-    then false
-    else
-      let max_idx_a = len_a - 1 in
-      let rec loop i =
-        if i > max_idx_a
-        then true
-        else if
-          not
-            (Char.equal
-               (unsafe_get suffix (len_a - 1 - i))
-               (unsafe_get s (len_s - 1 - i)))
-        then false
-        else loop (i + 1)
-      in
-      loop 0
-
   let drop_prefix ~prefix s =
     let plen = String.length prefix in
     if plen > String.length s
@@ -664,16 +502,6 @@ module String = struct
         Some (String.sub s plen (String.length s - plen))
       with Exit -> None
 
-  let for_all =
-    let rec loop s ~f ~last i =
-      if i > last
-      then true
-      else if f (String.unsafe_get s i)
-      then loop s ~f ~last (i + 1)
-      else false
-    in
-    fun s ~f -> loop s ~f ~last:(String.length s - 1) 0
-
   let is_ascii s =
     let res = ref true in
     for i = 0 to String.length s - 1 do
@@ -690,69 +518,6 @@ module String = struct
     done;
     !res
 
-  let split_char ~sep p = String.split_on_char sep p
-
-  (* copied from https://github.com/ocaml/ocaml/pull/10 *)
-  let split ~sep s =
-    let sep_len = String.length sep in
-    if sep_len = 1
-    then split_char ~sep:sep.[0] s
-    else
-      let sep_max = sep_len - 1 in
-      if sep_max < 0
-      then invalid_arg "String.split: empty separator"
-      else
-        let s_max = String.length s - 1 in
-        if s_max < 0
-        then [ "" ]
-        else
-          let acc = ref [] in
-          let sub_start = ref 0 in
-          let k = ref 0 in
-          let i = ref 0 in
-          (* We build the substrings by running from the start of [s] to the
-             end with [i] trying to match the first character of [sep] in
-             [s]. If this matches, we verify that the whole [sep] is matched
-             using [k]. If this matches we extract a substring from the start
-             of the current substring [sub_start] to [!i - 1] (the position
-             before the [sep] we found).  We then continue to try to match
-             with [i] by starting after the [sep] we just found, this is also
-             becomes the start position of the next substring. If [i] is such
-             that no separator can be found we exit the loop and make a
-             substring from [sub_start] until the end of the string. *)
-          while !i + sep_max <= s_max do
-            if not (Char.equal (String.unsafe_get s !i) (String.unsafe_get sep 0))
-            then incr i
-            else (
-              (* Check remaining [sep] chars match, access to unsafe s (!i + !k) is
-                   guaranteed by loop invariant. *)
-              k := 1;
-              while
-                !k <= sep_max
-                && Char.equal (String.unsafe_get s (!i + !k)) (String.unsafe_get sep !k)
-              do
-                incr k
-              done;
-              if !k <= sep_max
-              then (* no match *) incr i
-              else
-                let new_sub_start = !i + sep_max + 1 in
-                let sub_end = !i - 1 in
-                let sub_len = sub_end - !sub_start + 1 in
-                acc := String.sub s !sub_start sub_len :: !acc;
-                sub_start := new_sub_start;
-                i := new_sub_start)
-          done;
-          List.rev (String.sub s !sub_start (s_max - !sub_start + 1) :: !acc)
-
-  let apply1 f (s : string) : string =
-    let b = Bytes.of_string s in
-    if Bytes.length b = 0
-    then s
-    else (
-      Bytes.unsafe_set b 0 (f (Bytes.unsafe_get b 0));
-      Bytes.to_string b)
-
   let lsplit2 line ~on:delim =
     try
       let pos = index line delim in
@@ -765,10 +530,6 @@ module String = struct
       Some (sub line ~pos:0 ~len:pos, sub line ~pos:(pos + 1) ~len:(length line - pos - 1))
     with Not_found -> None
 
-  let capitalize_ascii s = apply1 Char.uppercase_ascii s
-
-  let uncapitalize_ascii s = apply1 Char.lowercase_ascii s
-
   let[@inline] not_in_x80_to_xBF b = b lsr 6 <> 0b10
 
   let[@inline] not_in_xA0_to_xBF b = b lsr 5 <> 0b101
@@ -1025,20 +786,6 @@ module String = struct
         | _ -> false
     in
     loop (length b - 1) b 0
-
-  let fold_left ~f ~init s =
-    let r = ref init in
-    for i = 0 to length s - 1 do
-      r := f !r (unsafe_get s i)
-    done;
-    !r
-
-  let fold_right ~f s ~init =
-    let r = ref init in
-    for i = length s - 1 downto 0 do
-      r := f (unsafe_get s i) !r
-    done;
-    !r
 end
 
 module Utf8_string : sig
@@ -1063,11 +810,7 @@ end = struct
 end
 
 module Int = struct
-  type t = int
-
-  let compare (x : int) y = compare x y
-
-  let equal (x : t) y = x = y
+  include Int
 
   let hash (x : t) = Hashtbl.hash x
 end
@@ -1106,14 +849,14 @@ end = struct
 
   let create () = { arr = Array.make 1 0 }
 
-  let create' n = { arr = Array.make ((n / int_num_bits) + 1) 0 }
+  let create' n = { arr = Array.make ((n / Sys.int_size) + 1) 0 }
 
-  let size t = Array.length t.arr * int_num_bits
+  let size t = Array.length t.arr * Sys.int_size
 
   let mem t i =
     let arr = t.arr in
-    let idx = i / int_num_bits in
-    let off = i mod int_num_bits in
+    let idx = i / Sys.int_size in
+    let off = i mod Sys.int_size in
     idx < Array.length arr
     &&
     let x = Array.unsafe_get arr idx in
@@ -1130,15 +873,15 @@ end = struct
     t.arr <- a
 
   let set t i =
-    let idx = i / int_num_bits in
-    let off = i mod int_num_bits in
+    let idx = i / Sys.int_size in
+    let off = i mod Sys.int_size in
     let size = Array.length t.arr in
     if idx >= size then resize t idx;
     Array.unsafe_set t.arr idx (Array.unsafe_get t.arr idx lor (1 lsl off))
 
   let unset t i =
-    let idx = i / int_num_bits in
-    let off = i mod int_num_bits in
+    let idx = i / Sys.int_size in
+    let off = i mod Sys.int_size in
     let size = Array.length t.arr in
     if idx >= size
     then ()
@@ -1172,17 +915,6 @@ end
 module Array = struct
   include ArrayLabels
 
