diff --git a/.github/workflows/build-wasm_of_ocaml.yml b/.github/workflows/build-wasm_of_ocaml.yml
index 877b34ca4f..177556089b 100644
--- a/.github/workflows/build-wasm_of_ocaml.yml
+++ b/.github/workflows/build-wasm_of_ocaml.yml
@@ -46,7 +46,7 @@ jobs:
       - name: Install node
         uses: actions/setup-node@v4
         with:
-          node-version: 23
+          node-version: 'v24.0.0-v8-canary202412116884e26428'
 
       - name: Install OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v3
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index 9957756643..c51850c03b 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -73,6 +73,32 @@ let output_gen output_file f =
   Code.Var.set_stable (Config.Flag.stable_var ());
   Filename.gen_file output_file f
 
+let build_runtime ~runtime_file =
+  let variables = [ "use-js-string", Config.Flag.use_js_string () ] in
+  List.fold_left
+    ~f:(fun cont (name, contents) ->
+      fun inputs ->
+       Fs.with_intermediate_file (Filename.temp_file name ".wat")
+       @@ fun filename ->
+       Fs.write_file
+         ~name:filename
+         ~contents:(Wat_preprocess.f ~variables ~filename ~contents);
+       cont ((filename, name) :: inputs))
+    ~init:(fun inputs ->
+      Fs.with_intermediate_file (Filename.temp_file "runtime-merged" ".wasm")
+      @@ fun merge_file ->
+      Binaryen.link ~opt_output_sourcemap:None ~inputs ~output_file:merge_file;
+      Binaryen.optimize
+        ~profile:None
+        ~options:[ "-O2" ]
+        ~opt_input_sourcemap:None
+        ~input_file:merge_file
+        ~opt_output_sourcemap:None
+        ~output_file:runtime_file
+        ())
+    Runtime_files.wat_files
+    []
+
 let link_and_optimize
     ~profile
     ~sourcemap_root
@@ -91,7 +117,7 @@ let link_and_optimize
   let enable_source_maps = Option.is_some opt_sourcemap_file in
   Fs.with_intermediate_file (Filename.temp_file "runtime" ".wasm")
   @@ fun runtime_file ->
-  Fs.write_file ~name:runtime_file ~contents:Wa_runtime.wasm_runtime;
+  build_runtime ~runtime_file;
   Fs.with_intermediate_file (Filename.temp_file "wasm-merged" ".wasm")
   @@ fun temp_file ->
   opt_with
@@ -101,8 +127,9 @@ let link_and_optimize
      else None)
   @@ fun opt_temp_sourcemap ->
   Binaryen.link
-    ~runtime_files:(runtime_file :: runtime_wasm_files)
-    ~input_files:wat_files
+    ~inputs:
+      (List.map ~f:(fun f -> f, "env") (runtime_file :: runtime_wasm_files)
+      @ List.map ~f:(fun f -> f, "OCaml") wat_files)
     ~opt_output_sourcemap:opt_temp_sourcemap
     ~output_file:temp_file;
   Fs.with_intermediate_file (Filename.temp_file "wasm-dce" ".wasm")
@@ -113,7 +140,7 @@ let link_and_optimize
   @@ fun opt_temp_sourcemap' ->
   let primitives =
     Binaryen.dead_code_elimination
-      ~dependencies:Wa_runtime.dependencies
+      ~dependencies:Runtime_files.dependencies
       ~opt_input_sourcemap:opt_temp_sourcemap
       ~opt_output_sourcemap:opt_temp_sourcemap'
       ~input_file:temp_file
@@ -124,7 +151,8 @@ let link_and_optimize
     ~opt_input_sourcemap:opt_temp_sourcemap'
     ~opt_output_sourcemap:opt_sourcemap
     ~input_file:temp_file'
-    ~output_file;
+    ~output_file
+    ();
   Option.iter
     ~f:(update_sourcemap ~sourcemap_root ~sourcemap_don't_inline_content)
     opt_sourcemap_file;
@@ -133,13 +161,12 @@ let link_and_optimize
 let link_runtime ~profile runtime_wasm_files output_file =
   Fs.with_intermediate_file (Filename.temp_file "runtime" ".wasm")
   @@ fun runtime_file ->
-  Fs.write_file ~name:runtime_file ~contents:Wa_runtime.wasm_runtime;
+  build_runtime ~runtime_file;
   Fs.with_intermediate_file (Filename.temp_file "wasm-merged" ".wasm")
   @@ fun temp_file ->
   Binaryen.link
+    ~inputs:(List.map ~f:(fun f -> f, "env") (runtime_file :: runtime_wasm_files))
     ~opt_output_sourcemap:None
-    ~runtime_files:(runtime_file :: runtime_wasm_files)
-    ~input_files:[]
     ~output_file:temp_file;
   Binaryen.optimize
     ~profile
@@ -147,6 +174,7 @@ let link_runtime ~profile runtime_wasm_files output_file =
     ~opt_output_sourcemap:None
     ~input_file:temp_file
     ~output_file
+    ()
 
 let generate_prelude ~out_file =
   Filename.gen_file out_file
@@ -162,10 +190,10 @@ let generate_prelude ~out_file =
   in
   let context = Generate.start () in
   let debug = Parse_bytecode.Debug.create ~include_cmis:false false in
-  let _ =
+  let _, generated_js =
     Generate.f
       ~context
-      ~unit_name:(Some "prelude")
+      ~unit_name:(Some "wasmoo_prelude")
       ~live_vars:variable_uses
       ~in_cps
       ~deadcode_sentinal
@@ -173,22 +201,23 @@ let generate_prelude ~out_file =
       program
   in
   Generate.output ch ~context;
-  uinfo.provides
+  uinfo.provides, generated_js
 
 let build_prelude z =
   Fs.with_intermediate_file (Filename.temp_file "prelude" ".wasm")
   @@ fun prelude_file ->
   Fs.with_intermediate_file (Filename.temp_file "prelude_file" ".wasm")
   @@ fun tmp_prelude_file ->
-  let predefined_exceptions = generate_prelude ~out_file:prelude_file in
+  let info = generate_prelude ~out_file:prelude_file in
   Binaryen.optimize
     ~profile:(Driver.profile 1)
     ~input_file:prelude_file
     ~output_file:tmp_prelude_file
     ~opt_input_sourcemap:None
-    ~opt_output_sourcemap:None;
+    ~opt_output_sourcemap:None
+    ();
   Zip.add_file z ~name:"prelude.wasm" ~file:tmp_prelude_file;
-  predefined_exceptions
+  info
 
 let build_js_runtime ~primitives ?runtime_arguments () =
   let always_required_js, primitives =
@@ -216,7 +245,7 @@ let build_js_runtime ~primitives ?runtime_arguments () =
   in
   let prelude = Link.output_js always_required_js in
   let init_fun =
-    match Parse_js.parse (Parse_js.Lexer.of_string Wa_runtime.js_runtime) with
+    match Parse_js.parse (Parse_js.Lexer.of_string Runtime_files.js_runtime) with
     | [ (Expression_statement f, _) ] -> f
     | _ -> assert false
   in
@@ -364,12 +393,18 @@ let run
      let z = Zip.open_out tmp_output_file in
      Zip.add_file z ~name:"runtime.wasm" ~file:tmp_wasm_file;
      Zip.add_entry z ~name:"runtime.js" ~contents:js_runtime;
-     let predefined_exceptions = build_prelude z in
+     let predefined_exceptions, (strings, fragments) = build_prelude z in
      Link.add_info
        z
        ~predefined_exceptions
        ~build_info:(Build_info.create `Runtime)
-       ~unit_data:[]
+       ~unit_data:
+         [ { Link.unit_name = "wasmoo_prelude"
+           ; unit_info = Unit_info.empty
+           ; strings
+           ; fragments
+           }
+         ]
        ();
      Zip.close_out z)
    else
@@ -411,7 +446,8 @@ let run
            ~opt_input_sourcemap:None
            ~opt_output_sourcemap:opt_tmp_map_file
            ~input_file:wat_file
-           ~output_file:tmp_wasm_file;
+           ~output_file:tmp_wasm_file
+           ();
          { Link.unit_name; unit_info; strings; fragments }
        in
        cont unit_data unit_name tmp_wasm_file opt_tmp_map_file
diff --git a/compiler/bin-wasm_of_ocaml/dune b/compiler/bin-wasm_of_ocaml/dune
index cd5f1a468a..d80668bbd9 100644
--- a/compiler/bin-wasm_of_ocaml/dune
+++ b/compiler/bin-wasm_of_ocaml/dune
@@ -22,12 +22,12 @@
   (:standard -safe-string)))
 
 (rule
- (target wa_runtime.ml)
+ (target runtime_files.ml)
  (deps
   gen/gen.exe
-  ../../runtime/wasm/runtime.wasm
   ../../runtime/wasm/runtime.js
-  ../../runtime/wasm/deps.json)
+  ../../runtime/wasm/deps.json
+  (glob_files ../../runtime/wasm/*.wat))
  (action
   (with-stdout-to
    %{target}
diff --git a/compiler/bin-wasm_of_ocaml/gen/gen.ml b/compiler/bin-wasm_of_ocaml/gen/gen.ml
index b7a20c4e3e..a0b8f68e88 100644
--- a/compiler/bin-wasm_of_ocaml/gen/gen.ml
+++ b/compiler/bin-wasm_of_ocaml/gen/gen.ml
@@ -3,11 +3,18 @@ let read_file ic = really_input_string ic (in_channel_length ic)
 let () =
   let () = set_binary_mode_out stdout true in
   Format.printf
-    "let wasm_runtime = \"%s\"@."
+    "let js_runtime = \"%s\"@."
     (String.escaped (read_file (open_in_bin Sys.argv.(1))));
   Format.printf
-    "let js_runtime = \"%s\"@."
+    "let dependencies = \"%s\"@."
     (String.escaped (read_file (open_in_bin Sys.argv.(2))));
+  let a = Array.sub Sys.argv 3 (Array.length Sys.argv - 3) in
   Format.printf
-    "let dependencies = \"%s\"@."
-    (String.escaped (read_file (open_in_bin Sys.argv.(3))))
+    "let wat_files = [%a]@."
+    (Format.pp_print_list (fun f file ->
+         Format.fprintf
+           f
+           "\"%s\", \"%s\"; "
+           Filename.(chop_suffix (basename file) ".wat")
+           (String.escaped (read_file (open_in_bin file)))))
+    (Array.to_list a)
diff --git a/compiler/lib-wasm/binaryen.ml b/compiler/lib-wasm/binaryen.ml
index 93b0b7b7fb..eded3b43e3 100644
--- a/compiler/lib-wasm/binaryen.ml
+++ b/compiler/lib-wasm/binaryen.ml
@@ -45,18 +45,12 @@ let opt_flag flag v =
   | None -> []
   | Some v -> [ flag; Filename.quote v ]
 
-let link ~runtime_files ~input_files ~opt_output_sourcemap ~output_file =
+let link ~inputs ~opt_output_sourcemap ~output_file =
   command
     ("wasm-merge"
     :: (common_options ()
        @ List.flatten
-           (List.map
-              ~f:(fun runtime_file -> [ Filename.quote runtime_file; "env" ])
-              runtime_files)
-       @ List.flatten
-           (List.map
-              ~f:(fun input_file -> [ Filename.quote input_file; "OCaml" ])
-              input_files)
+           (List.map ~f:(fun (file, name) -> [ Filename.quote file; name ]) inputs)
        @ [ "-o"; Filename.quote output_file ]
        @ opt_flag "--output-source-map" opt_output_sourcemap))
 
@@ -114,8 +108,14 @@ let optimization_options =
    ; [ "-O3"; "--skip-pass=inlining-optimizing"; "--traps-never-happen" ]
   |]
 
-let optimize ~profile ~opt_input_sourcemap ~input_file ~opt_output_sourcemap ~output_file
-    =
+let optimize
+    ~profile
+    ?options
+    ~opt_input_sourcemap
+    ~input_file
+    ~opt_output_sourcemap
+    ~output_file
+    () =
   let level =
     match profile with
     | None -> 1
@@ -124,7 +124,7 @@ let optimize ~profile ~opt_input_sourcemap ~input_file ~opt_output_sourcemap ~ou
   command
     ("wasm-opt"
      :: (common_options ()
-        @ optimization_options.(level - 1)
+        @ Option.value ~default:optimization_options.(level - 1) options
         @ [ Filename.quote input_file; "-o"; Filename.quote output_file ])
     @ opt_flag "--input-source-map" opt_input_sourcemap
     @ opt_flag "--output-source-map" opt_output_sourcemap)
diff --git a/compiler/lib-wasm/binaryen.mli b/compiler/lib-wasm/binaryen.mli
index 3e07e06f88..addfef39c9 100644
--- a/compiler/lib-wasm/binaryen.mli
+++ b/compiler/lib-wasm/binaryen.mli
@@ -17,8 +17,7 @@
  *)
 
 val link :
-     runtime_files:string list
-  -> input_files:string list
+     inputs:(string * string (*file, module name*)) list
   -> opt_output_sourcemap:string option
   -> output_file:string
   -> unit
@@ -33,8 +32,10 @@ val dead_code_elimination :
 
 val optimize :
      profile:Driver.profile option
+  -> ?options:string list
   -> opt_input_sourcemap:string option
   -> input_file:string
   -> opt_output_sourcemap:string option
   -> output_file:string
   -> unit
+  -> unit
diff --git a/compiler/lib-wasm/code_generation.ml b/compiler/lib-wasm/code_generation.ml
index cd23a3db7e..03f279eb6b 100644
--- a/compiler/lib-wasm/code_generation.ml
+++ b/compiler/lib-wasm/code_generation.ml
@@ -453,7 +453,8 @@ let rec is_smi e =
   | RefNull _
   | Br_on_cast _
   | Br_on_cast_fail _
-  | Try _ -> false
+  | Try _
+  | ExternConvertAny _ -> false
   | BinOp ((F32 _ | F64 _), _, _) | RefTest _ | RefEq _ -> true
   | IfExpr (_, _, ift, iff) -> is_smi ift && is_smi iff
 
diff --git a/compiler/lib-wasm/dune b/compiler/lib-wasm/dune
index 2a54c9316f..90e6dcddee 100644
--- a/compiler/lib-wasm/dune
+++ b/compiler/lib-wasm/dune
@@ -4,4 +4,6 @@
  (synopsis "Wasm_of_ocaml compiler library")
  (libraries js_of_ocaml_compiler)
  (flags
-  (:standard -w -7-37 -safe-string -open Js_of_ocaml_compiler)))
+  (:standard -w -7-37 -safe-string -open Js_of_ocaml_compiler))
+ (preprocess
+  (pps sedlex.ppx)))
diff --git a/compiler/lib-wasm/gc_target.ml b/compiler/lib-wasm/gc_target.ml
index f2e6b7eccd..42af3722b2 100644
--- a/compiler/lib-wasm/gc_target.ml
+++ b/compiler/lib-wasm/gc_target.ml
@@ -35,14 +35,31 @@ module Type = struct
           ; typ = W.Array { mut = true; typ = Value value }
           })
 
-  let string_type =
-    register_type "string" (fun () ->
+  let bytes_type =
+    register_type "bytes" (fun () ->
         return
           { supertype = None
           ; final = true
           ; typ = W.Array { mut = true; typ = Packed I8 }
           })
 
+  let string_type =
+    register_type "string" (fun () ->
+        return
+          (if Config.Flag.use_js_string ()
+           then
+             { supertype = None
+             ; final = true
+             ; typ =
+                 W.Struct
+                   [ { mut = false; typ = Value (Ref { nullable = true; typ = Any }) } ]
+             }
+           else
+             { supertype = None
+             ; final = true
+             ; typ = W.Array { mut = true; typ = Packed I8 }
+             }))
+
   let float_type =
     register_type "float" (fun () ->
         return
@@ -121,7 +138,7 @@ module Type = struct
 
   let custom_operations_type =
     register_type "custom_operations" (fun () ->
-        let* string = string_type in
+        let* bytes = bytes_type in
         let* compare = compare_type in
         let* hash = hash_type in
         let* fixed_length = fixed_length_type in
@@ -134,7 +151,7 @@ module Type = struct
           ; typ =
               W.Struct
                 [ { mut = false
-                  ; typ = Value (Ref { nullable = false; typ = Type string })
+                  ; typ = Value (Ref { nullable = false; typ = Type bytes })
                   }
                 ; { mut = false
                   ; typ = Value (Ref { nullable = true; typ = Type compare })
@@ -513,7 +530,8 @@ module Value = struct
     | ArrayLen e'
     | StructGet (_, _, _, e')
     | RefCast (_, e')
-    | RefTest (_, e') -> effect_free e'
+    | RefTest (_, e')
+    | ExternConvertAny e' -> effect_free e'
     | BinOp (_, e1, e2)
     | ArrayNew (_, e1, e2)
     | ArrayNewData (_, _, e1, e2)
@@ -793,15 +811,50 @@ module Memory = struct
        wasm_array_set ~ty:Type.float_array_type (load a) (load i) (unbox_float (load v)))
 
   let bytes_length e =
-    let* ty = Type.string_type in
+    let* ty = Type.bytes_type in
     let* e = wasm_cast ty e in
     return (W.ArrayLen e)
 
   let bytes_get e e' =
-    Value.val_int (wasm_array_get ~ty:Type.string_type e (Value.int_val e'))
+    Value.val_int (wasm_array_get ~ty:Type.bytes_type e (Value.int_val e'))
 
   let bytes_set e e' e'' =
-    wasm_array_set ~ty:Type.string_type e (Value.int_val e') (Value.int_val e'')
+    wasm_array_set ~ty:Type.bytes_type e (Value.int_val e') (Value.int_val e'')
+
+  let string_value e =
+    let* string = Type.string_type in
+    let* e = wasm_struct_get string (wasm_cast string e) 0 in
+    return (W.ExternConvertAny e)
+
+  let string_length e =
+    if Config.Flag.use_js_string ()
+    then
+      let* f =
+        register_import
+          ~import_module:"wasm:js-string"
+          ~name:"length"
+          (Fun { W.params = [ Ref { nullable = true; typ = Extern } ]; result = [ I32 ] })
+      in
+      let* e = string_value e in
+      return (W.Call (f, [ e ]))
+    else bytes_length e
+
+  let string_get e e' =
+    if Config.Flag.use_js_string ()
+    then
+      let* f =
+        register_import
+          ~import_module:"wasm:js-string"
+          ~name:"charCodeAt"
+          (Fun
+             { W.params = [ Ref { nullable = true; typ = Extern }; I32 ]
+             ; result = [ I32 ]
+             })
+      in
+      let* e = string_value e in
+      let* e' = Value.int_val e' in
+      Value.val_int (return (W.Call (f, [ e; e' ])))
+    else bytes_get e e'
 
   let field e idx = wasm_array_get e (Arith.const (Int32.of_int (idx + 1)))
 
@@ -928,6 +981,21 @@ module Constant = struct
     | Const_named of string
     | Mutated
 
+  let translate_js_string s =
+    let* i = register_string s in
+    let* x =
+      let* name = unit_name in
+      register_import
+        ~import_module:
+          (match name with
+          | None -> "strings"
+          | Some name -> name ^ ".strings")
+        ~name:(string_of_int i)
+        (Global { mut = false; typ = Ref { nullable = false; typ = Any } })
+    in
+    let* ty = Type.js_type in
+    return (Const_named ("str_" ^ s), W.StructNew (ty, [ GlobalGet x ]))
+
   let rec translate_rec c =
     match c with
     | Code.Int i -> return (Const, W.RefI31 (Const (I32 (Targetint.to_int32 i))))
@@ -986,38 +1054,29 @@ module Constant = struct
           | Utf (Utf8 s) -> str_js_utf8 s
           | Byte s -> str_js_byte s
         in
-        let* i = register_string s in
-        let* x =
-          let* name = unit_name in
-          register_import
-            ~import_module:
-              (match name with
-              | None -> "strings"
-              | Some name -> name ^ ".strings")
-            ~name:(string_of_int i)
-            (Global { mut = false; typ = Ref { nullable = false; typ = Any } })
-        in
-        let* ty = Type.js_type in
-        return (Const_named ("str_" ^ s), W.StructNew (ty, [ GlobalGet x ]))
+        translate_js_string s
     | String s ->
-        let* ty = Type.string_type in
-        if String.length s >= string_length_threshold
-        then
-          let name = Code.Var.fresh_n "string" in
-          let* () = register_data_segment name s in
-          return
-            ( Mutated
-            , W.ArrayNewData
-                (ty, name, Const (I32 0l), Const (I32 (Int32.of_int (String.length s))))
-            )
+        if Config.Flag.use_js_string ()
+        then translate_js_string (str_js_byte s)
         else
-          let l =
-            String.fold_right
-              ~f:(fun c r -> W.Const (I32 (Int32.of_int (Char.code c))) :: r)
-              s
-              ~init:[]
-          in
-          return (Const_named ("str_" ^ s), W.ArrayNewFixed (ty, l))
+          let* ty = Type.string_type in
+          if String.length s >= string_length_threshold
+          then
+            let name = Code.Var.fresh_n "string" in
+            let* () = register_data_segment name s in
+            return
+              ( Mutated
+              , W.ArrayNewData
+                  (ty, name, Const (I32 0l), Const (I32 (Int32.of_int (String.length s))))
+              )
+          else
+            let l =
+              String.fold_right
+                ~f:(fun c r -> W.Const (I32 (Int32.of_int (Char.code c))) :: r)
+                s
+                ~init:[]
+            in
+            return (Const_named ("str_" ^ s), W.ArrayNewFixed (ty, l))
     | Float f ->
         let* ty = Type.float_type in
         return (Const, W.StructNew (ty, [ Const (F64 f) ]))
diff --git a/compiler/lib-wasm/generate.ml b/compiler/lib-wasm/generate.ml
index d5e590dff2..803c41f91e 100644
--- a/compiler/lib-wasm/generate.ml
+++ b/compiler/lib-wasm/generate.ml
@@ -285,22 +285,29 @@ module Generate (Target : Target_sig.S) = struct
                 seq (Memory.array_set x y z) Value.unit
             | Extern "caml_floatarray_unsafe_set", [ x; y; z ] ->
                 seq (Memory.float_array_set x y z) Value.unit
-            | Extern ("caml_string_unsafe_get" | "caml_bytes_unsafe_get"), [ x; y ] ->
-                Memory.bytes_get x y
-            | Extern ("caml_string_unsafe_set" | "caml_bytes_unsafe_set"), [ x; y; z ] ->
+            | Extern "caml_string_unsafe_get", [ x; y ] -> Memory.string_get x y
+            | Extern "caml_bytes_unsafe_get", [ x; y ] -> Memory.bytes_get x y
+            | Extern "caml_bytes_unsafe_set", [ x; y; z ] ->
                 seq (Memory.bytes_set x y z) Value.unit
-            | Extern ("caml_string_get" | "caml_bytes_get"), [ x; y ] ->
+            | Extern "caml_string_get", [ x; y ] ->
+                seq
+                  (let* cond = Arith.uge (Value.int_val y) (Memory.string_length x) in
+                   instr (W.Br_if (label_index context bound_error_pc, cond)))
+                  (Memory.string_get x y)
+            | Extern "caml_bytes_get", [ x; y ] ->
                 seq
                   (let* cond = Arith.uge (Value.int_val y) (Memory.bytes_length x) in
                    instr (W.Br_if (label_index context bound_error_pc, cond)))
                   (Memory.bytes_get x y)
-            | Extern ("caml_string_set" | "caml_bytes_set"), [ x; y; z ] ->
+            | Extern "caml_bytes_set", [ x; y; z ] ->
                 seq
                   (let* cond = Arith.uge (Value.int_val y) (Memory.bytes_length x) in
                    let* () = instr (W.Br_if (label_index context bound_error_pc, cond)) in
                    Memory.bytes_set x y z)
                   Value.unit
-            | Extern ("caml_ml_string_length" | "caml_ml_bytes_length"), [ x ] ->
+            | Extern "caml_ml_string_length", [ x ] ->
+                Value.val_int (Memory.string_length x)
+            | Extern "caml_ml_bytes_length", [ x ] ->
                 Value.val_int (Memory.bytes_length x)
             | Extern "%int_add", [ x; y ] -> Value.int_add x y
             | Extern "%int_sub", [ x; y ] -> Value.int_sub x y
@@ -776,7 +783,6 @@ module Generate (Target : Target_sig.S) = struct
                     ( Extern
                         ( "caml_string_get"
                         | "caml_bytes_get"
-                        | "caml_string_set"
                         | "caml_bytes_set"
                         | "caml_check_bound"
                         | "caml_check_bound_gen"
diff --git a/compiler/lib-wasm/initialize_locals.ml b/compiler/lib-wasm/initialize_locals.ml
index 5e15235725..c4ecfe8c4e 100644
--- a/compiler/lib-wasm/initialize_locals.ml
+++ b/compiler/lib-wasm/initialize_locals.ml
@@ -46,7 +46,8 @@ let rec scan_expression ctx e =
   | RefCast (_, e')
   | RefTest (_, e')
   | Br_on_cast (_, _, _, e')
-  | Br_on_cast_fail (_, _, _, e') -> scan_expression ctx e'
+  | Br_on_cast_fail (_, _, _, e')
+  | ExternConvertAny e' -> scan_expression ctx e'
   | BinOp (_, e', e'')
   | ArrayNew (_, e', e'')
   | ArrayNewData (_, _, e', e'')
diff --git a/compiler/lib-wasm/link.ml b/compiler/lib-wasm/link.ml
index d043d0833e..fd8b574eb3 100644
--- a/compiler/lib-wasm/link.ml
+++ b/compiler/lib-wasm/link.ml
@@ -388,7 +388,7 @@ let generate_start_function ~to_link ~out_file =
   Filename.gen_file out_file
   @@ fun ch ->
   let context = Generate.start () in
-  Generate.add_init_function ~context ~to_link:("prelude" :: to_link);
+  Generate.add_init_function ~context ~to_link:("wasmoo_prelude" :: to_link);
   Generate.wasm_output ch ~context;
   if times () then Format.eprintf "    generate start: %a@." Timer.print t1
 
@@ -669,11 +669,11 @@ let load_information files =
   match files with
   | [] -> assert false
   | runtime :: other_files ->
-      let build_info, predefined_exceptions, _unit_data =
+      let build_info, predefined_exceptions, unit_data =
         Zip.with_open_in runtime read_info
       in
       ( predefined_exceptions
-      , (runtime, (build_info, []))
+      , (runtime, (build_info, unit_data))
         :: List.map other_files ~f:(fun file ->
                let build_info, _predefined_exceptions, unit_data =
                  Zip.with_open_in file read_info
@@ -775,7 +775,8 @@ let link ~output_file ~linkall ~enable_source_maps ~files =
               || cmo_file
               || linkall
               || unit_info.force_link
-              || not (StringSet.is_empty (StringSet.inter requires unit_info.provides))
+              || (not (StringSet.is_empty (StringSet.inter requires unit_info.provides)))
+              || String.equal unit_name "wasmoo_prelude"
             then
               ( StringSet.diff
                   (StringSet.union unit_info.requires requires)
diff --git a/compiler/lib-wasm/target_sig.ml b/compiler/lib-wasm/target_sig.ml
index 227da6d972..16c05164c0 100644
--- a/compiler/lib-wasm/target_sig.ml
+++ b/compiler/lib-wasm/target_sig.ml
@@ -77,6 +77,10 @@ module type S = sig
 
     val bytes_set : expression -> expression -> expression -> unit Code_generation.t
 
+    val string_length : expression -> expression
+
+    val string_get : expression -> expression -> expression
+
     val box_float : expression -> expression
 
     val unbox_float : expression -> expression
diff --git a/compiler/lib-wasm/wasm_ast.ml b/compiler/lib-wasm/wasm_ast.ml
index a23addc4a2..9ca66d6578 100644
--- a/compiler/lib-wasm/wasm_ast.ml
+++ b/compiler/lib-wasm/wasm_ast.ml
@@ -166,6 +166,7 @@ type expression =
   | Br_on_cast_fail of int * ref_type * ref_type * expression
   | IfExpr of value_type * expression * expression * expression
   | Try of func_type * instruction list * (var * int * value_type) list
+  | ExternConvertAny of expression
 
 and instruction =
   | Drop of expression
diff --git a/compiler/lib-wasm/wasm_output.ml b/compiler/lib-wasm/wasm_output.ml
index febd2c650e..78f38b9ac0 100644
--- a/compiler/lib-wasm/wasm_output.ml
+++ b/compiler/lib-wasm/wasm_output.ml
@@ -658,6 +658,11 @@ end = struct
             output_instruction st ch (Br (l + 1, Some (Pop ty))))
           catches;
         output_byte ch 0X0B
+    | ExternConvertAny e' ->
+        Feature.require gc;
+        output_expression st ch e';
+        output_byte ch 0xFB;
+        output_byte ch 0x1B
 