-  let find_opt ~f:p a =
-    let n = length a in
-    let rec loop i =
-      if i = n
-      then None
-      else
-        let x = unsafe_get a i in
-        if p x then Some x else loop (succ i)
-    in
-    loop 0
-
   let fold_right_i a ~f ~init:x =
     let r = ref x in
     for i = Array.length a - 1 downto 0 do
@@ -1405,4 +1137,4 @@ let file_lines_text file =
 
 let generated_name = function
   | "param" | "match" | "switcher" -> true
-  | s -> String.is_prefix ~prefix:"cst_" s
+  | s -> String.starts_with ~prefix:"cst_" s
diff --git a/compiler/lib/var_printer.ml b/compiler/lib/var_printer.ml
index 7cca64e730..b075803e37 100644
--- a/compiler/lib/var_printer.ml
+++ b/compiler/lib/var_printer.ml
@@ -93,7 +93,7 @@ let name t v nm_orig =
   then (
     let buf = Buffer.create (String.length nm_orig) in
     let idx = ref 0 in
-    while !idx < len && not (Char.is_alpha nm_orig.[!idx]) do
+    while !idx < len && not (Char.is_letter nm_orig.[!idx]) do
       incr idx
     done;
     let pending = ref false in
@@ -102,7 +102,7 @@ let name t v nm_orig =
       pending := true;
       idx := 0);
     for i = !idx to len - 1 do
-      if Char.is_alpha nm_orig.[i] || Char.is_num nm_orig.[i]
+      if Char.is_letter nm_orig.[i] || Char.is_digit nm_orig.[i]
       then (
         if !pending then Buffer.add_char buf '_';
         Buffer.add_char buf nm_orig.[i];
diff --git a/compiler/tests-check-prim/gen_dune.ml b/compiler/tests-check-prim/gen_dune.ml
index 64c2868ac9..401df00106 100644
--- a/compiler/tests-check-prim/gen_dune.ml
+++ b/compiler/tests-check-prim/gen_dune.ml
@@ -1,12 +1,7 @@
 (** *)
 
 type version =
-  [ `V4_08
-  | `V4_09
-  | `V4_10
-  | `V4_11
-  | `V4_12
-  | `V4_13
+  [ `V4_13
   | `V4_14
   | `V5_0
   | `V5_1
@@ -16,11 +11,6 @@ type version =
   ]
 
 let string_of_version : version -> string = function
-  | `V4_08 -> "4.08"
-  | `V4_09 -> "4.09"
-  | `V4_10 -> "4.10"
-  | `V4_11 -> "4.11"
-  | `V4_12 -> "4.12"
   | `V4_13 -> "4.13"
   | `V4_14 -> "4.14"
   | `V5_0 -> "5.0"
@@ -30,11 +20,6 @@ let string_of_version : version -> string = function
   | `V5_4 -> "5.4"
 
 let next_version : version -> version option = function
-  | `V4_08 -> Some `V4_09
-  | `V4_09 -> Some `V4_10
-  | `V4_10 -> Some `V4_11
-  | `V4_11 -> Some `V4_12
-  | `V4_12 -> Some `V4_13
   | `V4_13 -> Some `V4_14
   | `V4_14 -> Some `V5_0
   | `V5_0 -> Some `V5_1
diff --git a/compiler/tests-compiler/util/util.ml b/compiler/tests-compiler/util/util.ml
index 8b53796ecb..1e49e43d4d 100644
--- a/compiler/tests-compiler/util/util.ml
+++ b/compiler/tests-compiler/util/util.ml
@@ -183,7 +183,7 @@ let exec_to_string_exn ?input ~fail ~cmd () =
     let prev, env =
       Unix.environment ()
       |> Array.to_list
-      |> List.partition ~f:(String.is_prefix ~prefix:build_path_prefix_map)
+      |> List.partition ~f:(String.starts_with ~prefix:build_path_prefix_map)
     in
     let prev =
       List.filter_map ~f:(String.drop_prefix ~prefix:build_path_prefix_map) prev
diff --git a/compiler/tests-js-parser/run.ml b/compiler/tests-js-parser/run.ml
index 2acda654b8..37f07d4252 100644
--- a/compiler/tests-js-parser/run.ml
+++ b/compiler/tests-js-parser/run.ml
@@ -66,7 +66,7 @@ let normalize_string s =
   for i = 0 to l - 1 do
     let c = s.[i] in
     match c with
-    | '\000' when i = l - 1 || not (Char.is_num s.[i + 1]) -> Buffer.add_string b "\\0"
+    | '\000' when i = l - 1 || not (Char.is_digit s.[i + 1]) -> Buffer.add_string b "\\0"
     | '\b' -> Buffer.add_string b "\\b"
     | '\t' -> Buffer.add_string b "\\t"
     | '\n' -> Buffer.add_string b "\\n"
diff --git a/compiler/tests-jsoo/test_rec_mod.ml b/compiler/tests-jsoo/test_rec_mod.ml
index 950b98d91d..f68253d623 100644
--- a/compiler/tests-jsoo/test_rec_mod.ml
+++ b/compiler/tests-jsoo/test_rec_mod.ml
@@ -38,7 +38,7 @@ let%expect_test _ =
     ignore (IdSet.mem { id = 1 } basic_set : bool)
     (* diverge here *)
   with e ->
-    if String.is_suffix ~suffix:"Undefined recursive module" (Printexc.to_string e)
+    if String.ends_with ~suffix:"Undefined recursive module" (Printexc.to_string e)
     then ()
     else raise e
 
diff --git a/compiler/tests-ocaml/lib-sys/dune b/compiler/tests-ocaml/lib-sys/dune
index 9c8fdcd68b..a71af2d55c 100644
--- a/compiler/tests-ocaml/lib-sys/dune
+++ b/compiler/tests-ocaml/lib-sys/dune
@@ -5,7 +5,5 @@
 
 (tests
  (names immediate64)
- (build_if
-  (>= %{ocaml_version} 4.10))
  (libraries ocaml_testing)
  (modes js wasm))
diff --git a/dune-project b/dune-project
index b694b130a9..dcb969ad65 100644
--- a/dune-project
+++ b/dune-project
@@ -18,7 +18,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (and (>= 4.08) (< 5.4)))
+  (ocaml (and (>= 4.13) (< 5.4)))
   (num :with-test)
   (ppx_expect (and (>= v0.16.1) :with-test))
   (ppxlib (>= 0.15))
@@ -43,7 +43,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml (= :version))
   (js_of_ocaml-ppx (= :version))
   (lwt (>= 2.4.4))
@@ -62,7 +62,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml (= :version))
   (ppxlib (>= 0.15))
   (num :with-test)
@@ -76,7 +76,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml (= :version))
   (ppxlib (>= 0.15))
   (ppxlib (and (< 0.36) :with-test))
@@ -91,7 +91,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml-compiler (= :version))
   (ocamlfind (>= 1.5.1))
 ;;(cohttp-lwt-unix (and (>= 6.0.0) :with-test))
@@ -108,7 +108,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml (= :version))
   (js_of_ocaml-ppx (= :version))
   (react (>= 1.2.2))
@@ -126,7 +126,7 @@
  (description
   "Js_of_ocaml is a compiler from OCaml bytecode to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js")
  (depends
-  (ocaml (>= 4.08))
+  (ocaml (>= 4.13))
   (js_of_ocaml-compiler (= :version))
   (ppxlib (>= 0.15))
   (num :with-test)
diff --git a/dune-workspace.dev b/dune-workspace.dev
index 5b5373a125..08c5d13913 100644
--- a/dune-workspace.dev
+++ b/dune-workspace.dev
@@ -7,26 +7,6 @@
 ;;
 ;; This will build js_of_ocaml against all these version of OCaml
 
-(context
- (opam
-  (switch 4.08.1)))
-
-(context
- (opam
-  (switch 4.09.1)))
-
-(context
- (opam
-  (switch 4.10.2)))
-
-(context
- (opam
-  (switch 4.11.2)))
-
-(context
- (opam
-  (switch 4.12.1)))
-
 (context
  (opam
   (switch 4.13.1)))
diff --git a/js_of_ocaml-compiler.opam b/js_of_ocaml-compiler.opam
index 2ad2f90347..e7f30cd610 100644
--- a/js_of_ocaml-compiler.opam
+++ b/js_of_ocaml-compiler.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08" & < "5.4"}
+  "ocaml" {>= "4.13" & < "5.4"}
   "num" {with-test}
   "ppx_expect" {>= "v0.16.1" & with-test}
   "ppxlib" {>= "0.15"}
diff --git a/js_of_ocaml-lwt.opam b/js_of_ocaml-lwt.opam
index eb96726b78..b8f1f4fe92 100644
--- a/js_of_ocaml-lwt.opam
+++ b/js_of_ocaml-lwt.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml" {= version}
   "js_of_ocaml-ppx" {= version}
   "lwt" {>= "2.4.4"}
diff --git a/js_of_ocaml-ppx.opam b/js_of_ocaml-ppx.opam
index f5f059c6f2..df83005beb 100644
--- a/js_of_ocaml-ppx.opam
+++ b/js_of_ocaml-ppx.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml" {= version}
   "ppxlib" {>= "0.15"}
   "ppxlib" {< "0.36" & with-test}
diff --git a/js_of_ocaml-ppx_deriving_json.opam b/js_of_ocaml-ppx_deriving_json.opam
index 731bb76b45..c0c5ade8f7 100644
--- a/js_of_ocaml-ppx_deriving_json.opam
+++ b/js_of_ocaml-ppx_deriving_json.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml" {= version}
   "ppxlib" {>= "0.15"}
   "num" {with-test}
diff --git a/js_of_ocaml-toplevel.opam b/js_of_ocaml-toplevel.opam
index 08bfdc652f..931dab70f5 100644
--- a/js_of_ocaml-toplevel.opam
+++ b/js_of_ocaml-toplevel.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml-compiler" {= version}
   "ocamlfind" {>= "1.5.1"}
   "graphics" {with-test}
diff --git a/js_of_ocaml-tyxml.opam b/js_of_ocaml-tyxml.opam
index adc5995ffc..01c5a7fcdb 100644
--- a/js_of_ocaml-tyxml.opam
+++ b/js_of_ocaml-tyxml.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml" {= version}
   "js_of_ocaml-ppx" {= version}
   "react" {>= "1.2.2"}
diff --git a/js_of_ocaml.opam b/js_of_ocaml.opam
index 6de6024413..5326aa555d 100644
--- a/js_of_ocaml.opam
+++ b/js_of_ocaml.opam
@@ -13,7 +13,7 @@ doc: "https://ocsigen.org/js_of_ocaml/latest/manual/overview"
 bug-reports: "https://github.com/ocsigen/js_of_ocaml/issues"
 depends: [
   "dune" {>= "3.17"}
-  "ocaml" {>= "4.08"}
+  "ocaml" {>= "4.13"}
   "js_of_ocaml-compiler" {= version}
   "ppxlib" {>= "0.15"}
   "num" {with-test}
diff --git a/runtime/js/bigarray.js b/runtime/js/bigarray.js
index f60f07ec68..adf904d8b1 100644
--- a/runtime/js/bigarray.js
+++ b/runtime/js/bigarray.js
@@ -247,11 +247,6 @@ function caml_ba_create_buffer(kind, size) {
 }
 
 //Provides: caml_ba_custom_name
-//Version: < 4.11
-var caml_ba_custom_name = "_bigarray";
-
-//Provides: caml_ba_custom_name
-//Version: >= 4.11
 var caml_ba_custom_name = "_bigarr02";
 