   and output_instruction st ch i =
     match i with
@@ -871,7 +876,8 @@ end = struct
     | RefCast (_, e')
     | RefTest (_, e')
     | Br_on_cast (_, _, _, e')
-    | Br_on_cast_fail (_, _, _, e') -> expr_function_references e' set
+    | Br_on_cast_fail (_, _, _, e')
+    | ExternConvertAny e' -> expr_function_references e' set
     | BinOp (_, e', e'')
     | ArrayNew (_, e', e'')
     | ArrayNewData (_, _, e', e'')
diff --git a/compiler/lib-wasm/wat_output.ml b/compiler/lib-wasm/wat_output.ml
index 27c2307801..cc332d7d87 100644
--- a/compiler/lib-wasm/wat_output.ml
+++ b/compiler/lib-wasm/wat_output.ml
@@ -457,6 +457,7 @@ let expression_or_instructions ctx st in_function =
                              @ instruction (Wasm_ast.Br (i + 1, Some (Pop ty))))))
                       catches))
         ]
+    | ExternConvertAny e' -> [ List (Atom "extern.convert_any" :: expression e') ]
   and instruction i =
     match i with
     | Drop e -> [ List (Atom "drop" :: expression e) ]
diff --git a/compiler/lib-wasm/wat_preprocess.ml b/compiler/lib-wasm/wat_preprocess.ml
new file mode 100644
index 0000000000..8f72c10d11
--- /dev/null
+++ b/compiler/lib-wasm/wat_preprocess.ml
@@ -0,0 +1,394 @@
+exception Syntax_error of (Lexing.position * Lexing.position) * string
+
+let sign = [%sedlex.regexp? Opt ('+' | '-')]
+
+let digit = [%sedlex.regexp? '0' .. '9']
+
+let hexdigit = [%sedlex.regexp? '0' .. '9' | 'a' .. 'f' | 'A' .. 'F']
+
+let num = [%sedlex.regexp? digit, Star (Opt '_', digit)]
+
+let hexnum = [%sedlex.regexp? hexdigit, Star (Opt '_', hexdigit)]
+
+let uN = [%sedlex.regexp? num | "0x", hexnum]
+
+let sN = [%sedlex.regexp? sign, uN]
+
+let iN = [%sedlex.regexp? uN | sN]
+
+let float = [%sedlex.regexp? num, Opt ('.', Opt num), Opt (('e' | 'E'), sign, num)]
+
+let hexfloat =
+  [%sedlex.regexp? "0x", hexnum, Opt ('.', Opt hexnum), Opt (('p' | 'P'), sign, num)]
+
+let fN = [%sedlex.regexp? sign, (float | hexfloat | "inf" | "nan" | "nan:", hexnum)]
+
+let idchar =
+  [%sedlex.regexp?
+    ( '0' .. '9'
+    | 'A' .. 'Z'
+    | 'a' .. 'z'
+    | '!'
+    | '#'
+    | '$'
+    | '%'
+    | '&'
+    | '\''
+    | '*'
+    | '+'
+    | '-'
+    | '.'
+    | '/'
+    | ':'
+    | '<'
+    | '='
+    | '>'
+    | '?'
+    | '@'
+    | '\\'
+    | '^'
+    | '_'
+    | '`'
+    | '|'
+    | '~' )]
+
+let id = [%sedlex.regexp? '$', Plus idchar]
+
+let linechar = [%sedlex.regexp? Sub (any, (10 | 13))]
+
+let newline = [%sedlex.regexp? 10 | 13 | 13, 10]
+
+let linecomment = [%sedlex.regexp? ";;", Star linechar, (newline | eof)]
+
+let format = [%sedlex.regexp? '\n' | 9]
+
+(*
+let space = [%sedlex.regexp? ' ' | format | comment]
+*)
+let keyword = [%sedlex.regexp? Plus idchar]
+
+let rec comment lexbuf =
+  match%sedlex lexbuf with
+  | ";)" -> ()
+  | "(;" ->
+      comment lexbuf;
+      comment lexbuf
+  | ';' | '(' | Plus (Sub (any, (';' | '('))) -> comment lexbuf
+  | _ ->
+      raise
+        (Syntax_error
+           (Sedlexing.lexing_positions lexbuf, Printf.sprintf "Malformed comment.\n"))
+
+let string_buffer = Buffer.create 256
+
+let rec string lexbuf =
+  match%sedlex lexbuf with
+  | '"' ->
+      Buffer.add_string string_buffer (Sedlexing.Utf8.lexeme lexbuf);
+      let s = Buffer.contents string_buffer in
+      Buffer.clear string_buffer;
+      s
+  | Plus (Sub (any, (0 .. 31 | 0x7f | '"' | '\\')))
+  | "\\t" | "\\n" | "\\r" | "\\'" | "\\\"" | "\\\\"
+  | '\\', hexdigit, hexdigit
+  | "\\u{", hexnum, "}" ->
+      Buffer.add_string string_buffer (Sedlexing.Utf8.lexeme lexbuf);
+      string lexbuf
+  | _ ->
+      raise
+        (Syntax_error
+           (Sedlexing.lexing_positions lexbuf, Printf.sprintf "Malformed string.\n"))
+
+type pos =
+  { loc : Lexing.position
+  ; byte_loc : int
+  }
+
+type token =
+  | LPAREN
+  | RPAREN
+  | ATOM of string
+  | EOF
+
+let locs lexbuf =
+  let loc, loc' = Sedlexing.lexing_positions lexbuf in
+  let byte_loc, byte_loc' = Sedlexing.bytes_loc lexbuf in
+  { loc; byte_loc }, { loc = loc'; byte_loc = byte_loc' }
+
+let rec token lexbuf =
+  match%sedlex lexbuf with
+  | '(' -> LPAREN, locs lexbuf
+  | ')' -> RPAREN, locs lexbuf
+  | uN | sN | fN | keyword -> ATOM (Sedlexing.Utf8.lexeme lexbuf), locs lexbuf
+  | '"' ->
+      let string_start =
+        { loc = Sedlexing.lexing_position_start lexbuf
+        ; byte_loc = Sedlexing.lexeme_bytes_start lexbuf
+        }
+      in
+      Buffer.add_char string_buffer '"';
+      let str = string lexbuf in
+      ( ATOM str
+      , ( string_start
+        , { loc = Sedlexing.lexing_position_curr lexbuf
+          ; byte_loc = Sedlexing.lexeme_bytes_end lexbuf
+          } ) )
+  | newline | linecomment -> token lexbuf
+  | Plus (' ' | '\t') -> token lexbuf
+  | "(;" ->
+      comment lexbuf;
+      token lexbuf
+  | eof -> EOF, locs lexbuf
+  | _ ->
+      raise
+        (Syntax_error (Sedlexing.lexing_positions lexbuf, Printf.sprintf "Syntax error.\n"))
+
+type t =
+  { loc : pos * pos
+  ; desc : desc
+  }
+
+and desc =
+  | Atom of string
+  | List of t list
+
+let rec parse_list lexbuf toplevel start_loc acc =
+  let tok, (loc, loc') = token lexbuf in
+  match tok with
+  | LPAREN ->
+      let lst, loc = parse_list lexbuf false loc [] in
+      parse_list lexbuf toplevel start_loc ({ desc = List lst; loc } :: acc)
+  | RPAREN ->
+      if toplevel
+      then
+        raise
+          (Syntax_error
+             ( Sedlexing.lexing_positions lexbuf
+             , Printf.sprintf "Missing closing parenthesis.\n" ));
+      List.rev acc, (start_loc, loc')
+  | EOF ->
+      if not toplevel
+      then
+        raise
+          (Syntax_error
+             ( Sedlexing.lexing_positions lexbuf
+             , Printf.sprintf "Unexpected end of file.\n" ));
+      List.rev acc, (start_loc, loc')
+  | ATOM s ->
+      parse_list lexbuf toplevel start_loc ({ loc = loc, loc'; desc = Atom s } :: acc)
+
+let parse lexbuf =
+  parse_list
+    lexbuf
+    true
+    { loc = Sedlexing.lexing_position_start lexbuf
+    ; byte_loc = Sedlexing.lexeme_bytes_end lexbuf
+    }
+    []
+
+let report_syntax_error loc msg =
+  let location = MenhirLib.LexerUtil.range loc in
+  Format.eprintf "%s%s%!" location msg;
+  exit 1
+
+module StringMap = Map.Make (String)
+
+type st =
+  { text : string
+  ; mutable pos : pos
+  ; variables : bool StringMap.t
+  ; buf : Buffer.t
+  ; mutable head : int
+  ; head_buf : Buffer.t
+  }
+
+let write st pos' =
+  Buffer.add_substring st.buf st.text st.pos.byte_loc (pos'.byte_loc - st.pos.byte_loc);
+  st.pos <- pos'
+
+let skip st (pos' : pos) =
+  let lines = pos'.loc.pos_lnum - st.pos.loc.pos_lnum in
+  let cols =
+    pos'.loc.pos_cnum
+    - pos'.loc.pos_bol
+    - if lines > 0 then 0 else st.pos.loc.pos_cnum - st.pos.loc.pos_cnum
+  in
+  Buffer.add_string st.buf (String.make lines '\n');
+  Buffer.add_string st.buf (String.make cols ' ');
+  st.pos <- pos'
+
+let pred_position { loc; byte_loc } =
+  { loc = { loc with pos_cnum = loc.pos_cnum - 1 }; byte_loc = byte_loc - 1 }
+
+let eval st cond =
+  match cond with
+  | { desc = Atom s; loc = pos, pos' } ->
+      if not (StringMap.mem s st.variables)
+      then
+        raise
+          (Syntax_error ((pos.loc, pos'.loc), Printf.sprintf "Unknown variable '%s'.\n" s));
+      StringMap.find s st.variables
+  | { loc = pos, pos'; _ } ->
+      raise (Syntax_error ((pos.loc, pos'.loc), Printf.sprintf "Syntax error.\n"))
+
+let rec rewrite_list st l = List.iter (rewrite st) l
+
+and rewrite st elt =
+  match elt with
+  | { desc =
+        List
+          ({ desc = Atom "try"; _ }
+          :: ( { desc = List ({ desc = Atom "result"; _ } :: _); _ }
+               :: { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+                  ; loc = _, pos_after_body
+                  }
+               :: _
+             | { desc = List ({ desc = Atom "do"; loc = _, pos_after_do } :: body)
+               ; loc = _, pos_after_body
+               }
+               :: _ ))
+    ; loc = pos, pos'
+    }
+    when StringMap.find "trap-on-exception" st.variables ->
+      write st pos;
+      Buffer.add_string st.buf "(block";
+      skip st pos_after_do;
+      rewrite_list st body;
+      write st pos_after_body;
+      skip st pos'
+  | { desc = List ({ desc = Atom "throw"; _ } :: _); loc = pos, pos' }
+    when StringMap.find "trap-on-exception" st.variables ->
+      write st pos;
+      Buffer.add_string st.buf "(unreachable)";
+      skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "#if"; _ }
+          ; cond
+          ; { desc = List ({ desc = Atom "#then"; loc = _, pos_after_then } :: then_body)
+            ; loc = _, pos_after_then_body
+            }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      if eval st cond
+      then (
+        skip st pos_after_then;
+        rewrite_list st then_body;
+        write st (pred_position pos_after_then_body);
+        skip st pos')
+      else skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "#if"; _ }
+          ; cond
+          ; { desc = List ({ desc = Atom "#then"; loc = _, pos_after_then } :: then_body)
+            ; loc = _, pos_after_then_body
+            }
+          ; { desc = List ({ desc = Atom "#else"; loc = _, pos_after_else } :: else_body)
+            ; loc = _, pos_after_else_body
+            }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      if eval st cond
+      then (
+        skip st pos_after_then;
+        rewrite_list st then_body;
+        write st (pred_position pos_after_then_body);
+        skip st pos')
+      else (
+        skip st pos_after_else;
+        rewrite_list st else_body;
+        write st (pred_position pos_after_else_body);
+        skip st pos')
+  | { desc =
+        List
+          [ { desc = Atom "#string"; _ }
+          ; { desc = Atom name; _ }
+          ; { desc = Atom value; _ }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      (if
+         StringMap.mem "use-js-string" st.variables
+         && StringMap.find "use-js-string" st.variables
+       then (
+         Printf.bprintf
+           st.head_buf
+           "(import \"\" %s (global %s$string externref)) "
+           value
+           name;
+         Printf.bprintf
+           st.buf
+           "(global %s (ref eq) (struct.new $string (any.convert_extern (global.get \
+            %s$string))))"
+           name
+           name)
+       else
+         let s = String.sub value 1 (String.length value - 2) in
+         Printf.bprintf
+           st.buf
+           "(global %s (ref eq) (array.new_fixed $bytes %d%a))"
+           name
+           (String.length s)
+           (fun f s ->
+             String.iter (fun c -> Printf.bprintf f " (i32.const %d)" (Char.code c)) s)
+           s);
+      skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "#jsstring"; _ }
+          ; { desc = Atom name; _ }
+          ; { desc = Atom value; _ }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      Printf.bprintf
+        st.head_buf
+        "(import \"\" %s (global %s$string externref)) "
+        value
+        name;
+      Printf.bprintf
+        st.buf
+        "(global %s (ref eq) (struct.new $js (any.convert_extern (global.get \
+         %s$string))))"
+        name
+        name;
+      skip st pos'
+  | { desc = List ({ desc = Atom "module"; loc = _, pos } :: _ as l); _ } ->
+      st.head <- pos.byte_loc;
+      rewrite_list st l
+  | { desc = List l; _ } -> rewrite_list st l
+  | _ -> ()
+
+let f ~variables ~filename ~contents:text =
+  let variables = ("trap-on-exception", false) :: variables in
+  let variables =
+    List.fold_left (fun m (k, v) -> StringMap.add k v m) StringMap.empty variables
+  in
+  let lexbuf = Sedlexing.Utf8.from_string text in
+  Sedlexing.set_filename lexbuf filename;
+  try
+    let t, (pos, end_pos) = parse lexbuf in
+    let st =
+      { text
+      ; pos
+      ; variables
+      ; buf = Buffer.create (String.length text)
+      ; head_buf = Buffer.create 128
+      ; head = 0
+      }
+    in
+    rewrite_list st t;
+    write st end_pos;
+    let head = Buffer.contents st.head_buf in
+    let contents = Buffer.contents st.buf in
+    String.sub contents 0 st.head
+    ^ head
+    ^ String.sub contents st.head (String.length contents - st.head)
+  with Syntax_error (loc, msg) -> report_syntax_error loc msg
diff --git a/compiler/lib-wasm/wat_preprocess.mli b/compiler/lib-wasm/wat_preprocess.mli
new file mode 100644
index 0000000000..63e6f287a2
--- /dev/null
+++ b/compiler/lib-wasm/wat_preprocess.mli
@@ -0,0 +1 @@
+val f : variables:(string * bool) list -> filename:string -> contents:string -> string
diff --git a/compiler/lib/config.ml b/compiler/lib/config.ml
index 29f39a1f02..ec6fb742ac 100644
--- a/compiler/lib/config.ml
+++ b/compiler/lib/config.ml
@@ -193,5 +193,7 @@ let target () =
 let set_target (t : [ `JavaScript | `Wasm ]) =
   (match t with
   | `JavaScript -> Targetint.set_num_bits 32
-  | `Wasm -> Targetint.set_num_bits 31);
+  | `Wasm ->
+      Targetint.set_num_bits 31;
+      Flag.disable "use-js-string");
   target_ := (t :> [ `JavaScript | `Wasm | `None ])
diff --git a/compiler/tests-jsoo/dune b/compiler/tests-jsoo/dune
index 26f79c9284..de7d202c4d 100644
--- a/compiler/tests-jsoo/dune
+++ b/compiler/tests-jsoo/dune
@@ -45,7 +45,7 @@
  (libraries unix compiler-libs.common js_of_ocaml-compiler)
  (foreign_stubs
   (language c)
-  (names bigarray_stubs))
+  (names bigarray_stubs jsoo_runtime_stubs))
  (inline_tests
   (modes js wasm best))
  (preprocess
diff --git a/compiler/tests-jsoo/jsoo_runtime_stubs.c b/compiler/tests-jsoo/jsoo_runtime_stubs.c
new file mode 100644
index 0000000000..aa8ff04e0f
--- /dev/null
+++ b/compiler/tests-jsoo/jsoo_runtime_stubs.c
@@ -0,0 +1,7 @@
+
+#include <caml/mlvalues.h>
+
+CAMLprim value caml_jsoo_flags_use_js_string(value v_unit)
+{
+  return Val_false;
+}
diff --git a/compiler/tests-jsoo/test_obj.ml b/compiler/tests-jsoo/test_obj.ml
index 84ea39f0da..269fe2bdc2 100644
--- a/compiler/tests-jsoo/test_obj.ml
+++ b/compiler/tests-jsoo/test_obj.ml
@@ -45,13 +45,10 @@ let%expect_test "is_int" =
 (* https://github.com/ocsigen/js_of_ocaml/issues/666 *)
 (* https://github.com/ocsigen/js_of_ocaml/pull/725 *)
 
+external use_js_string : unit -> bool = "caml_jsoo_flags_use_js_string"
+
 let%expect_test "dup" =
-  let magic = "abcd" in
-  let js_string_enabled =
-    match Sys.backend_type with
-    | Other "js_of_ocaml" -> Array.unsafe_get (Obj.magic magic) 0 == "b"
-    | _ -> false
-  in
+  let js_string_enabled = use_js_string () in
   let s = "Hello" in
   let s' : string = Obj.obj (Obj.dup (Obj.repr s)) in
   print_bool (s = s');
diff --git a/dune b/dune
index 292467727b..7fa05d513d 100644
--- a/dune
+++ b/dune
@@ -4,7 +4,10 @@
    (:standard -w +a-4-40-41-42-44-48-58-66-70))
   (binaries
    (tools/node_wrapper.exe as node)
-   (tools/node_wrapper.exe as node.exe)))
+   (tools/node_wrapper.exe as node.exe))
+  (wasm_of_ocaml
+   (flags
+    (:standard --enable use-js-string))))
  (with-effects
   (js_of_ocaml
    (compilation_mode separate)
diff --git a/runtime/js/marshal.js b/runtime/js/marshal.js
index 7e4aec531a..33676514bb 100644
--- a/runtime/js/marshal.js
+++ b/runtime/js/marshal.js
@@ -863,7 +863,7 @@ var caml_output_val = (function () {
     }
     if (intern_obj_table) writer.obj_counter = intern_obj_table.objs.length;
     writer.finalize();
-    return writer.chunk;
+    return new Uint8Array(writer.chunk);
   };
 })();
 
@@ -880,10 +880,10 @@ function caml_output_value_to_bytes(v, flags) {
 }
 
 //Provides: caml_output_value_to_buffer
-//Requires: caml_output_val, caml_failwith, caml_blit_bytes
+//Requires: caml_output_val, caml_failwith, caml_blit_bytes, caml_bytes_of_array
 function caml_output_value_to_buffer(s, ofs, len, v, flags) {
   var t = caml_output_val(v, flags);
   if (t.length > len) caml_failwith("Marshal.to_buffer: buffer overflow");
-  caml_blit_bytes(t, 0, s, ofs, t.length);
+  caml_blit_bytes(caml_bytes_of_array(t), 0, s, ofs, t.length);
   return 0;
 }
diff --git a/runtime/js/mlBytes.js b/runtime/js/mlBytes.js
index 8566487548..aff366daab 100644
--- a/runtime/js/mlBytes.js
+++ b/runtime/js/mlBytes.js
@@ -78,7 +78,7 @@ function caml_subarray_to_jsbytes(a, i, len) {
   if (i === 0 && len <= 4096 && len === a.length) return f.apply(null, a);
   var s = "";
   for (; 0 < len; i += 1024, len -= 1024)
-    s += f.apply(null, a.slice(i, i + Math.min(len, 1024)));
+    s += f.apply(null, a.subarray(i, i + Math.min(len, 1024)));
   return s;
 }
 
diff --git a/runtime/wasm/args.ml b/runtime/wasm/args.ml
deleted file mode 100644
index 16cd0418a5..0000000000
--- a/runtime/wasm/args.ml
+++ /dev/null
@@ -1,4 +0,0 @@
-let () =
-  for i = 1 to Array.length Sys.argv - 1 do
-    Format.printf "%s@.%s@." Sys.argv.(i) (Filename.chop_suffix Sys.argv.(i) ".wat")
-  done
diff --git a/runtime/wasm/array.wat b/runtime/wasm/array.wat
index 00d01e8082..6c41a84257 100644
--- a/runtime/wasm/array.wat
+++ b/runtime/wasm/array.wat
@@ -20,11 +20,12 @@
       (func $caml_invalid_argument (param (ref eq))))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
 
-   (data $Array_make "Array.make")
+   (#string $Array_make "Array.make")
 
    (global $empty_array (ref eq)
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
@@ -34,10 +35,7 @@
       (local $sz i32) (local $b (ref $block)) (local $f f64)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.lt_s (local.get $sz) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (drop (block $not_float (result (ref eq))
          (local.set $f
@@ -56,10 +54,7 @@
       (local $sz i32) (local $f f64)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.lt_s (local.get $sz) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (local.set $f
          (struct.get $float 0
@@ -73,10 +68,7 @@
       (local $sz i32)
       (local.set $sz (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (if (i32.lt_s (local.get $sz) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $Array_make
-                               (i32.const 0) (i32.const 10)))))
+         (then (call $caml_invalid_argument (global.get $Array_make))))
       (if (i32.eqz (local.get $sz)) (then (return (global.get $empty_array))))
       (array.new $float_array (f64.const 0) (local.get $sz)))
 
diff --git a/runtime/wasm/backtrace.wat b/runtime/wasm/backtrace.wat
index ea4d0e46aa..b90e8f02b4 100644
--- a/runtime/wasm/backtrace.wat
+++ b/runtime/wasm/backtrace.wat
@@ -20,7 +20,8 @@
       (func $caml_invalid_argument (param (ref eq))))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (func (export "caml_get_exception_raw_backtrace")
       (param (ref eq)) (result (ref eq))
@@ -38,14 +39,12 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $raw_backtrace_slot_err
+   (#string $raw_backtrace_slot_err
       "Printexc.get_raw_backtrace_slot: index out of bounds")
 
    (func (export "caml_raw_backtrace_slot")
       (param (ref eq) (ref eq)) (result (ref eq))
-      (call $caml_invalid_argument
-          (array.new_data $string $raw_backtrace_slot_err
-             (i32.const 0) (i32.const 52)))
+      (call $caml_invalid_argument (global.get $raw_backtrace_slot_err))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_convert_raw_backtrace_slot")
diff --git a/runtime/wasm/bigarray.wat b/runtime/wasm/bigarray.wat
index 812055eca7..0c545eb476 100644
--- a/runtime/wasm/bigarray.wat
+++ b/runtime/wasm/bigarray.wat
@@ -69,14 +69,20 @@
    (import "bindings" "ta_subarray"
       (func $ta_subarray
          (param (ref extern)) (param i32) (param i32) (result (ref extern))))
+   (import "bindings" "ta_blit_from_bytes"
+      (func $ta_blit_from_bytes
+         (param (ref $bytes)) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
+   (import "bindings" "ta_blit_to_bytes"
+      (func $ta_blit_to_bytes
+         (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
+         (param i32)))
    (import "bindings" "ta_blit_from_string"
       (func $ta_blit_from_string
-         (param (ref $string)) (param i32) (param (ref extern)) (param i32)
-         (param i32)))
-   (import "bindings" "ta_blit_to_string"
-      (func $ta_blit_to_string
-         (param (ref extern)) (param i32) (param (ref $string)) (param i32)
+         (param anyref) (param i32) (param (ref extern)) (param i32)
          (param i32)))
+   (import "bindings" "ta_to_string"
+      (func $ta_to_string (param (ref extern)) (result (ref any))))
    (import "fail" "caml_bound_error" (func $caml_bound_error))
    (import "fail" "caml_raise_out_of_memory" (func $caml_raise_out_of_memory))
    (import "fail" "caml_invalid_argument"
@@ -124,9 +130,12 @@
       (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
    (import "marshal" "caml_deserialize_int_8"
       (func $caml_deserialize_int_8 (param (ref eq)) (result i64)))
+   (import "jsstring" "jsstring_length"
+      (func $jsstring_length (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
 
@@ -141,7 +150,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -153,7 +162,7 @@
 
    (global $bigarray_ops (export "bigarray_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 9 ;; "_bigarr02"
+         (array.new_fixed $bytes 9 ;; "_bigarr02"
             (i32.const 95) (i32.const 98) (i32.const 105) (i32.const 103)
             (i32.const 97) (i32.const 114) (i32.const 114) (i32.const 48)
             (i32.const 50))
@@ -516,7 +525,7 @@
          (i32.mul (i32.add (i32.const 4) (local.get $num_dims)) (i32.const 4))
          (i32.mul (i32.add (i32.const 4) (local.get $num_dims)) (i32.const 8))))
 
-   (data $intern_overflow
+   (#string $intern_overflow
       "input_value: cannot read bigarray with 64-bit OCaml ints")
 
    (func $bigarray_deserialize
@@ -589,10 +598,7 @@
                (br $done))
               ;; int
               (if (call $caml_deserialize_uint_1 (local.get $s))
-                 (then
-                    (call $caml_failwith
-                       (array.new_data $string $intern_overflow
-                          (i32.const 0) (i32.const 56))))))
+                 (then (call $caml_failwith (global.get $intern_overflow)))))
              ;; int32
              (loop $loop
                 (if (i32.lt_u (local.get $i) (local.get $len))
@@ -701,8 +707,8 @@
 
    (global $CAML_BA_MAX_NUM_DIMS i32 (i32.const 16))
 
-   (data $ba_create_bad_dims "Bigarray.create: bad number of dimensions")
-   (data $ba_create_negative_dim "Bigarray.create: negative dimension")
+   (#string $ba_create_bad_dims "Bigarray.create: bad number of dimensions")
+   (#string $ba_create_negative_dim "Bigarray.create: negative dimension")
 