 //Provides: Ml_Bigarray
diff --git a/runtime/js/dune b/runtime/js/dune
index f31ffd1b6a..17b7db4117 100644
--- a/runtime/js/dune
+++ b/runtime/js/dune
@@ -12,7 +12,6 @@
   graphics.js
   ieee_754.js
   int64.js
-  internalMod.js
   io.js
   jslib.js
   jslib_js_of_ocaml.js
diff --git a/runtime/js/dynlink.js b/runtime/js/dynlink.js
index 8ffa521996..70f9473317 100644
--- a/runtime/js/dynlink.js
+++ b/runtime/js/dynlink.js
@@ -71,21 +71,3 @@ function caml_dynlink_get_current_libs() {
   for (var i = 0; i < len; i++) a[i] = i;
   return a;
 }
-
-//Provides: caml_register_code_fragment
-//Version: < 4.10
-function caml_register_code_fragment(code, codesize, digest) {
-  return 0;
-}
-
-//Provides: caml_add_debug_info
-//Version: < 4.13
-function caml_add_debug_info(code, size, events) {
-  return 0;
-}
-
-//Provides: caml_remove_debug_info
-//Version: < 4.13
-function caml_remove_debug_info(code) {
-  return 0;
-}
diff --git a/runtime/js/gc.js b/runtime/js/gc.js
index 3692f89970..a3883508c5 100644
--- a/runtime/js/gc.js
+++ b/runtime/js/gc.js
@@ -25,16 +25,10 @@ function caml_gc_counters() {
   return [254, 0, 0, 0];
 }
 //Provides: caml_gc_quick_stat
-//Version: >= 4.12
 function caml_gc_quick_stat() {
   return [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
 }
 
-//Provides: caml_gc_quick_stat
-//Version: < 4.12
-function caml_gc_quick_stat() {
-  return [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
-}
 //Provides: caml_gc_stat
 //Requires: caml_gc_quick_stat
 function caml_gc_stat() {
@@ -51,12 +45,6 @@ function caml_gc_get() {
   return [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
 }
 
-//Provides: caml_memprof_set
-//Version: = 4.10
-function caml_memprof_set(_control) {
-  return 0;
-}
-
 //Provides: caml_final_register const
 function caml_final_register() {
   return 0;
diff --git a/runtime/js/hash.js b/runtime/js/hash.js
index a1f499adf8..dcd9f9825f 100644
--- a/runtime/js/hash.js
+++ b/runtime/js/hash.js
@@ -17,75 +17,6 @@
 
 ///////////// Hashtbl
 
-//Provides: caml_hash_univ_param mutable
-//Requires: caml_is_ml_string, caml_is_ml_bytes
-//Requires: caml_ml_bytes_content
-//Requires: caml_int64_to_bytes, caml_int64_bits_of_float, caml_custom_ops
-//Requires: caml_ml_bytes_length, caml_jsbytes_of_string
-//Version: < 4.12
-function caml_hash_univ_param(count, limit, obj) {
-  var hash_accu = 0;
-  function hash_aux(obj) {
-    limit--;
-    if (count < 0 || limit < 0) return;
-    if (Array.isArray(obj) && obj[0] === (obj[0] | 0)) {
-      switch (obj[0]) {
-        case 248:
-          // Object
-          count--;
-          hash_accu = (hash_accu * 65599 + obj[2]) | 0;
-          break;
-        case 250:
-          // Forward
-          limit++;
-          hash_aux(obj);
-          break;
-        default:
-          count--;
-          hash_accu = (hash_accu * 19 + obj[0]) | 0;
-          for (var i = obj.length - 1; i > 0; i--) hash_aux(obj[i]);
-      }
-    } else if (caml_is_ml_bytes(obj)) {
-      count--;
-      var content = caml_ml_bytes_content(obj);
-      if (typeof content === "string") {
-        for (var b = content, l = b.length, i = 0; i < l; i++)
-          hash_accu = (hash_accu * 19 + b.charCodeAt(i)) | 0;
-      } else {
-        /* ARRAY */
-        for (var a = content, l = a.length, i = 0; i < l; i++)
-          hash_accu = (hash_accu * 19 + a[i]) | 0;
-      }
-    } else if (caml_is_ml_string(obj)) {
-      var jsbytes = caml_jsbytes_of_string(obj);
-      for (var b = jsbytes, l = jsbytes.length, i = 0; i < l; i++)
-        hash_accu = (hash_accu * 19 + b.charCodeAt(i)) | 0;
-    } else if (typeof obj === "string") {
-      for (var b = obj, l = obj.length, i = 0; i < l; i++)
-        hash_accu = (hash_accu * 19 + b.charCodeAt(i)) | 0;
-    } else if (obj === (obj | 0)) {
-      // Integer
-      count--;
-      hash_accu = (hash_accu * 65599 + obj) | 0;
-    } else if (obj === +obj) {
-      // Float
-      count--;
-      var p = caml_int64_to_bytes(caml_int64_bits_of_float(obj));
-      for (var i = 7; i >= 0; i--) hash_accu = (hash_accu * 19 + p[i]) | 0;
-    } else if (obj?.caml_custom) {
-      if (
-        caml_custom_ops[obj.caml_custom] &&
-        caml_custom_ops[obj.caml_custom].hash
-      ) {
-        var h = caml_custom_ops[obj.caml_custom].hash(obj) | 0;
-        hash_accu = (hash_accu * 65599 + h) | 0;
-      }
-    }
-  }
-  hash_aux(obj);
-  return hash_accu & 0x3fffffff;
-}
-
 //function ROTL32(x,n) { return ((x << n) | (x >>> (32-n))); }
 //Provides: caml_hash_mix_int
 //Requires: caml_mul
diff --git a/runtime/js/internalMod.js b/runtime/js/internalMod.js
deleted file mode 100644
index bf09932850..0000000000
--- a/runtime/js/internalMod.js
+++ /dev/null
@@ -1,139 +0,0 @@
-// Js_of_ocaml runtime support
-// http://www.ocsigen.org/js_of_ocaml/
-// Copyright (C) 2014 Jérôme Vouillon, Hugo Heuzard
-// Laboratoire PPS - CNRS Université Paris Diderot
-//
-// This program is free software; you can redistribute it and/or modify
-// it under the terms of the GNU Lesser General Public License as published by
-// the Free Software Foundation, with linking exception;
-// either version 2.1 of the License, or (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU Lesser General Public License for more details.
-//
-// You should have received a copy of the GNU Lesser General Public License
-// along with this program; if not, write to the Free Software
-// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
-
-//Provides: caml_CamlinternalMod_init_mod
-//Requires: caml_raise_with_arg, caml_global_data, caml_alloc_dummy_infix
-//If: !effects
-//Version: < 4.13
-function caml_CamlinternalMod_init_mod(loc, shape) {
-  function undef_module(_x) {
-    caml_raise_with_arg(caml_global_data.Undefined_recursive_module, loc);
-  }
-  function loop(shape, struct, idx) {
-    if (typeof shape === "number")
-      switch (shape) {
-        case 0: //function
-          var dummy = caml_alloc_dummy_infix();
-          dummy.fun = undef_module;
-          struct[idx] = dummy;
-          break;
-        case 1: //lazy
-          struct[idx] = [246, undef_module];
-          break;
-        default: //case 2://class
-          struct[idx] = [];
-      }
-    else
-      switch (shape[0]) {
-        case 0: //module
-          struct[idx] = [0];
-          for (var i = 1; i < shape[1].length; i++)
-            loop(shape[1][i], struct[idx], i);
-          break;
-        default: //case 1://Value
-          struct[idx] = shape[1];
-      }
-  }
-  var res = [];
-  loop(shape, res, 0);
-  return res[0];
-}
-//Provides: caml_CamlinternalMod_update_mod
-//Requires: caml_update_dummy
-//If: !effects
-//Version: < 4.13
-function caml_CamlinternalMod_update_mod(shape, real, x) {
-  if (typeof shape === "number")
-    //function
-    //lazy
-    //class
-    caml_update_dummy(real, x);
-  else
-    switch (shape[0]) {
-      case 0: //module
-        for (var i = 1; i < shape[1].length; i++)
-          caml_CamlinternalMod_update_mod(shape[1][i], real[i], x[i]);
-        break;
-      //case 1://Value
-      default:
-    }
-  return 0;
-}
-
-//Provides: caml_CamlinternalMod_init_mod
-//Requires: caml_raise_with_arg, caml_global_data, caml_alloc_dummy_infix
-//If: effects
-//Version: < 4.13
-function caml_CamlinternalMod_init_mod(loc, shape, cont) {
-  function undef_module(_x, _cont) {
-    caml_raise_with_arg(caml_global_data.Undefined_recursive_module, loc);
-  }
-  function loop(shape, struct, idx) {
-    if (typeof shape === "number")
-      switch (shape) {
-        case 0: //function
-          var dummy = caml_alloc_dummy_infix();
-          dummy.fun = undef_module;
-          struct[idx] = dummy;
-          break;
-        case 1: //lazy
-          struct[idx] = [246, undef_module];
-          break;
-        default: //case 2://class
-          struct[idx] = [];
-      }
-    else
-      switch (shape[0]) {
-        case 0: //module
-          struct[idx] = [0];
-          for (var i = 1; i < shape[1].length; i++)
-            loop(shape[1][i], struct[idx], i);
-          break;
-        default: //case 1://Value
-          struct[idx] = shape[1];
-      }
-  }
-  var res = [];
-  loop(shape, res, 0);
-  return cont(res[0]);
-}
-//Provides: caml_CamlinternalMod_update_mod
-//Requires: caml_update_dummy
-//If: effects
-//Version: < 4.13
-function caml_CamlinternalMod_update_mod(shape, real, x, cont) {
-  function loop(shape, real, x) {
-    if (typeof shape === "number")
-      //function
-      //lazy
-      //class
-      caml_update_dummy(real, x);
-    else
-      switch (shape[0]) {
-        case 0: //module
-          for (var i = 1; i < shape[1].length; i++)
-            loop(shape[1][i], real[i], x[i]);
-          break;
-        //case 1://Value
-        default:
-      }
-  }
-  loop(shape, real, x);
-  return cont(0);
-}
diff --git a/runtime/js/marshal.js b/runtime/js/marshal.js
index 39f2da30eb..208dff8275 100644
--- a/runtime/js/marshal.js
+++ b/runtime/js/marshal.js
@@ -182,21 +182,6 @@ function caml_float_of_bytes(a) {
   return caml_int64_float_of_bits(caml_int64_of_bytes(a));
 }
 
-//Provides: caml_input_value_from_string mutable
-//Requires: JsStringReader, UInt8ArrayReader
-//Requires: caml_input_value_from_reader
-//Requires: caml_ml_bytes_content
-//Version: < 4.12
-function caml_input_value_from_string(s, ofs) {
-  var c = typeof s === "string" ? s : caml_ml_bytes_content(s);
-  var ofs = typeof ofs === "number" ? ofs : ofs[0];
-  var reader =
-    c instanceof Uint8Array
-      ? new UInt8ArrayReader(c, ofs)
-      : new JsStringReader(c, ofs);
-  return caml_input_value_from_reader(reader);
-}
-
 //Provides: caml_input_value_from_bytes mutable
 //Requires: JsStringReader, UInt8ArrayReader
 //Requires: caml_input_value_from_reader
diff --git a/runtime/js/obj.js b/runtime/js/obj.js
index d0df309b40..2d2ab48b0d 100644
--- a/runtime/js/obj.js
+++ b/runtime/js/obj.js
@@ -38,12 +38,6 @@ function caml_alloc_dummy_infix() {
   };
 }
 
-//Provides: caml_obj_is_block const (const)
-//Version: < 4.12
-function caml_obj_is_block(x) {
-  return +Array.isArray(x);
-}
-
 //Provides: caml_obj_tag
 //Requires: caml_is_ml_bytes, caml_is_ml_string
 function caml_obj_tag(x) {
diff --git a/runtime/js/sys.js b/runtime/js/sys.js
index 0217e902ee..16ecee6ad6 100644
--- a/runtime/js/sys.js
+++ b/runtime/js/sys.js
@@ -332,30 +332,11 @@ function caml_ml_runtime_warnings_enabled(_unit) {
   return caml_runtime_warnings;
 }
 