    (func (export "caml_ba_create")
       (param $vkind (ref eq)) (param $layout (ref eq)) (param $d (ref eq))
@@ -716,8 +722,7 @@
       (if (i32.gt_u (local.get $num_dims) (global.get $CAML_BA_MAX_NUM_DIMS))
          (then
             (call $caml_invalid_argument
-               (array.new_data $string $ba_create_bad_dims
-                  (i32.const 0) (i32.const 41)))))
+               (global.get $ba_create_bad_dims))))
       (local.set $dim
          (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (local.set $i (i32.const 0))
@@ -732,8 +737,7 @@
                (if (i32.lt_s (local.get $n) (i32.const 0))
                   (then
                      (call $caml_invalid_argument
-                        (array.new_data $string $ba_create_negative_dim
-                              (i32.const 0) (i32.const 35)))))
+                        (global.get $ba_create_negative_dim))))
                (array.set $int_array
                   (local.get $dim) (local.get $i) (local.get $n))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -747,8 +751,8 @@
          (local.get $kind)
          (i31.get_s (ref.cast (ref i31) (local.get $layout)))))
 
-   (data $ta_unsupported_kind "Typed_array.to_genarray: unsupported kind")
-   (data $ta_too_large "Typed_array.to_genarray: too large")
+   (#string $ta_unsupported_kind "Typed_array.to_genarray: unsupported kind")
+   (#string $ta_too_large "Typed_array.to_genarray: too large")
 
    (func (export "caml_ba_from_typed_array") (param (ref eq)) (result (ref eq))
       (local $data (ref extern))
@@ -759,18 +763,12 @@
             (ref.as_non_null (extern.convert_any (call $unwrap (local.get 0))))))
       (local.set $kind (call $ta_kind (local.get $data)))
       (if (i32.lt_s (local.get $kind) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $ta_unsupported_kind
-                  (i32.const 0) (i32.const 41)))))
+         (then (call $caml_invalid_argument (global.get $ta_unsupported_kind))))
       (if (i32.eq (local.get $kind) (i32.const 13)) ;; Uint8ClampedArray
          (then (local.set $kind (i32.const 3))))
       (local.set $len (call $ta_length (local.get $data)))
       (if (i32.lt_s (local.get $len) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $ta_too_large
-                  (i32.const 0) (i32.const 34)))))
+         (then (call $caml_invalid_argument (global.get $ta_too_large))))
       (struct.new $bigarray
          (global.get $bigarray_ops)
          (local.get $data)
@@ -944,7 +942,7 @@
          (struct.get $float 0 (ref.cast (ref $float) (local.get $v))))
       (return))
 
-   (data $Bigarray_dim "Bigarray.dim")
+   (#string $Bigarray_dim "Bigarray.dim")
 
    (func $caml_ba_dim (export "caml_ba_dim")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
@@ -955,9 +953,7 @@
             (ref.cast (ref $bigarray) (local.get 0))))
       (local.set $i (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (if (i32.ge_u (local.get $i) (array.len (local.get $dim)))
-         (then (call $caml_invalid_argument
-                  (array.new_data $string $Bigarray_dim
-                     (i32.const 0) (i32.const 12)))))
+         (then (call $caml_invalid_argument (global.get $Bigarray_dim))))
       (ref.i31 (array.get $int_array (local.get $dim) (local.get $i))))
 
    (func (export "caml_ba_dim_1") (param (ref eq)) (result (ref eq))
@@ -1303,7 +1299,7 @@
          (local.get $v))
       (ref.i31 (i32.const 0)))
 
-   (data $too_many_indices "Bigarray.slice: too many indices")
+   (#string $too_many_indices "Bigarray.slice: too many indices")
 
    (func (export "caml_ba_slice")
       (param $vb (ref eq)) (param $vind (ref eq)) (result (ref eq))
@@ -1319,10 +1315,7 @@
       (local.set $num_dims (struct.get $bigarray $ba_num_dims (local.get $b)))
       (if (i32.gt_u (local.get $num_inds)
              (struct.get $bigarray $ba_num_dims (local.get $b)))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $too_many_indices
-                  (i32.const 0) (i32.const 32)))))
+         (then (call $caml_invalid_argument (global.get $too_many_indices))))
       (local.set $sub_dim
          (array.new $int_array (i32.const 0)
             (i32.sub (local.get $num_dims) (local.get $num_inds))))
@@ -1386,7 +1379,7 @@
          (struct.get $bigarray $ba_kind (local.get $b))
          (struct.get $bigarray $ba_layout (local.get $b))))
 
-   (data $bad_subarray "Bigarray.sub: bad sub-array")
+   (#string $bad_subarray "Bigarray.sub: bad sub-array")
 
    (func (export "caml_ba_sub")
       (param $vba (ref eq)) (param $vofs (ref eq)) (param $vlen (ref eq))
@@ -1436,10 +1429,7 @@
              (i32.gt_s (i32.add (local.get $ofs) (local.get $len))
                 (array.get $int_array (local.get $dim)
                    (local.get $changed_dim))))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $bad_subarray
-                  (i32.const 0) (i32.const 27)))))
+         (then (call $caml_invalid_argument (global.get $bad_subarray))))
       (local.set $new_dim
          (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (array.copy $int_array $int_array
@@ -1546,7 +1536,7 @@
           (struct.get $float 0 (ref.cast (ref $float) (local.get $v))))
        (return (ref.i31 (i32.const 0))))
 
-   (data $dim_mismatch "Bigarray.blit: dimension mismatch")
+   (#string $dim_mismatch "Bigarray.blit: dimension mismatch")
 
    (func (export "caml_ba_blit")
       (param $vsrc (ref eq)) (param $vdst (ref eq)) (result (ref eq))
@@ -1560,10 +1550,7 @@
       (local.set $len (struct.get $bigarray $ba_num_dims (local.get $dst)))
       (if (i32.ne (local.get $len)
              (struct.get $bigarray $ba_num_dims (local.get $src)))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $dim_mismatch
-                  (i32.const 0) (i32.const 33)))))
+         (then (call $caml_invalid_argument (global.get $dim_mismatch))))
       (local.set $sdim (struct.get $bigarray $ba_dim (local.get $src)))
       (local.set $ddim (struct.get $bigarray $ba_dim (local.get $dst)))
       (loop $loop
@@ -1573,9 +1560,7 @@
                       (array.get $int_array (local.get $sdim) (local.get $i))
                       (array.get $int_array (local.get $ddim) (local.get $i)))
                   (then
-                     (call $caml_invalid_argument
-                        (array.new_data $string $dim_mismatch
-                           (i32.const 0) (i32.const 33)))))
+                     (call $caml_invalid_argument (global.get $dim_mismatch))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (call $ta_blit
@@ -1583,9 +1568,9 @@
          (struct.get $bigarray $ba_data (local.get $dst)))
      (ref.i31 (i32.const 0)))
 
-   (data $bad_number_dim "Bigarray.reshape: bad number of dimensions")
-   (data $negative_dim "Bigarray.reshape: negative dimension")
-   (data $size_mismatch "Bigarray.reshape: size mismatch")
+   (#string $bad_number_dim "Bigarray.reshape: bad number of dimensions")
+   (#string $negative_dim "Bigarray.reshape: negative dimension")
+   (#string $size_mismatch "Bigarray.reshape: size mismatch")
 
    (func (export "caml_ba_reshape")
       (param $vb (ref eq)) (param $vd (ref eq)) (result (ref eq))
@@ -1598,9 +1583,7 @@
       (local.set $b (ref.cast (ref $bigarray) (local.get $vb)))
       (if (i32.gt_u (local.get $num_dims) (global.get $CAML_BA_MAX_NUM_DIMS))
          (then
-            (call $caml_invalid_argument
-               (array.new_data $string $bad_number_dim
-                  (i32.const 0) (i32.const 42)))))
+            (call $caml_invalid_argument (global.get $bad_number_dim))))
       (local.set $num_elts (i64.const 1))
       (local.set $dim (array.new $int_array (i32.const 0) (local.get $num_dims)))
       (loop $loop
@@ -1613,9 +1596,7 @@
                            (i32.add (local.get $i) (i32.const 1))))))
                (if (i32.lt_s (local.get $d) (i32.const 0))
                   (then
-                     (call $caml_invalid_argument
-                        (array.new_data $string $negative_dim
-                           (i32.const 0) (i32.const 36)))))
+                     (call $caml_invalid_argument (global.get $negative_dim))))
                (array.set $int_array (local.get $dim) (local.get $i)
                   (local.get $d))
                (local.set $num_elts
@@ -1629,10 +1610,7 @@
       (if (i32.ne (i32.wrap_i64 (local.get $num_elts))
              (call $caml_ba_get_size
                 (struct.get $bigarray $ba_dim (local.get $b))))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $size_mismatch
-                  (i32.const 0) (i32.const 31)))))
+         (then (call $caml_invalid_argument (global.get $size_mismatch))))
       (struct.new $bigarray
          (global.get $bigarray_ops)
          (struct.get $bigarray $ba_data (local.get $b))
@@ -2020,29 +1998,44 @@
          (i32.wrap_i64 (i64.shr_u (local.get $d) (i64.const 32))))
       (ref.i31 (i32.const 0)))
 
-   (export "caml_bytes_of_array" (func $caml_string_of_array))
-   (func $caml_string_of_array (export "caml_string_of_array")
+(#if use-js-string
+(#then
+   (func (export "caml_string_of_array")
       (param (ref eq)) (result (ref eq))
       ;; used to convert a typed array to a string
+      (local $a (ref extern))
+      (local.set $a
+         (ref.as_non_null (extern.convert_any (call $unwrap (local.get 0)))))
+      (struct.new $string (call $ta_to_string (local.get $a))))
+)
+(#else
+   (export "caml_string_of_array" (func $caml_bytes_of_array))
+))
+
+   (func $caml_bytes_of_array (export "caml_bytes_of_array")
+      (param (ref eq)) (result (ref eq))
+      ;; used to convert a typed array to bytes
       (local $a (ref extern)) (local $len i32)
-      (local $s (ref $string))
+      (local $s (ref $bytes))
       (local.set $a
          (ref.as_non_null (extern.convert_any (call $unwrap (local.get 0)))))
       (local.set $len (call $ta_length (local.get $a)))
-      (local.set $s (array.new $string (i32.const 0) (local.get $len)))
-      (call $ta_blit_to_string
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
+      (call $ta_blit_to_bytes
          (local.get $a) (i32.const 0) (local.get $s) (i32.const 0)
          (local.get $len))
       (local.get $s))
 
-   (export "caml_uint8_array_of_bytes" (func $caml_uint8_array_of_string))
-   (func $caml_uint8_array_of_string (export "caml_uint8_array_of_string")
+(#if use-js-string
+(#then
+   (func (export "caml_uint8_array_of_string")
       (param (ref eq)) (result (ref eq))
       ;; Convert a string to a typed array
       (local $ta (ref extern)) (local $len i32)
-      (local $s (ref $string))
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
-      (local.set $len (array.len (local.get $s)))
+      (local $s anyref)
+      (local.set $s
+         (struct.get $string 0 (ref.cast (ref $string) (local.get 0))))
+      (local.set $len (call $jsstring_length (local.get $s)))
       (local.set $ta
          (call $ta_create
             (i32.const 3) ;; Uint8Array
@@ -2051,6 +2044,26 @@
          (local.get $s) (i32.const 0) (local.get $ta) (i32.const 0)
          (local.get $len))
       (call $wrap (any.convert_extern (local.get $ta))))
+)
+(#else
+   (export "caml_uint8_array_of_string" (func $caml_uint8_array_of_bytes))
+))
+
+   (func $caml_uint8_array_of_bytes (export "caml_uint8_array_of_bytes")
+      (param (ref eq)) (result (ref eq))
+      ;; Convert bytes to a typed array
+      (local $ta (ref extern)) (local $len i32)
+      (local $s (ref $bytes))
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
+      (local.set $len (array.len (local.get $s)))
+      (local.set $ta
+         (call $ta_create
+            (i32.const 3) ;; Uint8Array
+            (local.get $len)))
+      (call $ta_blit_from_bytes
+         (local.get $s) (i32.const 0) (local.get $ta) (i32.const 0)
+         (local.get $len))
+      (call $wrap (any.convert_extern (local.get $ta))))
 
    (func (export "caml_ba_get_kind") (param (ref eq)) (result i32)
       (struct.get $bigarray $ba_kind (ref.cast (ref $bigarray) (local.get 0))))
@@ -2083,13 +2096,13 @@
 
    (func (export "string_set")
       (param $s externref) (param $i i32) (param $v i32)
-      (array.set $string
-         (ref.cast (ref null $string) (any.convert_extern (local.get $s)))
+      (array.set $bytes
+         (ref.cast (ref null $bytes) (any.convert_extern (local.get $s)))
          (local.get $i) (local.get $v)))
 
    (func (export "string_get")
       (param $s externref) (param $i i32) (result i32)
-      (array.get $string
-         (ref.cast (ref null $string) (any.convert_extern (local.get $s)))
+      (array.get $bytes
+         (ref.cast (ref null $bytes) (any.convert_extern (local.get $s)))
          (local.get $i)))
 )
diff --git a/runtime/wasm/bigstring.wat b/runtime/wasm/bigstring.wat
index 94a9621743..a0dc1f3f8e 100644
--- a/runtime/wasm/bigstring.wat
+++ b/runtime/wasm/bigstring.wat
@@ -48,18 +48,39 @@
       (func $ta_length (param (ref extern)) (result i32)))
    (import "bindings" "ta_bytes"
       (func $ta_bytes (param anyref) (result anyref)))
+   (import "bindings" "ta_blit_from_bytes"
+      (func $ta_blit_from_bytes
+         (param (ref $bytes)) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
+   (import "bindings" "ta_blit_to_bytes"
+      (func $ta_blit_to_bytes
+         (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
+         (param i32)))
    (import "bindings" "ta_blit_from_string"
       (func $ta_blit_from_string
-         (param (ref $string)) (param i32) (param (ref extern)) (param i32)
-         (param i32)))
-   (import "bindings" "ta_blit_to_string"
-      (func $ta_blit_to_string
-         (param (ref extern)) (param i32) (param (ref $string)) (param i32)
+         (param anyref) (param i32) (param (ref extern)) (param i32)
          (param i32)))
    (import "hash" "caml_hash_mix_int"
       (func $caml_hash_mix_int (param i32) (param i32) (result i32)))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
+   (type $js (struct (field anyref)))
+
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
 
    (func (export "caml_hash_mix_bigstring")
       (param $h i32) (param $b (ref eq)) (result i32)
@@ -98,13 +119,13 @@
          (local.set $h (call $caml_hash_mix_int (local.get $h) (local.get $w))))
       (i32.xor (local.get $h) (local.get $len)))
 
-   (data $buffer "buffer")
+   (#jsstring $buffer "buffer")
 
    (func (export "bigstring_to_array_buffer")
       (param $bs (ref eq)) (result (ref eq))
       (return_call $caml_js_get
          (call $caml_ba_to_typed_array (local.get $bs))
-         (array.new_data $string $buffer (i32.const 0) (i32.const 6))))
+         (global.get $buffer)))
 
    (export "bigstring_to_typed_array" (func $caml_ba_to_typed_array))
 
@@ -160,10 +181,16 @@
       (local $i i32) (local $pos1 i32) (local $pos2 i32) (local $len i32)
       (local $c1 i32) (local $c2 i32)
       (local $d1 (ref extern))
-      (local $s2 (ref $string))
+(#if use-js-string
+(#then
+      (local $s2 externref)
+)
+(#else
+      (local $s2 (ref $bytes))
+))
       (local.set $d1 (call $caml_ba_get_data (local.get $s1)))
       (local.set $pos1 (i31.get_s (ref.cast (ref i31) (local.get $vpos1))))
-      (local.set $s2 (ref.cast (ref $string) (local.get $vs2)))
+      (local.set $s2 (call $string_val (local.get $vs2)))
       (local.set $pos2 (i31.get_s (ref.cast (ref i31) (local.get $vpos2))))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
       (loop $loop
@@ -173,7 +200,7 @@
                   (call $ta_get_ui8 (local.get $d1)
                      (i32.add (local.get $pos1) (local.get $i))))
                (local.set $c2
-                  (array.get_u $string (local.get $s2)
+                  (call $string_get (local.get $s2)
                      (i32.add (local.get $pos2) (local.get $i))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $loop (i32.eq (local.get $c1) (local.get $c2)))
@@ -203,22 +230,46 @@
                (br $loop))))
      (ref.i31 (i32.const -1)))
 
+(#if use-js-string
+(#then
+   (func (export "caml_bigstring_blit_string_to_ba")
+      (param $str1 (ref eq)) (param $vpos1 (ref eq))
+      (param $ba2 (ref eq)) (param $vpos2 (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $pos1 i32) (local $pos2 i32) (local $len i32)
+      (local $s1 anyref)
+      (local $d2 (ref extern))
+      (local.set $s1
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $str1))))
+      (local.set $pos1 (i31.get_s (ref.cast (ref i31) (local.get $vpos1))))
+      (local.set $d2 (call $caml_ba_get_data (local.get $ba2)))
+      (local.set $pos2 (i31.get_s (ref.cast (ref i31) (local.get $vpos2))))
+      (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
+      (call $ta_blit_from_string
+         (local.get $s1) (local.get $pos1)
+         (local.get $d2) (local.get $pos2)
+         (local.get $len))
+      (ref.i31 (i32.const 0)))
+)
+(#else
    (export "caml_bigstring_blit_string_to_ba"
       (func $caml_bigstring_blit_bytes_to_ba))
+))
+
    (func $caml_bigstring_blit_bytes_to_ba
       (export "caml_bigstring_blit_bytes_to_ba")
       (param $str1 (ref eq)) (param $vpos1 (ref eq))
       (param $ba2 (ref eq)) (param $vpos2 (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
       (local $pos1 i32) (local $pos2 i32) (local $len i32)
-      (local $s1 (ref $string))
+      (local $s1 (ref $bytes))
       (local $d2 (ref extern))
-      (local.set $s1 (ref.cast (ref $string) (local.get $str1)))
+      (local.set $s1 (ref.cast (ref $bytes) (local.get $str1)))
       (local.set $pos1 (i31.get_s (ref.cast (ref i31) (local.get $vpos1))))
       (local.set $d2 (call $caml_ba_get_data (local.get $ba2)))
       (local.set $pos2 (i31.get_s (ref.cast (ref i31) (local.get $vpos2))))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
-      (call $ta_blit_from_string
+      (call $ta_blit_from_bytes
          (local.get $s1) (local.get $pos1)
          (local.get $d2) (local.get $pos2)
          (local.get $len))
@@ -230,13 +281,13 @@
       (param $vlen (ref eq)) (result (ref eq))
       (local $pos1 i32) (local $pos2 i32) (local $len i32)
       (local $d1 (ref extern))
-      (local $s2 (ref $string))
+      (local $s2 (ref $bytes))
       (local.set $d1 (call $caml_ba_get_data (local.get $ba1)))
       (local.set $pos1 (i31.get_s (ref.cast (ref i31) (local.get $vpos1))))
-      (local.set $s2 (ref.cast (ref $string) (local.get $str2)))
+      (local.set $s2 (ref.cast (ref $bytes) (local.get $str2)))
       (local.set $pos2 (i31.get_s (ref.cast (ref i31) (local.get $vpos2))))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
-      (call $ta_blit_to_string
+      (call $ta_blit_to_bytes
          (local.get $d1) (local.get $pos1)
          (local.get $s2) (local.get $pos2)
          (local.get $len))
diff --git a/runtime/wasm/compare.wat b/runtime/wasm/compare.wat
index affa2a2765..5e4252adf7 100644
--- a/runtime/wasm/compare.wat
+++ b/runtime/wasm/compare.wat
@@ -38,7 +38,8 @@
       (func $jsstring_compare (param anyref) (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
    (type $js (struct (field anyref)))
@@ -62,7 +63,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -161,7 +162,7 @@
    (global $unordered (export "unordered") i32 (i32.const 0x80000000))
 
    (func $compare_strings
-      (param $s1 (ref $string)) (param $s2 (ref $string)) (result i32)
+      (param $s1 (ref $bytes)) (param $s2 (ref $bytes)) (result i32)
       (local $l1 i32) (local $l2 i32) (local $len i32) (local $i i32)
       (local $c1 i32) (local $c2 i32)
       (if (ref.eq (local.get $s1) (local.get $s2))
@@ -175,9 +176,9 @@
          (if (i32.lt_s (local.get $i) (local.get $len))
             (then
                (local.set $c1
-                  (array.get_u $string (local.get $s1) (local.get $i)))
+                  (array.get_u $bytes (local.get $s1) (local.get $i)))
                (local.set $c2
-                  (array.get_u $string (local.get $s2) (local.get $i)))
+                  (array.get_u $bytes (local.get $s2) (local.get $i)))
                (if (i32.ne (local.get $c1) (local.get $c2))
                   (then
                      (if (i32.le_u (local.get $c1) (local.get $c2))
@@ -215,9 +216,9 @@
       (call $clear_compare_stack)
       (local.get $res))
 
-   (data $abstract_value "compare: abstract value")
-   (data $functional_value "compare: functional value")
-   (data $continuation_value "compare: continuation value")
+   (#string $abstract_value "compare: abstract value")
+   (#string $functional_value "compare: functional value")
+   (#string $continuation_value "compare: continuation value")
 
    (func $do_compare_val
       (param $stack (ref $compare_stack))
@@ -228,7 +229,7 @@
       (local $s1 i32) (local $s2 i32)
       (local $f1 f64) (local $f2 f64)
       (local $fa1 (ref $float_array)) (local $fa2 (ref $float_array))
-      (local $str1 (ref $string)) (local $str2 (ref $string))
+      (local $str1 (ref $bytes)) (local $str2 (ref $bytes))
       (local $c1 (ref $custom)) (local $c2 (ref $custom))
       (local $js1 anyref) (local $js2 anyref)
       (local $tuple (tuple (ref eq) (ref eq)))
@@ -394,10 +395,10 @@
                   (br $next_item)))
                (drop (block $v1_not_string (result (ref eq))
                   (local.set $str1
-                     (br_on_cast_fail $v1_not_string (ref eq) (ref $string)
+                     (br_on_cast_fail $v1_not_string (ref eq) (ref $bytes)
                         (local.get $v1)))
                   (local.set $str2
-                      (br_on_cast_fail $heterogeneous (ref eq) (ref $string)
+                      (br_on_cast_fail $heterogeneous (ref eq) (ref $bytes)
                          (local.get $v2)))
                   (local.set $res
                      (call $compare_strings (local.get $str1) (local.get $str2)))
@@ -477,9 +478,7 @@
                      (br_if $next_item (i32.eqz (local.get $res)))
                      (return (local.get $res)))
                   (call $clear_compare_stack)
-                  (call $caml_invalid_argument
-                     (array.new_data $string $abstract_value
-                        (i32.const 0) (i32.const 23)))
+                  (call $caml_invalid_argument (global.get $abstract_value))
                   (ref.i31 (i32.const 0))))
                (drop (block $v1_not_js (result (ref eq))
                   (local.set $js1
@@ -514,8 +513,7 @@
                               (i32.eqz (call $caml_is_closure (local.get $v2)))))
                      (call $clear_compare_stack)
                      (call $caml_invalid_argument
-                        (array.new_data $string $functional_value
-                           (i32.const 0) (i32.const 25)))))
+                        (global.get $functional_value))))
                (if (call $caml_is_continuation (local.get $v1))
                   (then
                      (drop (br_if $heterogeneous(ref.i31 (i32.const 0))
@@ -523,8 +521,7 @@
                                  (call $caml_is_continuation (local.get $v2)))))
                      (call $clear_compare_stack)
                      (call $caml_invalid_argument
-                        (array.new_data $string $continuation_value
-                           (i32.const 0) (i32.const 27)))))
+                        (global.get $continuation_value))))
                (ref.i31 (i32.const 0)))) ;; fall through
             ;; heterogeneous comparison
             (local.set $t1
@@ -550,8 +547,7 @@
                (then
                   (call $clear_compare_stack)
                   (call $caml_invalid_argument
-                     (array.new_data $string $abstract_value
-                        (i32.const 0) (i32.const 23)))))
+                     (global.get $abstract_value))))
             (return (local.get $res)))
          (if (call $compare_stack_is_not_empty (local.get $stack))
             (then
diff --git a/runtime/wasm/custom.wat b/runtime/wasm/custom.wat
index 4d48e9e075..526d36ca62 100644
--- a/runtime/wasm/custom.wat
+++ b/runtime/wasm/custom.wat
@@ -22,11 +22,11 @@
    (import "int64" "int64_ops" (global $int64_ops (ref $custom_operations)))
    (import "bigarray" "bigarray_ops"
       (global $bigarray_ops (ref $custom_operations)))
-   (import "string" "caml_string_equal"
-      (func $caml_string_equal
+   (import "string" "caml_bytes_equal"
+      (func $caml_bytes_equal
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
    (type $compare
       (func (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
    (type $hash
@@ -38,7 +38,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -109,14 +109,14 @@
             (local.get $ops) (global.get $custom_operations))))
 
    (func (export "caml_find_custom_operations")
-      (param $id (ref $string)) (result (ref null $custom_operations))
+      (param $id (ref $bytes)) (result (ref null $custom_operations))
       (local $l (ref null $custom_operations_list))
       (block $not_found
          (local.set $l (br_on_null $not_found (global.get $custom_operations)))
          (loop $loop
             (if (i31.get_u
                    (ref.cast (ref i31)
-                       (call $caml_string_equal (local.get $id)
+                       (call $caml_bytes_equal (local.get $id)
                          (struct.get $custom_operations $id
                             (struct.get $custom_operations_list $ops
                                (local.get $l))))))
diff --git a/runtime/wasm/dune b/runtime/wasm/dune
index 90422c0f9d..d20394961f 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -1,7 +1,9 @@
 (install
  (section lib)
  (package wasm_of_ocaml-compiler)
- (files runtime.wasm runtime.js))
+ (files
+  (glob_files *.wat)
+  runtime.js))
 
 (rule
  (target version-dependent.wat)
@@ -28,57 +30,3 @@
   (< %{ocaml_version} 5.1.0))
  (action
   (copy %{deps} %{target})))
-
-(rule
- (target runtime.wasm)
- (deps runtime.merged.wasm)
- (action
-  (run
-   wasm-opt
-   -g
-   --enable-gc
-   --enable-exception-handling
-   --enable-reference-types
-   --enable-tail-call
-   --enable-strings
-   --enable-multivalue
-   --enable-bulk-memory
-   %{deps}
-   -O3
-   -o
-   %{target})))
-
-(rule
- (target runtime.merged.wasm)
- (deps
-  args
-  (glob_files *.wat))
- (action
-  (progn
-   (bash
-    "which wasm-merge > /dev/null || (echo 'Error: Binaryen tools not found in the PATH'; false)")
-   (bash
-    "wasm-merge --version | grep -q 'version \\(11[89]\\|1[2-9][0-9]\\)' || (echo 'Error: Binaryen version 118 or greater is currently required'; false)")
-   (run
-    wasm-merge
-    -g
-    --enable-gc
-    --enable-exception-handling
-    --enable-reference-types
-    --enable-tail-call
-    --enable-strings
-    --enable-multivalue
-    --enable-bulk-memory
-    %{read-lines:args}
-    -o
-    %{target}))))
-
-(rule
- (target args)
- (deps
-  args.ml
-  (glob_files *.wat))
- (action
-  (with-stdout-to
-   %{target}
-   (run ocaml %{deps}))))
diff --git a/runtime/wasm/effect.wat b/runtime/wasm/effect.wat
index f061f0800b..fe34c789a8 100644
--- a/runtime/wasm/effect.wat
+++ b/runtime/wasm/effect.wat
@@ -38,7 +38,8 @@
       (func $resume_fiber (param externref) (param (ref eq))))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
    (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
    (type $function_3
@@ -118,23 +119,19 @@
             (field $cont (ref $cont))
             (field $next (ref null $fiber)))))
 
-   (data $effect_unhandled "Effect.Unhandled")
+   (#string $effect_unhandled "Effect.Unhandled")
 
    (func $raise_unhandled
       (param $eff (ref eq)) (param (ref eq)) (result (ref eq))
-      (local $effect_unhandled (ref $string))
-      (local.set $effect_unhandled
-         (array.new_data $string $effect_unhandled
-            (i32.const 0) (i32.const 16)))
+      (local $effect_unhandled (ref $bytes))
       (block $null
          (call $caml_raise_with_arg
             (br_on_null $null
-               (call $caml_named_value
-                  (local.get $effect_unhandled)))
+               (call $caml_named_value (global.get $effect_unhandled)))
             (local.get $eff)))
       (call $caml_raise_constant
          (array.new_fixed $block 3 (ref.i31 (i32.const 248))
-            (local.get $effect_unhandled)
+            (global.get $effect_unhandled)
             (call $caml_fresh_oo_id (ref.i31 (i32.const 0)))))
       (ref.i31 (i32.const 0)))
 
@@ -215,7 +212,7 @@
       (return_call_ref $cont_func (local.get $p) (local.get $k)
          (struct.get $cont $cont_func (local.get $k))))
 
-   (data $already_resumed "Effect.Continuation_already_resumed")
+   (#string $already_resumed "Effect.Continuation_already_resumed")
 
    (func (export "%resume")
       (param $stack (ref eq)) (param $f (ref eq)) (param $v (ref eq))
@@ -227,8 +224,7 @@
             (call $caml_raise_constant
                (ref.as_non_null
                   (call $caml_named_value
-                     (array.new_data $string $already_resumed
-                        (i32.const 0) (i32.const 35)))))))
+                     (global.get $already_resumed))))))
       (return_call $capture_continuation
          (ref.func $do_resume)
           (struct.new $pair
@@ -638,9 +634,7 @@
          (return (local.get $k))))
       (call $caml_raise_constant
          (ref.as_non_null
-            (call $caml_named_value
-               (array.new_data $string $already_resumed
-                  (i32.const 0) (i32.const 35)))))
+            (call $caml_named_value (global.get $already_resumed))))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_perform_effect")
diff --git a/runtime/wasm/fail.wat b/runtime/wasm/fail.wat
index e3dc000d55..ae69a113b5 100644
--- a/runtime/wasm/fail.wat
+++ b/runtime/wasm/fail.wat
@@ -21,7 +21,8 @@
    (import "bindings" "jstag" (tag $javascript_exception (param externref)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (tag $ocaml_exception (export "ocaml_exception") (param (ref eq)))
    (export "javascript_exception" (tag $javascript_exception))
@@ -60,7 +61,7 @@
        (return_call $caml_raise_with_arg
            (array.get $block (global.get $caml_global_data)
               (global.get $FAILURE_EXN))
-           (local.get 0)))
+           (local.get $arg)))
 
    (global $INVALID_EXN i32 (i32.const 3))
 
@@ -71,12 +72,10 @@
               (global.get $INVALID_EXN))
            (local.get 0)))
 
-   (data $index_out_of_bounds "index out of bounds")
+   (#string $index_out_of_bounds "index out of bounds")
 
    (func (export "caml_bound_error")
-      (return_call $caml_invalid_argument
-         (array.new_data $string $index_out_of_bounds
-            (i32.const 0) (i32.const 19))))
+      (return_call $caml_invalid_argument (global.get $index_out_of_bounds)))
 
    (global $END_OF_FILE_EXN i32 (i32.const 4))
 
diff --git a/runtime/wasm/float.wat b/runtime/wasm/float.wat
index 50e0b200d0..92de6d33d0 100644
--- a/runtime/wasm/float.wat
+++ b/runtime/wasm/float.wat
@@ -16,7 +16,6 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "bindings" "format_float"
       (func $format_float
          (param i32) (param i32) (param i32) (param f64) (result anyref)))
@@ -28,13 +27,36 @@
       (func $caml_invalid_argument (param (ref eq))))
    (import "ints" "lowercase_hex_table"
       (global $lowercase_hex_table (ref $chars)))
-   (import "jsstring" "jsstring_of_string"
-      (func $jsstring_of_string (param (ref $string)) (result anyref)))
-   (import "jsstring" "string_of_jsstring"
-      (func $string_of_jsstring (param anyref) (result (ref $string))))
+   (import "jsstring" "jsstring_of_bytes"
+      (func $jsstring_of_bytes (param (ref $bytes)) (result anyref)))
+   (import "jsstring" "bytes_of_jsstring"
+      (func $bytes_of_jsstring (param anyref) (result (ref $bytes))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
 
-   (type $float (struct (field f64)))
-   (type $string (array (mut i8)))
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_length (param $s (ref $bytes)) (result i32)
+      (array.len (local.get $s)))
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
+
+  (type $float (struct (field f64)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $block (array (mut (ref eq))))
 
    (type $chars (array i8))
@@ -55,7 +77,7 @@
       (local $b i64) (local $prec i32) (local $style i32)
       (local $sign i32) (local $exp i32) (local $m i64)
       (local $i i32) (local $j i32) (local $d i32) (local $txt (ref $chars))
-      (local $len i32) (local $s (ref $string))
+      (local $len i32) (local $s (ref $bytes))
       (local $unit i64) (local $half i64) (local $mask i64) (local $frac i64)
       (local.set $prec (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (local.set $style (i31.get_s (ref.cast (ref i31) (local.get 2))))
@@ -73,7 +95,7 @@
          (i32.or (local.get $sign)
             (i32.ne (local.get $style) (i32.const 45)))) ;; '-'
       (local.set $s
-         (block $sign (result (ref $string))
+         (block $sign (result (ref $bytes))
             (if (i32.eq (local.get $exp) (i32.const 0x7FF))
                (then
                   (local.set $txt
@@ -84,9 +106,9 @@
                            (global.get $nan))))
                   (local.set $len (array.len (local.get $txt)))
                   (local.set $s
-                     (array.new $string (i32.const 0)
+                     (array.new $bytes (i32.const 0)
                         (i32.add (local.get $i) (local.get $len))))
-                  (array.copy $string $chars
+                  (array.copy $bytes $chars
                      (local.get $s) (local.get $i) (local.get $txt) (i32.const 0)
                      (local.get $len))
                   (br $sign (local.get $s))))
@@ -144,37 +166,37 @@
                                   (i32.add (i32.const 6) (local.get $j))))
          (if (i32.eqz (local.get $prec))
             (then (local.set $len (i32.sub (local.get $len) (i32.const 1)))))
-         (local.set $s (array.new $string (i32.const 0) (local.get $len)))
+         (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
          (if (i32.ge_s (local.get $exp) (i32.const 0))
             (then (local.set $d (local.get $exp)))
             (else (local.set $d (i32.sub (i32.const 0) (local.get $exp)))))
          (loop $write
             (local.set $len (i32.sub (local.get $len) (i32.const 1)))
-            (array.set $string (local.get $s) (local.get $len)
+            (array.set $bytes (local.get $s) (local.get $len)
                (i32.add (i32.const 48)
                   (i32.rem_u (local.get $d) (i32.const 10))))
             (local.set $d (i32.div_u (local.get $d) (i32.const 10)))
             (br_if $write (local.get $d)))
-         (array.set $string (local.get $s)
+         (array.set $bytes (local.get $s)
             (i32.sub (local.get $len) (i32.const 1))
             (select (i32.const 43) (i32.const 45)
                (i32.ge_s (local.get $exp) (i32.const 0))))
-         (array.set $string (local.get $s) (local.get $i) (i32.const 48)) ;; '0'
-         (array.set $string (local.get $s) (i32.add (local.get $i) (i32.const 1))
+         (array.set $bytes (local.get $s) (local.get $i) (i32.const 48)) ;; '0'
+         (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 1))
             (i32.const 120)) ;; 'x'
-         (array.set $string (local.get $s) (i32.add (local.get $i) (i32.const 2))
+         (array.set $bytes (local.get $s) (i32.add (local.get $i) (i32.const 2))
             (i32.add
                (i32.wrap_i64 (i64.shr_u (local.get $m) (i64.const 52)))
                (i32.const 48))) ;; '0'
          (local.set $i (i32.add (local.get $i) (i32.const 3)))
          (if (i32.gt_s (local.get $prec) (i32.const 0))
             (then
-               (array.set $string (local.get $s) (local.get $i)
+               (array.set $bytes (local.get $s) (local.get $i)
                   (i32.const 46)) ;; '.'
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (local.set $frac (i64.shl (local.get $m) (i64.const 12)))
                (loop $write
-                  (array.set $string (local.get $s) (local.get $i)
+                  (array.set $bytes (local.get $s) (local.get $i)
                      (array.get_u $chars (global.get $lowercase_hex_table)
                         (i32.wrap_i64
                            (i64.shr_u (local.get $frac) (i64.const 60)))))
@@ -182,35 +204,43 @@
                   (local.set $prec (i32.sub (local.get $prec) (i32.const 1)))
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (br_if $write (i32.gt_s (local.get $prec) (i32.const 0))))))
-         (array.set $string (local.get $s) (local.get $i) (i32.const 112))
+         (array.set $bytes (local.get $s) (local.get $i) (i32.const 112))
          (local.get $s)))
       (if (local.get $sign)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45))) ;; '-'
          (else
             (if (i32.ne (local.get $style) (i32.const 45)) ;; '-'
                (then
-                  (array.set $string (local.get $s) (i32.const 0)
+                  (array.set $bytes (local.get $s) (i32.const 0)
                      (local.get $style))))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
-   (data $format_error "format_float: bad format")
+   (#string $format_error "format_float: bad format")
 