-//Provides: caml_spacetime_enabled const (const)
-//Version: < 4.12
-function caml_spacetime_enabled(_unit) {
-  return 0;
-}
-
 //Provides: caml_sys_const_naked_pointers_checked const (const)
 function caml_sys_const_naked_pointers_checked(_unit) {
   return 0;
 }
 
-//Provides: caml_register_channel_for_spacetime const (const)
-//Version: < 4.12
-function caml_register_channel_for_spacetime(_channel) {
-  return 0;
-}
-
-//Provides: caml_spacetime_only_works_for_native_code
-//Requires: caml_failwith
-//Version: < 4.12
-function caml_spacetime_only_works_for_native_code() {
-  caml_failwith("Spacetime profiling only works for native code");
-}
-
 //Provides: caml_xdg_defaults
 //Version: >= 5.2
 function caml_xdg_defaults(_unit) {
diff --git a/runtime/js/toplevel.js b/runtime/js/toplevel.js
index 2e4547ee4e..0a74139b97 100644
--- a/runtime/js/toplevel.js
+++ b/runtime/js/toplevel.js
@@ -133,19 +133,6 @@ function caml_static_release_bytecode() {
   return 0;
 }
 
-//Provides: caml_static_alloc
-//Requires: caml_create_bytes
-//Version: < 4.12
-function caml_static_alloc(len) {
-  return caml_create_bytes(len);
-}
-
-//Provides: caml_static_free
-//Version: < 4.12
-function caml_static_free() {
-  return 0;
-}
-
 //Provides: caml_realloc_global
 //Requires: caml_global_data
 function caml_realloc_global(len) {
diff --git a/tools/toplevel_expect/gen.ml b/tools/toplevel_expect/gen.ml
index 04661fc990..a905d5e575 100644
--- a/tools/toplevel_expect/gen.ml
+++ b/tools/toplevel_expect/gen.ml
@@ -49,7 +49,6 @@ let () =
     | _ -> assert false
   in
   match maj, min with
-  | 4, 8 | 4, 9 | 4, 10 -> dump_file "toplevel_expect_test.ml-4.08"
   | 4, min ->
       assert (min >= 11);
       dump_file "toplevel_expect_test.ml-4.11"
diff --git a/tools/toplevel_expect/toplevel_expect_test.ml-4.08 b/tools/toplevel_expect/toplevel_expect_test.ml-4.08
deleted file mode 100644
index 0741a007fb..0000000000
--- a/tools/toplevel_expect/toplevel_expect_test.ml-4.08
+++ /dev/null
@@ -1,382 +0,0 @@
-(**************************************************************************)
-(*                                                                        *)
-(*                                 OCaml                                  *)
-(*                                                                        *)
-(*                   Jeremie Dimino, Jane Street Europe                   *)
-(*                                                                        *)
-(*   Copyright 2016 Jane Street Group LLC                                 *)
-(*                                                                        *)
-(*   All rights reserved.  This file is distributed under the terms of    *)
-(*   the GNU Lesser General Public License version 2.1, with the          *)
-(*   special exception on linking described in the file LICENSE.          *)
-(*                                                                        *)
-(**************************************************************************)
-
-(* Execute a list of phrases from a .ml file and compare the result to the
-   expected output, written inside [%%expect ...] nodes. At the end, create
-   a .corrected file containing the corrected expectations. The test is
-   successful if there is no differences between the two files.
-
-   An [%%expect] node always contains both the expected outcome with and
-   without -principal. When the two differ the expectation is written as
-   follows:
-
-   {[
-     [%%expect {|
-     output without -principal
-     |}, Principal{|
-     output with -principal
-     |}]
-   ]}
-*)
-
-[@@@ocaml.warning "-40"]
-
-open StdLabels
-
-(* representation of: {tag|str|tag} *)
-type string_constant =
-  { str : string
-  ; tag : string
-  }
-
-type expectation =
-  { extid_loc   : Location.t (* Location of "expect" in "[%%expect ...]" *)
-  ; payload_loc : Location.t (* Location of the whole payload *)
-  ; normal      : string_constant (* expectation without -principal *)
-  ; principal   : string_constant (* expectation with -principal *)
-  }
-
-(* A list of phrases with the expected toplevel output *)
-type chunk =
-  { phrases     : Parsetree.toplevel_phrase list
-  ; expectation : expectation
-  }
-
-type correction =
-  { corrected_expectations : expectation list
-  ; trailing_output        : string
-  }
-
-let match_expect_extension (ext : Parsetree.extension) =
-  match ext with
-  | ({Asttypes.txt="expect"|"ocaml.expect"; loc = extid_loc}, payload) ->
-    let invalid_payload () =
-      Location.raise_errorf ~loc:extid_loc
-        "invalid [%%%%expect payload]"
-    in
-    let string_constant (e : Parsetree.expression) =
-      match e.pexp_desc with
-      | Pexp_constant (Pconst_string (str, Some tag)) ->
-        { str; tag }
-      | _ -> invalid_payload ()
-    in
-    let expectation =
-      match payload with
-      | PStr [{ pstr_desc = Pstr_eval (e, []); _ }] ->
-        let normal, principal =
-          match e.pexp_desc with
-          | Pexp_tuple
-              [ a
-              ; { pexp_desc = Pexp_construct
-                                ({ txt = Lident "Principal"; _ }, Some b); _ }
-              ] ->
-            (string_constant a, string_constant b)
-          | _ -> let s = string_constant e in (s, s)
-        in
-        { extid_loc
-        ; payload_loc = e.pexp_loc
-        ; normal
-        ; principal
-        }
-      | PStr [] ->