    (func $parse_format
-      (param $s (ref $string)) (result i32 i32 i32 i32)
+      (param $v (ref eq)) (result i32 i32 i32 i32)
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $i i32) (local $len i32) (local $c i32)
       (local $sign_style i32) (local $precision i32)
       (local $conversion i32) (local $uppercase i32)
-      (local.set $len (array.len (local.get $s)))
+      (local.set $s (call $string_val (local.get $v)))
+      (local.set $len (call $string_length (local.get $s)))
       (local.set $i (i32.const 1))
       (block $return
          (block $bad_format
             (br_if $bad_format (i32.lt_u (local.get $len) (i32.const 2)))
             (br_if $bad_format
-               (i32.ne (array.get_u $string (local.get $s) (i32.const 0))
+               (i32.ne (call $string_get (local.get $s) (i32.const 0))
                        (i32.const 37))) ;; '%'
-            (local.set $c (array.get_u $string (local.get $s) (i32.const 1)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 1)))
             (if (i32.eq (local.get $c) (i32.const 43)) ;; '+'
                (then
                   (local.set $sign_style (i32.const 1))
@@ -221,13 +251,13 @@
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))))
             (br_if $bad_format (i32.eq (local.get $i) (local.get $len)))
             (br_if $bad_format
-               (i32.ne (array.get_u $string (local.get $s) (local.get $i))
+               (i32.ne (call $string_get (local.get $s) (local.get $i))
                        (i32.const 46))) ;; '.'
             (loop $precision
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $bad_format (i32.eq (local.get $i) (local.get $len)))
                (local.set $c
-                  (array.get_u $string (local.get $s) (local.get $i)))
+                  (call $string_get (local.get $s) (local.get $i)))
                (if (i32.and (i32.ge_u (local.get $c) (i32.const 48))  ;; '0'
                             (i32.le_u (local.get $c) (i32.const 57))) ;; '9'
                   (then
@@ -243,9 +273,7 @@
                   (i32.and (local.get $c) (i32.const 0xdf))
                   (i32.const 69))) ;; 'E'
             (br_if $return (i32.le_u (local.get $conversion) (i32.const 2))))
-         (call $caml_invalid_argument
-            (array.new_data $string $format_error
-               (i32.const 0) (i32.const 22))))
+         (call $caml_invalid_argument (global.get $format_error)))
       (tuple.make 4
          (local.get $sign_style)
          (local.get $precision)
@@ -264,12 +292,11 @@
       (local $negative i32)
       (local $exp i32) (local $m i64)
       (local $i i32) (local $len i32) (local $c i32)
-      (local $s (ref $string)) (local $txt (ref $chars))
+      (local $s (ref $bytes)) (local $txt (ref $chars))
       (local $num anyref)
       (local.set $f (struct.get $float 0 (ref.cast (ref $float) (local.get 1))))
       (local.set $b (i64.reinterpret_f64 (local.get $f)))
-      (local.set $format
-         (call $parse_format (ref.cast (ref $string) (local.get 0))))
+      (local.set $format (call $parse_format (local.get 0)))
       (local.set $sign_style (tuple.extract 4 0 (local.get $format)))
       (local.set $precision (tuple.extract 4 1 (local.get $format)))
       (local.set $conversion (tuple.extract 4 2 (local.get $format)))
@@ -280,7 +307,7 @@
          (i32.or (local.get $negative)
                  (i32.ne (local.get $sign_style) (i32.const 0))))
       (local.set $s
-         (block $sign (result (ref $string))
+         (block $sign (result (ref $bytes))
             (local.set $exp
                 (i32.and (i32.wrap_i64 (i64.shr_u (local.get $b) (i64.const 52)))
                          (i32.const 0x7FF)))
@@ -298,9 +325,9 @@
                            (global.get $nan))))
                   (local.set $len (array.len (local.get $txt)))
                   (local.set $s
-                     (array.new $string (i32.const 0)
+                     (array.new $bytes (i32.const 0)
                         (i32.add (local.get $i) (local.get $len))))
-                  (array.copy $string $chars
+                  (array.copy $bytes $chars
                      (local.get $s) (local.get $i) (local.get $txt) (i32.const 0)
                      (local.get $len))
                   (br $sign (local.get $s))))
@@ -309,52 +336,60 @@
                   (local.get $precision) (local.get $conversion)
                   (local.get $i)
                   (f64.abs (local.get $f))))
-            (local.set $s (call $string_of_jsstring (local.get $num)))
+            (local.set $s (call $bytes_of_jsstring (local.get $num)))
             (br $sign (local.get $s))))
       (if (local.get $negative)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45))) ;; '-'
          (else
             (if (local.get $sign_style)
                (then
                   (if (i32.eq (local.get $sign_style) (i32.const 1))
                      (then
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 43))) ;; '+'
                      (else
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 32)))))))) ;; ' '
       (if (local.get $uppercase)
          (then
             (local.set $i (i32.const 0))
             (local.set $len (array.len (local.get $s)))
             (loop $uppercase
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+               (local.set $c (array.get_u $bytes (local.get $s) (local.get $i)))
                (if (i32.and (i32.ge_u (local.get $c) (i32.const 97))   ;; 'a'
                             (i32.le_u (local.get $c) (i32.const 122))) ;; 'z'
                   (then
-                     (array.set $string (local.get $s) (local.get $i)
+                     (array.set $bytes (local.get $s) (local.get $i)
                         (i32.sub (local.get $c) (i32.const 32)))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $uppercase (i32.lt_u (local.get $i) (local.get $len))))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
-   (data $float_of_string "float_of_string")
+   (#string $float_of_string "float_of_string")
 
-   (func $caml_float_of_hex (param $s (ref $string)) (param $i i32) (result f64)
+   (func $caml_float_of_hex
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (result f64)
       (local $len i32) (local $c i32) (local $d i32) (local $m i64)
       (local $f f64) (local $negative i32)
       (local $dec_point i32) (local $exp i32) (local $adj i32)
       (local $n_bits i32) (local $m_bits i32) (local $x_bits i32)
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (local.set $dec_point (i32.const -1))
       (block $error
          (loop $parse
             (if (i32.lt_u (local.get $i) (local.get $len))
                (then
                   (local.set $c
-                     (array.get_u $string (local.get $s) (local.get $i)))
+                     (call $string_get (local.get $s) (local.get $i)))
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (if (i32.eq (local.get $c) (i32.const 46)) ;; '.'
                      (then
@@ -367,7 +402,7 @@
                      (then
                         (br_if $error (i32.eq (local.get $i) (local.get $len)))
                         (local.set $c
-                           (array.get_u $string (local.get $s) (local.get $i)))
+                           (call $string_get (local.get $s) (local.get $i)))
                         (local.set $i (i32.add (local.get $i) (i32.const 1)))
                         (if (i32.eq (local.get $c) (i32.const 45)) ;; '-'
                            (then
@@ -375,7 +410,7 @@
                               (br_if $error
                                  (i32.eq (local.get $i) (local.get $len)))
                               (local.set $c
-                                 (array.get_u $string
+                                 (call $string_get
                                     (local.get $s) (local.get $i)))
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))))
@@ -384,7 +419,7 @@
                               (br_if $error
                                  (i32.eq (local.get $i) (local.get $len)))
                               (local.set $c
-                                 (array.get_u $string
+                                 (call $string_get
                                     (local.get $s) (local.get $i)))
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))))
@@ -404,7 +439,7 @@
                               (if (i32.ne (local.get $i) (local.get $len))
                                  (then
                                     (local.set $c
-                                       (array.get_u $string
+                                       (call $string_get
                                           (local.get $s) (local.get $i)))
                                     (local.set $i
                                        (i32.add (local.get $i) (i32.const 1)))
@@ -479,28 +514,42 @@
          (if (local.get $exp)
             (then (local.set $f (call $ldexp (local.get $f) (local.get $exp)))))
          (return (local.get $f)))
-      (call $caml_failwith
-         (array.new_data $string $float_of_string (i32.const 0) (i32.const 15)))
+      (call $caml_failwith (global.get $float_of_string))
       (f64.const 0))
 
-   (func $on_whitespace (param $s (ref $string)) (param $i i32) (result i32)
+   (func $on_whitespace
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (result i32)
       (local $c i32)
-      (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+      (local.set $c (call $string_get (local.get $s) (local.get $i)))
       (i32.or (i32.eq (local.get $c) (i32.const 32)) ;; ' '
          (i32.le_u (i32.sub (local.get $c) (i32.const 9)) (i32.const 4))))
 
    (func (export "caml_float_of_string") (param (ref eq)) (result (ref eq))
-      (local $s (ref $string)) (local $len i32) (local $i i32) (local $j i32)
-      (local $s' (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
+      (local $len i32) (local $i i32) (local $j i32)
+      (local $s' (ref $bytes))
       (local $negative i32) (local $c i32)
       (local $f f64)
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $s (call $string_val (local.get 0)))
+      (local.set $len (call $string_length (local.get $s)))
       (loop $count
          (if (i32.lt_u (local.get $i) (local.get $len))
             (then
                (if (i32.eq (i32.const 95) ;; '_'
-                      (array.get_u $string (local.get $s) (local.get $i)))
+                      (call $string_get (local.get $s) (local.get $i)))
                   (then
                      (local.set $j (i32.add (local.get $j) (i32.const 1)))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -508,7 +557,7 @@
       (if (local.get $j)
          (then
             (local.set $s'
-               (array.new $string (i32.const 0)
+               (array.new $bytes (i32.const 0)
                   (i32.sub (local.get $len) (local.get $j))))
             (local.set $i (i32.const 0))
             (local.set $j (i32.const 0))
@@ -516,17 +565,25 @@
                (if (i32.lt_u (local.get $i) (local.get $len))
                   (then
                      (local.set $c
-                        (array.get_u $string (local.get $s) (local.get $i)))
+                        (call $string_get (local.get $s) (local.get $i)))
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (if (i32.ne (local.get $c) (i32.const 95)) ;; '_'
                         (then
-                           (array.set $string (local.get $s')
+                           (array.set $bytes (local.get $s')
                               (local.get $j) (local.get $c))
                            (local.set $j
                               (i32.add (local.get $j) (i32.const 1)))))
                      (br $copy))))
             (local.set $len (array.len (local.get $s')))
-            (local.set $s (local.get $s'))))
+(#if use-js-string
+(#then
+            (local.set $s
+               (extern.convert_any (call $jsstring_of_bytes (local.get $s'))))
+)
+(#else
+            (local.set $s (local.get $s'))
+))
+         ))
       (local.set $i (i32.const 0))
       (loop $skip_spaces
          (if (i32.lt_u (local.get $i) (local.get $len))
@@ -540,7 +597,7 @@
          (br_if $error
             (call $on_whitespace
                (local.get $s) (i32.sub (local.get $len) (i32.const 1))))
-         (local.set $c (array.get_u $string (local.get $s) (i32.const 0)))
+         (local.set $c (call $string_get (local.get $s) (i32.const 0)))
          (if (i32.eq (local.get $c) (i32.const 45)) ;; '-'
             (then
                (local.set $negative (i32.const 1))
@@ -550,11 +607,11 @@
                (local.set $i (i32.const 1))))
          (if (i32.lt_u (i32.add (local.get $i) (i32.const 2)) (local.get $len))
             (then
-               (if (i32.eq (array.get_u $string (local.get $s) (local.get $i))
+               (if (i32.eq (call $string_get (local.get $s) (local.get $i))
                            (i32.const 48)) ;; '0'
                   (then
                      (if (i32.eq (i32.and
-                                    (array.get_u $string (local.get $s)
+                                    (call $string_get (local.get $s)
                                        (i32.add (local.get $i) (i32.const 1)))
                                     (i32.const 0xdf))
                                  (i32.const 88)) ;; 'X'
@@ -567,17 +624,17 @@
                            (return (struct.new $float (local.get $f)))))))))
          (if (i32.eq (i32.add (local.get $i) (i32.const 3)) (local.get $len))
             (then
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+               (local.set $c (call $string_get (local.get $s) (local.get $i)))
                (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                            (i32.const 78)) (then ;; 'N'
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $string (local.get $s) (local.get $i)))
+                     (call $string_get (local.get $s) (local.get $i)))
                   (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                               (i32.const 65)) (then ;; 'A'
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $string (local.get $s) (local.get $i)))
+                        (call $string_get (local.get $s) (local.get $i)))
                      (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                  (i32.const 78)) ;; 'N'
                         (then
@@ -587,12 +644,12 @@
                            (i32.const 73)) (then ;; 'I'
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $string (local.get $s) (local.get $i)))
+                     (call $string_get (local.get $s) (local.get $i)))
                   (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                               (i32.const 78)) (then ;; 'N'
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $string (local.get $s) (local.get $i)))
+                        (call $string_get (local.get $s) (local.get $i)))
                      (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                  (i32.const 70)) ;; 'F'
                         (then
@@ -604,34 +661,34 @@
                                     (local.get $negative))))))))))))
          (if (i32.eq (i32.add (local.get $i) (i32.const 8)) (local.get $len))
             (then
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+               (local.set $c (call $string_get (local.get $s) (local.get $i)))
                (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                            (i32.const 73)) (then ;; 'I'
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $string (local.get $s) (local.get $i)))
+                     (call $string_get (local.get $s) (local.get $i)))
                   (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                               (i32.const 78)) (then ;; 'N'
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $string (local.get $s) (local.get $i)))
+                        (call $string_get (local.get $s) (local.get $i)))
                      (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                  (i32.const 70)) (then ;; 'F'
                         (local.set $i (i32.add (local.get $i) (i32.const 1)))
                         (local.set $c
-                           (array.get_u $string (local.get $s) (local.get $i)))
+                           (call $string_get (local.get $s) (local.get $i)))
                         (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                     (i32.const 73)) (then ;; 'I'
                            (local.set $i (i32.add (local.get $i) (i32.const 1)))
                            (local.set $c
-                              (array.get_u $string
+                              (call $string_get
                                  (local.get $s) (local.get $i)))
                            (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                        (i32.const 78)) (then ;; 'N'
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))
                               (local.set $c
-                                 (array.get_u $string
+                                 (call $string_get
                                      (local.get $s) (local.get $i)))
                               (if (i32.eq
                                      (i32.and (local.get $c) (i32.const 0xdf))
@@ -639,7 +696,7 @@
                                  (local.set $i
                                     (i32.add (local.get $i) (i32.const 1)))
                                  (local.set $c
-                                    (array.get_u $string
+                                    (call $string_get
                                        (local.get $s) (local.get $i)))
                                  (if (i32.eq
                                         (i32.and (local.get $c) (i32.const 0xdf))
@@ -647,7 +704,7 @@
                                     (local.set $i
                                        (i32.add (local.get $i) (i32.const 1)))
                                     (local.set $c
-                                       (array.get_u $string
+                                       (call $string_get
                                            (local.get $s) (local.get $i)))
                                     (if (i32.eq
                                            (i32.and (local.get $c)
@@ -661,11 +718,18 @@
                                                 (local.get $negative))))
                                        ))))))))))))))))))
          (local.set $f
-            (call $parse_float (call $jsstring_of_string (local.get $s))))
+            (call $parse_float
+(#if use-js-string
+(#then
+               (any.convert_extern (local.get $s))
+)
+(#else
+               (call $jsstring_of_bytes (local.get $s))
+))
+            ))
          (br_if $error (f64.ne (local.get $f) (local.get $f)))
          (return (struct.new $float (local.get $f))))
-      (call $caml_failwith
-         (array.new_data $string $float_of_string (i32.const 0) (i32.const 15)))
+      (call $caml_failwith (global.get $float_of_string))
       (return (ref.i31 (i32.const 0))))
 
    (func (export "caml_nextafter_float")
diff --git a/runtime/wasm/fs.wat b/runtime/wasm/fs.wat
index f01d5612d0..203f90c8ae 100644
--- a/runtime/wasm/fs.wat
+++ b/runtime/wasm/fs.wat
@@ -16,7 +16,6 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "jslib" "log_str" (func $log_str (param (ref $string))))
    (import "bindings" "getcwd" (func $getcwd (result anyref)))
    (import "bindings" "chdir" (func $chdir (param anyref)))
    (import "bindings" "mkdir" (func $mkdir (param anyref) (param i32)))
@@ -42,8 +41,11 @@
       (tag $javascript_exception (param externref)))
    (import "sys" "caml_handle_sys_error"
       (func $caml_handle_sys_error (param externref)))
+   (import "string" "caml_string_concat"
+      (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (func (export "caml_sys_getcwd")
       (param (ref eq)) (result (ref eq))
@@ -109,23 +111,12 @@
       (return_call $file_exists
          (call $unwrap (call $caml_jsstring_of_string (local.get $name)))))
 
-   (data $no_such_file ": No such file or directory")
-
-   (func $caml_raise_no_such_file (param $vname (ref eq))
-      (local $name (ref $string)) (local $msg (ref $string))
-      (local $len i32)
-      (local.set $name (ref.cast (ref $string) (local.get $vname)))
-      (local.set $len (array.len (local.get $name)))
-      (local.set $msg
-         (array.new $string (i32.const 0)
-            (i32.add (local.get $len) (i32.const 27))))
-      (array.copy $string $string
-         (local.get $msg) (i32.const 0)
-         (local.get $name) (i32.const 0)
-         (local.get $len))
-      (array.init_data $string $no_such_file
-         (local.get $msg) (local.get $len) (i32.const 0) (i32.const 27))
-      (call $caml_raise_sys_error (local.get $msg)))
+   (#string $no_such_file ": No such file or directory")
+
+   (func $caml_raise_no_such_file (param $name (ref eq))
+      (call $caml_raise_sys_error
+         (call $caml_string_concat
+            (local.get $name) (global.get $no_such_file))))
 
    (data $caml_read_file_content "caml_read_file_content")
 
diff --git a/runtime/wasm/hash.wat b/runtime/wasm/hash.wat
index 543df44919..c6c5630e76 100644
--- a/runtime/wasm/hash.wat
+++ b/runtime/wasm/hash.wat
@@ -24,7 +24,8 @@
       (func $jsstring_hash (param i32) (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $js (struct (field anyref)))
 
@@ -39,7 +40,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -109,8 +110,8 @@
          (then (local.set $i (i32.const 0))))
       (return_call $caml_hash_mix_int (local.get $h) (local.get $i)))
 
-   (func $caml_hash_mix_string (export "caml_hash_mix_string")
-      (param $h i32) (param $s (ref $string)) (result i32)
+   (func $caml_hash_mix_bytes
+      (param $h i32) (param $s (ref $bytes)) (result i32)
       (local $i i32) (local $len i32) (local $w i32)
       (local.set $len (array.len (local.get $s)))
       (local.set $i (i32.const 0))
@@ -122,15 +123,15 @@
                      (local.get $h)
                      (i32.or
                         (i32.or
-                           (array.get_u $string (local.get $s) (local.get $i))
-                           (i32.shl (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s) (local.get $i))
+                           (i32.shl (array.get_u $bytes (local.get $s)
                                        (i32.add (local.get $i) (i32.const 1)))
                                     (i32.const 8)))
                         (i32.or
-                           (i32.shl (array.get_u $string (local.get $s)
+                           (i32.shl (array.get_u $bytes (local.get $s)
                                        (i32.add (local.get $i) (i32.const 2)))
                                     (i32.const 16))
-                           (i32.shl (array.get_u $string (local.get $s)
+                           (i32.shl (array.get_u $bytes (local.get $s)
                                        (i32.add (local.get $i) (i32.const 3)))
                                     (i32.const 24))))))
                (local.set $i (i32.add (local.get $i) (i32.const 4)))
@@ -143,20 +144,36 @@
                   (br_table $0_bytes $1_byte $2_bytes $3_bytes
                      (i32.and (local.get $len) (i32.const 3))))
                (local.set $w
-                  (i32.shl (array.get_u $string (local.get $s)
+                  (i32.shl (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $i) (i32.const 2)))
                            (i32.const 16))))
             (local.set $w
                (i32.or (local.get $w)
-                  (i32.shl (array.get_u $string (local.get $s)
+                  (i32.shl (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $i) (i32.const 1)))
                            (i32.const 8)))))
          (local.set $w
             (i32.or (local.get $w)
-               (array.get_u $string (local.get $s) (local.get $i))))
+               (array.get_u $bytes (local.get $s) (local.get $i))))
          (local.set $h (call $caml_hash_mix_int (local.get $h) (local.get $w))))
       (i32.xor (local.get $h) (local.get $len)))
 
+   (func $caml_hash_mix_string
+      (param $h i32) (param $s (ref $string)) (result i32)
+      (return_call $jsstring_hash
+         (local.get $h) (struct.get $js 0 (local.get $s))))
+
+(;
+(#if use-js-string
+(#then
+   (export "caml_hash_mix_string" (func $caml_hash_mix_string))
+)
+(#else
+   (export "caml_hash_mix_string" (func $caml_hash_mix_bytes))
+))
+;)
+   (export "caml_hash_mix_string" (func $caml_hash_mix_bytes)) ;; ZZZ Fix base
+
    (global $HASH_QUEUE_SIZE i32 (i32.const 256))
    (global $MAX_FORWARD_DEREFERENCE i32 (i32.const 1000))
 
@@ -205,10 +222,10 @@
                               (i32.const 1))))
                      (local.set $num (i32.sub (local.get $num) (i32.const 1)))
                      (br $loop)))
-                  (drop (block $not_string (result (ref eq))
+                  (drop (block $not_bytes (result (ref eq))
                      (local.set $h
-                        (call $caml_hash_mix_string (local.get $h)
-                           (br_on_cast_fail $not_string (ref eq) (ref $string)
+                        (call $caml_hash_mix_bytes (local.get $h)
+                           (br_on_cast_fail $not_bytes (ref eq) (ref $bytes)
                               (local.get $v))))
                      (local.set $num (i32.sub (local.get $num) (i32.const 1)))
                      (br $loop)))
@@ -312,6 +329,8 @@
       (ref.i31 (i32.and (call $caml_hash_mix_final (local.get $h))
                         (i32.const 0x3FFFFFFF))))
 
+(#if use-js-string
+(#then
    (func (export "caml_string_hash")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (local $h i32)
@@ -323,3 +342,16 @@
                   (ref.cast (ref $string) (local.get 1))))
             (i32.const 0x3FFFFFFF))))
 )
+(#else
+   (func (export "caml_string_hash")
+      (param (ref eq)) (param (ref eq)) (result (ref eq))
+      (local $h i32)
+      (ref.i31
+         (i32.and
+            (call $caml_hash_mix_final
+               (call $caml_hash_mix_bytes
+                  (i31.get_s (ref.cast (ref i31) (local.get 0)))
+                  (ref.cast (ref $bytes) (local.get 1))))
+            (i32.const 0x3FFFFFFF))))
+))
+)
diff --git a/runtime/wasm/int32.wat b/runtime/wasm/int32.wat
index 79ce5095d8..a1d362df1d 100644
--- a/runtime/wasm/int32.wat
+++ b/runtime/wasm/int32.wat
@@ -18,7 +18,7 @@
 (module
    (import "ints" "parse_int"
       (func $parse_int
-         (param (ref eq)) (param i32) (param (ref $string)) (result i32)))
+         (param (ref eq)) (param i32) (param (ref eq)) (result i32)))
    (import "ints" "format_int"
       (func $format_int
          (param (ref eq)) (param i32) (param i32) (result (ref eq))))
@@ -32,7 +32,8 @@
    (import "marshal" "caml_deserialize_int_4"
       (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $compare
       (func (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
    (type $hash
@@ -44,7 +45,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -56,7 +57,7 @@
 
    (global $int32_ops (export "int32_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 2 (i32.const 95) (i32.const 105)) ;; "_i"
+         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 105)) ;; "_i"
          (ref.func $int32_cmp)
          (ref.null $compare)
          (ref.func $int32_hash)
@@ -117,12 +118,7 @@
          (i32.rotl (i32.and (local.get $i) (i32.const 0xFF00FF00))
                    (i32.const 8))))
 
-   (global $INT32_ERRMSG (ref $string)
-      (array.new_fixed $string 15 ;; "Int32.of_string"
-         (i32.const 73) (i32.const 110) (i32.const 116) (i32.const 51)
-         (i32.const 50) (i32.const 46) (i32.const 111) (i32.const 102)
-         (i32.const 95) (i32.const 115) (i32.const 116) (i32.const 114)
-         (i32.const 105) (i32.const 110) (i32.const 103)))
+   (#string $INT32_ERRMSG "Int32.of_string")
 
    (func (export "caml_int32_of_string") (param $v (ref eq)) (result (ref eq))
       (return_call $caml_copy_int32
@@ -137,7 +133,7 @@
 
    (global $nativeint_ops (export "nativeint_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 2 (i32.const 95) (i32.const 110)) ;; "_n"
+         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 110)) ;; "_n"
          (ref.func $int32_cmp)
          (ref.null $compare)
          (ref.func $int32_hash)
@@ -153,15 +149,12 @@
          (struct.get $int32 1 (ref.cast (ref $int32) (local.get $v))))
       (tuple.make 2 (i32.const 4) (i32.const 8)))
 
-   (data $integer_too_large "input_value: native integer value too large")
+   (#string $integer_too_large "input_value: native integer value too large")
 
    (func $nativeint_deserialize
       (param $s (ref eq)) (result (ref eq)) (result i32)
       (if (i32.ne (call $caml_deserialize_uint_1 (local.get $s)) (i32.const 1))
-         (then
-            (call $caml_failwith
-               (array.new_data $string $integer_too_large
-                  (i32.const 0) (i32.const 43)))))
+         (then (call $caml_failwith (global.get $integer_too_large))))
       (tuple.make 2
          (struct.new $int32 (global.get $nativeint_ops)
             (call $caml_deserialize_int_4 (local.get $s)))
@@ -171,12 +164,7 @@
       (param $i i32) (result (ref eq))
       (struct.new $int32 (global.get $nativeint_ops) (local.get $i)))
 
-   (global $NATIVEINT_ERRMSG (ref $string)
-      (array.new_fixed $string 16 ;; "Nativeint.of_string"
-         (i32.const 78) (i32.const 97) (i32.const 116) (i32.const 105)
-         (i32.const 118) (i32.const 101) (i32.const 46) (i32.const 111)
-         (i32.const 102) (i32.const 95) (i32.const 115) (i32.const 116)
-         (i32.const 114) (i32.const 105) (i32.const 110) (i32.const 103)))
+   (#string $NATIVEINT_ERRMSG "Nativeint.of_string")
 
    (func (export "caml_nativeint_of_string")
       (param $v (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/int64.wat b/runtime/wasm/int64.wat
index de7e64c52b..e7926a1065 100644
--- a/runtime/wasm/int64.wat
+++ b/runtime/wasm/int64.wat
@@ -18,11 +18,25 @@
 (module
    (import "ints" "parse_sign_and_base"
       (func $parse_sign_and_base
-         (param (ref $string)) (result i32 i32 i32 i32)))
+(#if use-js-string
+(#then
+         (param externref)
+)
+(#else
+         (param (ref $bytes))
+))
+         (result i32 i32 i32 i32)))
    (import "ints" "parse_digit" (func $parse_digit (param i32) (result i32)))
    (import "ints" "parse_int_format"
       (func $parse_int_format
-         (param (ref $string)) (result i32 i32 i32 i32 i32)))
+(#if use-js-string
+(#then
+         (param externref)
+)
+(#else
+         (param (ref $bytes))
+))
+         (result i32 i32 i32 i32 i32)))
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
    (import "marshal" "caml_serialize_int_8"
       (func $caml_serialize_int_8 (param (ref eq)) (param i64)))
@@ -32,8 +46,31 @@
       (global $lowercase_hex_table (ref $chars)))
    (import "ints" "uppercase_hex_table"
       (global $uppercase_hex_table (ref $chars)))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
+
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_length (param $s (ref $bytes)) (result i32)
+      (array.len (local.get $s)))
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $compare
       (func (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
    (type $hash
@@ -45,7 +82,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -57,7 +94,7 @@
 
    (global $int64_ops (export "int64_ops") (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 2 (i32.const 95) (i32.const 106)) ;; "_j"
+         (array.new_fixed $bytes 2 (i32.const 95) (i32.const 106)) ;; "_j"
          (ref.func $int64_cmp)
          (ref.null $compare)
          (ref.func $int64_hash)
@@ -128,12 +165,7 @@
       (ref.i31 (i32.sub (i64.gt_s (local.get $i1) (local.get $i2))
                         (i64.lt_s (local.get $i1) (local.get $i2)))))
 
-   (global $INT64_ERRMSG (ref $string)
-      (array.new_fixed $string 15 ;; "Int64.of_string"
-         (i32.const 73) (i32.const 110) (i32.const 116) (i32.const 54)
-         (i32.const 52) (i32.const 46) (i32.const 111) (i32.const 102)
-         (i32.const 95) (i32.const 115) (i32.const 116) (i32.const 114)
-         (i32.const 105) (i32.const 110) (i32.const 103)))
+   (#string $INT64_ERRMSG "Int64.of_string")
 
    ;; Parse a sequence of digits into an i64 as dicted by $base,
    ;; $signedness and $sign. The sequence is read in $s starting from $i.
@@ -142,17 +174,24 @@
    ;; package "integers".
    (func $caml_i64_of_digits (export "caml_i64_of_digits")
       (param $base i32) (param $signedness i32) (param $sign i32)
-      (param $s (ref $string)) (param $i i32) (param $errmsg (ref $string))
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (param $errmsg (ref eq))
       (result i64)
       (local $len i32) (local $d i32) (local $c i32)
       (local $res i64) (local $threshold i64)
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.eqz (local.get $len))
         (then (call $caml_failwith (local.get $errmsg))))
       (local.set $threshold
          (i64.div_u (i64.const -1) (i64.extend_i32_u (local.get $base))))
       (local.set $d
-         (call $parse_digit (array.get_u $string (local.get $s) (local.get $i))))
+         (call $parse_digit (call $string_get (local.get $s) (local.get $i))))
       (if (i32.ge_u (local.get $d) (local.get $base))
          (then (call $caml_failwith (local.get $errmsg))))
       (local.set $res (i64.extend_i32_u (local.get $d)))
@@ -160,7 +199,7 @@
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (if (i32.lt_s (local.get $i) (local.get $len))
             (then
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+               (local.set $c (call $string_get (local.get $s) (local.get $i)))
                (br_if $loop (i32.eq (local.get $c) (i32.const 95))) ;; '_'
                (local.set $d (call $parse_digit (local.get $c)))
                (if (i32.ge_u (local.get $d) (local.get $base))
@@ -191,10 +230,16 @@
       (local.get $res))
 
    (func (export "caml_int64_of_string") (param $v (ref eq)) (result (ref eq))
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $i i32) (local $signedness i32) (local $sign i32) (local $base i32)
       (local $t (tuple i32 i32 i32 i32))
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
+      (local.set $s (call $string_val (local.get $v)))
       (local.set $t (call $parse_sign_and_base (local.get $s)))
       (local.set $i (tuple.extract 4 0 (local.get $t)))
       (local.set $signedness (tuple.extract 4 1 (local.get $t)))
@@ -212,7 +257,7 @@
    (data $caml_int64_create_lo_mi_hi "caml_int64_create_lo_mi_hi")
 
    (func $format_int64_default (param $d i64) (result (ref eq))
-      (local $s (ref $string))
+      (local $s (ref $bytes))
       (local $negative i32) (local $i i32) (local $n i64)
       (if (i64.lt_s (local.get $d) (i64.const 0))
          (then
@@ -224,26 +269,33 @@
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (local.set $n (i64.div_u (local.get $n) (i64.const 10)))
          (br_if $count (i64.ne (local.get $n) (i64.const 0))))
-      (local.set $s (array.new $string (i32.const 0) (local.get $i)))
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $i)))
       (loop $write
          (local.set $i (i32.sub (local.get $i) (i32.const 1)))
-         (array.set $string (local.get $s) (local.get $i)
+         (array.set $bytes (local.get $s) (local.get $i)
             (i32.add (i32.const 48)
                (i32.wrap_i64 (i64.rem_u (local.get $d) (i64.const 10)))))
          (local.set $d (i64.div_u (local.get $d) (i64.const 10)))
          (br_if $write (i64.ne (local.get $d) (i64.const 0))))
       (if (local.get $negative)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45)))) ;; '-'
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
    (type $chars (array i8))
 
    (func (export "caml_int64_format")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
       (local $d i64)
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $fmt externref)
+)
+(#else
+      (local $fmt (ref $bytes))
+))
+      (local $s (ref $bytes))
       (local $format (tuple i32 i32 i32 i32 i32))
       (local $sign_style i32) (local $alternate i32) (local $signed i32)
       (local $base i64) (local $uppercase i32)
@@ -251,14 +303,14 @@
       (local $i i32)
       (local $n i64)
       (local $chars (ref $chars))
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
+      (local.set $fmt (call $string_val (local.get 0)))
       (local.set $d (struct.get $int64 1 (ref.cast (ref $int64) (local.get 1))))
-      (if (i32.eq (array.len (local.get $s)) (i32.const 2))
+      (if (i32.eq (call $string_length (local.get $fmt)) (i32.const 2))
          (then
-            (if (i32.eq (array.get_u $string (local.get $s) (i32.const 1))
+            (if (i32.eq (call $string_get (local.get $fmt) (i32.const 1))
                         (i32.const 100)) ;; 'd'
                (then (return_call $format_int64_default (local.get $d))))))
-      (local.set $format (call $parse_int_format (local.get $s)))
+      (local.set $format (call $parse_int_format (local.get $fmt)))
       (local.set $sign_style (tuple.extract 5 0 (local.get $format)))
       (local.set $alternate (tuple.extract 5 1 (local.get $format)))
       (local.set $signed (tuple.extract 5 2 (local.get $format)))
@@ -293,39 +345,39 @@
             (global.get $uppercase_hex_table)
             (global.get $lowercase_hex_table)
             (local.get $uppercase)))
-      (local.set $s (array.new $string (i32.const 0) (local.get $i)))
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $i)))
       (loop $write
          (local.set $i (i32.sub (local.get $i) (i32.const 1)))
-         (array.set $string (local.get $s) (local.get $i)
+         (array.set $bytes (local.get $s) (local.get $i)
             (array.get_u $chars (local.get $chars)
                (i32.wrap_i64 (i64.rem_u (local.get $d) (local.get $base)))))
          (local.set $d (i64.div_u (local.get $d) (local.get $base)))
          (br_if $write (i64.ne (local.get $d) (i64.const 0))))
       (if (local.get $negative)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45))) ;; '-'
          (else
             (if (local.get $sign_style)
                (then
                   (if (i32.eq (local.get $sign_style) (i32.const 1))
                      (then
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 43))) ;; '+'
                      (else
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 32)))))))) ;; ' '
       (if (local.get $alternate)
          (then
             (if (local.get $i)
                (then
-                  (array.set $string (local.get $s) (i32.const 0)
+                  (array.set $bytes (local.get $s) (i32.const 0)
                      (i32.const 48)) ;; '0'
                   (if (i64.eq (local.get $base) (i64.const 16))
                      (then
-                        (array.set $string (local.get $s) (i32.const 1)
+                        (array.set $bytes (local.get $s) (i32.const 1)
                            (select (i32.const 88) (i32.const 120) ;; 'X' 'x'
                               (local.get $uppercase)))))))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
 )
diff --git a/runtime/wasm/ints.wat b/runtime/wasm/ints.wat
index 1744e733f7..65f8a205b8 100644
--- a/runtime/wasm/ints.wat
+++ b/runtime/wasm/ints.wat
@@ -19,8 +19,31 @@
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
 
-   (type $string (array (mut i8)))
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_length (param $s (ref $bytes)) (result i32)
+      (array.len (local.get $s)))
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
+
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (func (export "caml_format_int")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
@@ -29,17 +52,24 @@
          (i31.get_s (ref.cast (ref i31) (local.get 1))) (i32.const 1)))
 