-        let s = { tag = ""; str = "" } in
-        { extid_loc
-        ; payload_loc  = { extid_loc with loc_start = extid_loc.loc_end }
-        ; normal    = s
-        ; principal = s
-        }
-      | _ -> invalid_payload ()
-    in
-    Some expectation
-  | _ ->
-    None
-
-(* Split a list of phrases from a .ml file *)
-let split_chunks phrases =
-  let rec loop (phrases : Parsetree.toplevel_phrase list) code_acc acc =
-    match phrases with
-    | [] ->
-      if code_acc = [] then
-        (List.rev acc, None)
-      else
-        (List.rev acc, Some (List.rev code_acc))
-    | phrase :: phrases ->
-      match phrase with
-      | Ptop_def [] -> loop phrases code_acc acc
-      | Ptop_def [{pstr_desc = Pstr_extension(ext, []); _}] -> begin
-          match match_expect_extension ext with
-          | None -> loop phrases (phrase :: code_acc) acc
-          | Some expectation ->
-            let chunk =
-              { phrases     = List.rev code_acc
-              ; expectation
-              }
-            in
-            loop phrases [] (chunk :: acc)
-        end
-      | _ -> loop phrases (phrase :: code_acc) acc
-  in
-  loop phrases [] []
-
-module Compiler_messages = struct
-  let print_loc ppf (loc : Location.t) =
-    let startchar = loc.loc_start.pos_cnum - loc.loc_start.pos_bol in
-    let endchar = loc.loc_end.pos_cnum - loc.loc_start.pos_cnum + startchar in
-    Format.fprintf ppf "Line _";
-    if startchar >= 0 then
-      Format.fprintf ppf ", characters %d-%d" startchar endchar;
-    Format.fprintf ppf ":@."
-
-  let () =
-    let default = !Location.report_printer () in
-    Location.report_printer := (fun _ ->
-      { default with
-        Location.pp_main_loc = (fun _ _ fmt loc -> print_loc fmt loc);
-        Location.pp_submsg_loc = (fun _ _ fmt loc -> print_loc fmt loc);
-      })
-
-  let capture ppf ~f =
-    Misc.protect_refs
-      [ R (Location.formatter_for_warnings , ppf            )
-      ]
-      f
-end
-
-let collect_formatters buf pps ~f =
-  List.iter ~f:(fun pp -> Format.pp_print_flush pp ()) pps;
-  let save =
-    List.map ~f:(fun pp -> Format.pp_get_formatter_out_functions pp ()) pps
-  in
-  let restore () =
-    List.iter2
-      ~f:(fun pp out_functions ->
-         Format.pp_print_flush pp ();
-         Format.pp_set_formatter_out_functions pp out_functions)
-      pps save
-  in
-  let out_string str ofs len = Buffer.add_substring buf str ofs len
-  and out_flush = ignore
-  and out_newline () = Buffer.add_char buf '\n'
-  and out_spaces n = for _i = 1 to n do Buffer.add_char buf ' ' done
-  and out_indent n = for _i = 1 to n do Buffer.add_char buf ' ' done in
-  let out_functions =
-    { Format.out_string; out_flush; out_newline; out_spaces; out_indent }
-  in
-  List.iter
-    ~f:(fun pp -> Format.pp_set_formatter_out_functions pp out_functions)
-    pps;
-  match f () with
-  | x             -> restore (); x
-  | exception exn -> restore (); raise exn
-
-(* Invariant: ppf = Format.formatter_of_buffer buf *)
-let capture_everything buf ppf ~f =
-  collect_formatters buf [Format.std_formatter; Format.err_formatter]
-                     ~f:(fun () -> Compiler_messages.capture ppf ~f)
-
-let exec_phrase ppf phrase =
-  if !Clflags.dump_parsetree then Printast. top_phrase ppf phrase;
-  if !Clflags.dump_source    then Pprintast.top_phrase ppf phrase;
-  Toploop.execute_phrase true ppf phrase
-
-let parse_contents ~fname contents =
-  let lexbuf = Lexing.from_string contents in
-  Location.init lexbuf fname;
-  Location.input_name := fname;
-  Parse.use_file lexbuf
-
-let eval_expectation expectation ~output =
-  let s =
-    if !Clflags.principal then
-      expectation.principal
-    else
-      expectation.normal
-  in
-  if s.str = output then
-    None
-  else
-    let s = { s with str = output } in
-    Some (
-      if !Clflags.principal then
-        { expectation with principal = s }
-      else
-        { expectation with normal = s }
-    )
-
-let preprocess_structure mappers str =
-  let open Ast_mapper in
-  List.fold_right
-    ~f:(fun ppx_rewriter str  ->
-       let mapper : Ast_mapper.mapper = ppx_rewriter [] in
-       mapper.structure mapper str)
-    mappers
-    ~init:str
-
-let preprocess_phrase mappers phrase =
-  let open Parsetree in
-  match phrase with
-  | Ptop_def str -> Ptop_def (preprocess_structure mappers str)
-  | Ptop_dir _ as x -> x
-
-
-let shift_lines delta =
-  let position (pos : Lexing.position) =
-    { pos with pos_lnum = pos.pos_lnum + delta }
-  in
-  let location _this (loc : Location.t) =
-    { loc with
-      loc_start = position loc.loc_start
-    ; loc_end   = position loc.loc_end
-    }
-  in
-  fun _ -> { Ast_mapper.default_mapper with location }
-
-let rec min_line_number : Parsetree.toplevel_phrase list -> int option =
-function
-  | [] -> None
-  | (Ptop_dir _  | Ptop_def []) :: l -> min_line_number l
-  | Ptop_def (st :: _) :: _ -> Some st.pstr_loc.loc_start.pos_lnum
-
-let eval_expect_file mapper fname ~file_contents =
-  Warnings.reset_fatal ();
-  let chunks, trailing_code =
-    parse_contents ~fname:fname file_contents |> split_chunks
-  in
-  let buf = Buffer.create 1024 in
-  let ppf = Format.formatter_of_buffer buf in
-  let out_fun = Format.pp_get_formatter_out_functions ppf () in
-  Format.pp_set_formatter_out_functions Format.std_formatter out_fun;
-
-  let exec_phrases phrases =
-
-    let mappers =
-      match min_line_number phrases with
-      | None -> []
-      | Some lnum -> [shift_lines (1 - lnum)]
-    in
-    let mappers = mapper :: mappers in
-    let phrases = List.map ~f:(preprocess_phrase mappers) phrases in
-
-    (* For formatting purposes *)
-    Buffer.add_char buf '\n';
-    let _ : bool =
-      List.fold_left phrases ~init:true ~f:(fun acc phrase ->
-        acc &&
-        try
-          exec_phrase ppf phrase
-        with exn ->
-          Location.report_exception ppf exn;
-          false)
-    in
-    Format.pp_print_flush ppf ();
-    let len = Buffer.length buf in
-    if len > 0 && Buffer.nth buf (len - 1) <> '\n' then
-      (* For formatting purposes *)
-      Buffer.add_char buf '\n';
-    let s = Buffer.contents buf in
-    Buffer.clear buf;
-    Misc.delete_eol_spaces s
-  in
-  let corrected_expectations =
-    capture_everything buf ppf ~f:(fun () ->
-      List.fold_left chunks ~init:[] ~f:(fun acc chunk ->
-        let output = exec_phrases chunk.phrases in
-        match eval_expectation chunk.expectation ~output with
-        | None -> acc
-        | Some correction -> correction :: acc)
-      |> List.rev)
-  in
-  let trailing_output =
-    match trailing_code with
-    | None -> ""
-    | Some phrases ->
-      capture_everything buf ppf ~f:(fun () -> exec_phrases phrases)
-  in
-  { corrected_expectations; trailing_output }
-
-let output_slice oc s a b =
-  output_string oc (String.sub s ~pos:a ~len:(b - a))
-
-let output_corrected oc ~file_contents correction =
-  let output_body oc { str; tag } =
-    Printf.fprintf oc "{%s|%s|%s}" tag str tag
-  in
-  let ofs =
-    List.fold_left correction.corrected_expectations ~init:0
-      ~f:(fun ofs c ->
-        output_slice oc file_contents ofs c.payload_loc.loc_start.pos_cnum;
-        output_body oc c.normal;
-        if !Clflags.principal && c.normal.str <> c.principal.str then begin
-          output_string oc ", Principal";
-          output_body oc c.principal
-        end;
-        c.payload_loc.loc_end.pos_cnum)
-  in
-  output_slice oc file_contents ofs (String.length file_contents);
-  match correction.trailing_output with
-  | "" -> ()
-  | s  -> Printf.fprintf oc "\n[%%%%expect{|%s|}]\n" s
-
-let write_corrected ~file ~file_contents correction =
-  let oc = open_out file in
-  output_corrected oc ~file_contents correction;
-  close_out oc
-
-let process_expect_file mapper fname =
-  let corrected_fname = fname ^ ".corrected" in
-  let file_contents =
-    let ic = open_in_bin fname in
-    match really_input_string ic (in_channel_length ic) with
-    | s           -> close_in ic; Misc.normalise_eol s
-    | exception e -> close_in ic; raise e
-  in
-  let correction = eval_expect_file mapper fname ~file_contents in
-  write_corrected ~file:corrected_fname ~file_contents correction
-
-let repo_root = ref ""
-
-let main mapper fname =
-  Toploop.override_sys_argv
-    (Array.sub Sys.argv ~pos:!Arg.current
-       ~len:(Array.length Sys.argv - !Arg.current));
-  (* Ignore OCAMLRUNPARAM=b to be reproducible *)
-  Printexc.record_backtrace false;
-  List.iter [ "stdlib" ] ~f:(fun s ->
-    Topdirs.dir_directory (Filename.concat !repo_root s));
-  Toploop.initialize_toplevel_env ();
-  Sys.interactive := false;
-  process_expect_file mapper fname;
-  exit 0
-
-let args =
-  Arg.align
-    [ "-repo-root", Set_string repo_root,
-      "<dir> root of the OCaml repository"
-    ; "-principal", Set Clflags.principal,
-      " Evaluate the file with -principal set"
-    ]
-
-let usage = "Usage: expect_test <options> [script-file [arguments]]\n\
-             options are:"
-
-let run mapper =
-  Toploop.set_paths ();
-  Clflags.error_style := Some Misc.Error_style.Short;
-  try
-    Arg.parse args (main mapper) usage;
-    Printf.eprintf "expect_test: no input file\n";
-    exit 2
-  with exn ->
-    Location.report_exception Format.err_formatter exn;
-    exit 2
diff --git a/toplevel/bin/jsoo_common.ml b/toplevel/bin/jsoo_common.ml
index 9e79ac7521..b202a9e308 100644
--- a/toplevel/bin/jsoo_common.ml
+++ b/toplevel/bin/jsoo_common.ml
@@ -75,7 +75,7 @@ let cmis_of_package pkg : string list =
       try Findlib.package_property [ "byte" ] pkg "archive"
       with exc -> if String.equal pkg "stdlib" then "stdlib.cma" else raise exc
     in
-    let l = String.split_char ~sep:' ' archive in
+    let l = String.split_on_char ~sep:' ' archive in
     List.iter l ~f:(function
       | "" -> ()
       | x ->
@@ -107,7 +107,7 @@ let cmis files =
       match kind file with
       | `Pkg pkg -> cmis_of_package pkg @ fs
       | `Cmi s -> (
-          match String.split_char ~sep:':' s with
+          match String.split_on_char ~sep:':' s with
           | [ s ] -> read_cmi ~dir:"." s :: fs
           | [ pkg; s ] ->
               let dir = Findlib.package_directory pkg in
diff --git a/toplevel/bin/jsoo_mkcmis.ml b/toplevel/bin/jsoo_mkcmis.ml
index c1ab4d04e9..3dda46ee04 100644
--- a/toplevel/bin/jsoo_mkcmis.ml
+++ b/toplevel/bin/jsoo_mkcmis.ml
@@ -68,8 +68,8 @@ let () =
   let runtime_files, builtin =
     List.partition_map runtime_files ~f:(fun name ->
         match Js_of_ocaml_compiler.Builtins.find name with
-        | Some t -> `Snd t
-        | None -> `Fst name)
+        | Some t -> Right t
+        | None -> Left name)
   in
   let builtin =
     if !runtime then Js_of_ocaml_compiler_runtime_files.runtime @ builtin else builtin