    (func $parse_sign_and_base (export "parse_sign_and_base")
-      (param $s (ref $string)) (result i32 i32 i32 i32)
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (result i32 i32 i32 i32)
       (local $i i32) (local $len i32) (local $c i32)
       (local $signedness i32) (local $sign i32) (local $base i32)
       (local.set $i (i32.const 0))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (local.set $signedness (i32.const 1))
       (local.set $sign (i32.const 1))
       (local.set $base (i32.const 10))
       (if (i32.ne (local.get $len) (i32.const 0))
          (then
-            (local.set $c (array.get_u $string (local.get $s) (i32.const 0)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 0)))
             (if (i32.eq (local.get $c) (i32.const 45))
                (then
                   (local.set $sign (i32.const -1))
@@ -47,11 +77,11 @@
                (else (if (i32.eq (local.get $c) (i32.const 43))
                   (then (local.set $i (i32.const 1))))))))
       (if (i32.lt_s (i32.add (local.get $i) (i32.const 1)) (local.get $len))
-         (then (if (i32.eq (array.get_u $string (local.get $s) (local.get $i))
+         (then (if (i32.eq (call $string_get (local.get $s) (local.get $i))
                            (i32.const 48))
             (then
                (local.set $c
-                  (array.get_u $string (local.get $s)
+                  (call $string_get (local.get $s)
                      (i32.add (local.get $i) (i32.const 1))))
                (if (i32.or (i32.eq (local.get $c) (i32.const 88))
                            (i32.eq (local.get $c) (i32.const 120)))
@@ -94,15 +124,21 @@
       (return (i32.const -1)))
 
    (func $parse_int (export "parse_int")
-      (param $v (ref eq)) (param $nbits i32) (param $errmsg (ref $string))
+      (param $v (ref eq)) (param $nbits i32) (param $errmsg (ref eq))
       (result i32)
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $i i32) (local $len i32) (local $d i32) (local $c i32)
       (local $signedness i32) (local $sign i32) (local $base i32)
       (local $res i32) (local $threshold i32)
       (local $t (tuple i32 i32 i32 i32))
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $s (call $string_val (local.get $v)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.eqz (local.get $len))
         (then (call $caml_failwith (local.get $errmsg))))
       (local.set $t (call $parse_sign_and_base (local.get $s)))
@@ -114,7 +150,7 @@
       (if (i32.ge_s (local.get $i) (local.get $len))
          (then (call $caml_failwith (local.get $errmsg))))
       (local.set $d
-         (call $parse_digit (array.get_u $string (local.get $s) (local.get $i))))
+         (call $parse_digit (call $string_get (local.get $s) (local.get $i))))
       (if (i32.ge_u (local.get $d) (local.get $base))
          (then (call $caml_failwith (local.get $errmsg))))
       (local.set $res (local.get $d))
@@ -122,7 +158,7 @@
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (if (i32.lt_s (local.get $i) (local.get $len))
             (then
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+               (local.set $c (call $string_get (local.get $s) (local.get $i)))
                (br_if $loop (i32.eq (local.get $c) (i32.const 95))) ;; '_'
                (local.set $d (call $parse_digit (local.get $c)))
                (if (i32.ge_u (local.get $d) (local.get $base))
@@ -157,12 +193,7 @@
          (then (local.set $res (i32.sub (i32.const 0) (local.get $res)))))
       (local.get $res))
 
-   (global $INT_ERRMSG (ref $string)
-      (array.new_fixed $string 13 ;; "int.of_string"
-         (i32.const 105) (i32.const 110) (i32.const 116) (i32.const 95)
-         (i32.const 111) (i32.const 102) (i32.const 95) (i32.const 115)
-         (i32.const 116) (i32.const 114) (i32.const 105) (i32.const 110)
-         (i32.const 103)))
+   (#string $INT_ERRMSG "int_of_string")
 
    (func (export "caml_int_of_string")
       (param $v (ref eq)) (result (ref eq))
@@ -196,7 +227,7 @@
          (i32.const 67) (i32.const 68) (i32.const 69) (i32.const 70)))
 
    (func $format_int_default (param $d i32) (result (ref eq))
-      (local $s (ref $string))
+      (local $s (ref $bytes))
       (local $negative i32) (local $i i32) (local $n i32)
       (if (i32.lt_s (local.get $d) (i32.const 0))
          (then
@@ -208,36 +239,43 @@
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (local.set $n (i32.div_u (local.get $n) (i32.const 10)))
          (br_if $count (local.get $n)))
-      (local.set $s (array.new $string (i32.const 0) (local.get $i)))
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $i)))
       (loop $write
          (local.set $i (i32.sub (local.get $i) (i32.const 1)))
-         (array.set $string (local.get $s) (local.get $i)
+         (array.set $bytes (local.get $s) (local.get $i)
             (i32.add (i32.const 48)
                (i32.rem_u (local.get $d) (i32.const 10))))
          (local.set $d (i32.div_u (local.get $d) (i32.const 10)))
          (br_if $write (local.get $d)))
       (if (local.get $negative)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45)))) ;; '-'
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
-   (data $format_error "format_int: bad format")
+   (#string $format_error "format_int: bad format")
 
    (func $parse_int_format (export "parse_int_format")
-      (param $s (ref $string)) (result i32 i32 i32 i32 i32)
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (result i32 i32 i32 i32 i32)
       (local $i i32) (local $len i32) (local $c i32)
       (local $sign_style i32) (local $alternate i32) (local $base i32)
       (local $signed i32) (local $uppercase i32)
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (local.set $i (i32.const 1))
       (block $return
          (block $bad_format
             (br_if $bad_format (i32.lt_u (local.get $len) (i32.const 2)))
             (br_if $bad_format
-               (i32.ne (array.get_u $string (local.get $s) (i32.const 0))
+               (i32.ne (call $string_get (local.get $s) (i32.const 0))
                        (i32.const 37))) ;; '%'
-            (local.set $c (array.get_u $string (local.get $s) (i32.const 1)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 1)))
             (if (i32.eq (local.get $c) (i32.const 43)) ;; '+'
                (then
                   (local.set $sign_style (i32.const 1))
@@ -251,7 +289,7 @@
                   (local.set $alternate (i32.const 1))
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))))
             (br_if $bad_format (i32.eq (local.get $i) (local.get $len)))
-            (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
+            (local.set $c (call $string_get (local.get $s) (local.get $i)))
             (if (i32.or (i32.or (i32.eq (local.get $c) (i32.const 76)) ;; 'L'
                                 (i32.eq (local.get $c) (i32.const 108))) ;; 'l'
                         (i32.eq (local.get $c) (i32.const 110))) ;; 'n'
@@ -259,7 +297,7 @@
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (br_if $bad_format (i32.eq (local.get $i) (local.get $len)))
                   (local.set $c
-                     (array.get_u $string (local.get $s) (local.get $i)))))
+                     (call $string_get (local.get $s) (local.get $i)))))
             (br_if $bad_format
               (i32.ne (i32.add (local.get $i) (i32.const 1)) (local.get $len)))
             (if (i32.or (i32.eq (local.get $c) (i32.const 100)) ;; 'd'
@@ -283,9 +321,7 @@
             (else
                (br $bad_format)))))))))))
             (br $return))
-         (call $caml_invalid_argument
-            (array.new_data $string $format_error
-               (i32.const 0) (i32.const 22))))
+         (call $caml_invalid_argument (global.get $format_error)))
       (tuple.make 5
          (local.get $sign_style)
          (local.get $alternate)
@@ -295,7 +331,14 @@
 
    (func $format_int (export "format_int")
       (param (ref eq)) (param $d i32) (param $small i32) (result (ref eq))
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $fmt externref)
+)
+(#else
+      (local $fmt (ref $bytes))
+))
+      (local $s (ref $bytes))
       (local $format (tuple i32 i32 i32 i32 i32))
       (local $sign_style i32) (local $alternate i32) (local $signed i32)
       (local $base i32) (local $uppercase i32)
@@ -303,13 +346,13 @@
       (local $i i32)
       (local $n i32)
       (local $chars (ref $chars))
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
-      (if (i32.eq (array.len (local.get $s)) (i32.const 2))
+      (local.set $fmt (call $string_val (local.get 0)))
+      (if (i32.eq (call $string_length (local.get $fmt)) (i32.const 2))
          (then
-            (if (i32.eq (array.get_u $string (local.get $s) (i32.const 1))
+            (if (i32.eq (call $string_get (local.get $fmt) (i32.const 1))
                         (i32.const 100)) ;; 'd'
                (then (return_call $format_int_default (local.get $d))))))
-      (local.set $format (call $parse_int_format (local.get $s)))
+      (local.set $format (call $parse_int_format (local.get $fmt)))
       (local.set $sign_style (tuple.extract 5 0 (local.get $format)))
       (local.set $alternate (tuple.extract 5 1 (local.get $format)))
       (local.set $signed (tuple.extract 5 2 (local.get $format)))
@@ -350,38 +393,38 @@
             (global.get $uppercase_hex_table)
             (global.get $lowercase_hex_table)
             (local.get $uppercase)))
-      (local.set $s (array.new $string (i32.const 0) (local.get $i)))
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $i)))
       (loop $write
          (local.set $i (i32.sub (local.get $i) (i32.const 1)))
-         (array.set $string (local.get $s) (local.get $i)
+         (array.set $bytes (local.get $s) (local.get $i)
             (array.get_u $chars (local.get $chars)
                (i32.rem_u (local.get $d) (local.get $base))))
          (local.set $d (i32.div_u (local.get $d) (local.get $base)))
          (br_if $write (local.get $d)))
       (if (local.get $negative)
          (then
-            (array.set $string (local.get $s) (i32.const 0)
+            (array.set $bytes (local.get $s) (i32.const 0)
                (i32.const 45))) ;; '-'
          (else
             (if (local.get $sign_style)
                (then
                   (if (i32.eq (local.get $sign_style) (i32.const 1))
                      (then
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 43))) ;; '+'
                      (else
-                        (array.set $string (local.get $s) (i32.const 0)
+                        (array.set $bytes (local.get $s) (i32.const 0)
                            (i32.const 32)))))))) ;; ' '
       (if (local.get $alternate)
          (then
             (if (local.get $i)
                (then
-                  (array.set $string (local.get $s) (i32.const 0)
+                  (array.set $bytes (local.get $s) (i32.const 0)
                      (i32.const 48)) ;; '0'
                   (if (i32.eq (local.get $base) (i32.const 16))
                      (then
-                        (array.set $string (local.get $s) (i32.const 1)
+                        (array.set $bytes (local.get $s) (i32.const 1)
                            (select (i32.const 88) (i32.const 120) ;; 'X' 'x'
                               (local.get $uppercase)))))))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 )
diff --git a/runtime/wasm/io.wat b/runtime/wasm/io.wat
index 76e02a2d39..3fcc28b120 100644
--- a/runtime/wasm/io.wat
+++ b/runtime/wasm/io.wat
@@ -55,13 +55,17 @@
       (func $ta_set_ui8 (param (ref extern)) (param i32) (param i32))) ;; ZZZ ??
    (import "bindings" "ta_get_ui8"
       (func $ta_get_ui8 (param (ref extern)) (param i32) (result i32)))
+   (import "bindings" "ta_blit_from_bytes"
+      (func $ta_blit_from_bytes
+         (param (ref $bytes)) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
+   (import "bindings" "ta_blit_to_bytes"
+      (func $ta_blit_to_bytes
+         (param (ref extern)) (param i32) (param (ref $bytes)) (param i32)
+         (param i32)))
    (import "bindings" "ta_blit_from_string"
       (func $ta_blit_from_string
-         (param (ref $string)) (param i32) (param (ref extern)) (param i32)
-         (param i32)))
-   (import "bindings" "ta_blit_to_string"
-      (func $ta_blit_to_string
-         (param (ref extern)) (param i32) (param (ref $string)) (param i32)
+         (param anyref) (param i32) (param (ref extern)) (param i32)
          (param i32)))
    (import "custom" "custom_compare_id"
       (func $custom_compare_id
@@ -77,6 +81,8 @@
       (tag $javascript_exception (param externref)))
    (import "sys" "caml_handle_sys_error"
       (func $caml_handle_sys_error (param externref)))
+   (import "string" "caml_bytes_of_string"
+      (func $caml_bytes_of_string (param (ref eq)) (result (ref eq))))
 
    (import "bindings" "map_new" (func $map_new (result (ref extern))))
    (import "bindings" "map_get"
@@ -89,7 +95,8 @@
       (func $map_delete (param (ref extern)) (param i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $offset_array (array (mut i64)))
 
    (type $compare
@@ -103,7 +110,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -120,7 +127,7 @@
 
    (global $channel_ops (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 5 ;; "_chan"
+         (array.new_fixed $bytes 5 ;; "_chan"
             (i32.const 95) (i32.const 99) (i32.const 104) (i32.const 97)
             (i32.const 110))
          (ref.func $custom_compare_id)
@@ -351,7 +358,7 @@
       (return (call $ta_get_ui8 (local.get $buf) (i32.const 0))))
 
    (func $caml_getblock (export "caml_getblock")
-      (param $vch (ref eq)) (param $s (ref $string))
+      (param $vch (ref eq)) (param $s (ref $bytes))
       (param $pos i32) (param $len i32)
       (result i32)
       (local $ch (ref $channel))
@@ -367,7 +374,7 @@
          (then
             (if (i32.gt_u (local.get $len) (local.get $avail))
                (then (local.set $len (local.get $avail))))
-            (call $ta_blit_to_string
+            (call $ta_blit_to_bytes
                (struct.get $channel $buffer (local.get $ch))
                (struct.get $channel $curr (local.get $ch))
                (local.get $s) (local.get $pos)
@@ -382,7 +389,7 @@
       (struct.set $channel $max (local.get $ch) (local.get $nread))
       (if (i32.gt_u (local.get $len) (local.get $nread))
          (then (local.set $len (local.get $nread))))
-      (call $ta_blit_to_string
+      (call $ta_blit_to_bytes
          (struct.get $channel $buffer (local.get $ch))
          (i32.const 0)
          (local.get $s) (local.get $pos)
@@ -391,7 +398,7 @@
       (local.get $len))
 
    (func (export "caml_really_getblock")
-      (param $ch (ref eq)) (param $s (ref $string))
+      (param $ch (ref eq)) (param $s (ref $bytes))
       (param $pos i32) (param $len i32)
       (result i32)
       (local $read i32) (local $n i32)
@@ -412,12 +419,12 @@
    (func (export "caml_ml_input")
       (param $vch (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
-      (local $ch (ref $channel)) (local $s (ref $string))
+      (local $ch (ref $channel)) (local $s (ref $bytes))
       (local $pos i32) (local $len i32) (local $curr i32)
       (local $i i32) (local $avail i32) (local $nread i32)
       (local $buf (ref extern))
       (local.set $ch (ref.cast (ref $channel) (local.get $vch)))
-      (local.set $s (ref.cast (ref $string) (local.get $vs)))
+      (local.set $s (ref.cast (ref $bytes) (local.get $vs)))
       (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
       (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
       (local.set $buf (struct.get $channel $buffer (local.get $ch)))
@@ -438,7 +445,7 @@
                   (local.set $curr (i32.const 0))
                   (if (i32.gt_u (local.get $len) (local.get $nread))
                      (then (local.set $len (local.get $nread))))))))
-      (call $ta_blit_to_string
+      (call $ta_blit_to_bytes
          (local.get $buf) (local.get $curr)
          (local.get $s) (local.get $pos) (local.get $len))
       (struct.set $channel $curr (local.get $ch)
@@ -721,7 +728,27 @@
       (i32.eqz (local.get $towrite)))
 
    (func $caml_putblock
-      (param $ch (ref $channel)) (param $s (ref $string)) (param $pos i32)
+      (param $ch (ref $channel)) (param $s (ref $bytes)) (param $pos i32)
+      (param $len i32) (result i32)
+      (local $free i32) (local $curr i32)
+      (local $buf (ref extern))
+      (local.set $curr (struct.get $channel $curr (local.get $ch)))
+      (local.set $free
+         (i32.sub (struct.get $channel $size (local.get $ch)) (local.get $curr)))
+      (if (i32.ge_u (local.get $len) (local.get $free))
+         (then (local.set $len (local.get $free))))
+      (local.set $buf (struct.get $channel $buffer (local.get $ch)))
+      (call $ta_blit_from_bytes
+         (local.get $s) (local.get $pos)
+         (local.get $buf) (local.get $curr) (local.get $len))
+      (struct.set $channel $curr (local.get $ch)
+         (i32.add (local.get $curr) (local.get $len)))
+      (if (i32.ge_u (local.get $len) (local.get $free))
+         (then (drop (call $caml_flush_partial (local.get $ch)))))
+      (local.get $len))
+
+   (func $caml_putblock_string
+      (param $ch (ref $channel)) (param $s anyref) (param $pos i32)
       (param $len i32) (result i32)
       (local $free i32) (local $curr i32)
       (local $buf (ref extern))
@@ -741,7 +768,7 @@
       (local.get $len))
 
    (func (export "caml_really_putblock")
-      (param $ch (ref eq)) (param $s (ref $string))
+      (param $ch (ref eq)) (param $s (ref $bytes))
       (param $pos i32) (param $len i32)
       (local $written i32)
       (loop $loop
@@ -754,8 +781,34 @@
                (local.set $len (i32.sub (local.get $len) (local.get $written)))
                (br $loop)))))
 
-   (export "caml_ml_output_bytes" (func $caml_ml_output))
+(#if use-js-string
+(#then
    (func $caml_ml_output (export "caml_ml_output")
+      (param $ch (ref eq)) (param $s (ref eq)) (param $vpos (ref eq))
+      (param $vlen (ref eq)) (result (ref eq))
+      (local $pos i32) (local $len i32) (local $written i32)
+      (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
+      (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
+      (loop $loop
+         (if (i32.gt_u (local.get $len) (i32.const 0))
+            (then
+               (local.set $written
+                  (call $caml_putblock_string
+                     (ref.cast (ref $channel) (local.get $ch))
+                     (struct.get $string 0
+                       (ref.cast (ref $string) (local.get $s)))
+                     (local.get $pos) (local.get $len)))
+               (local.set $pos (i32.add (local.get $pos) (local.get $written)))
+               (local.set $len (i32.sub (local.get $len) (local.get $written)))
+               (br $loop))))
+      (call $caml_flush_if_unbuffered (local.get $ch))
+      (ref.i31 (i32.const 0)))
+)
+(#else
+   (export "caml_ml_output" (func $caml_ml_output_bytes))
+))
+
+   (func $caml_ml_output_bytes (export "caml_ml_output_bytes")
       (param $ch (ref eq)) (param $s (ref eq)) (param $vpos (ref eq))
       (param $vlen (ref eq)) (result (ref eq))
       (local $pos i32) (local $len i32) (local $written i32)
@@ -766,7 +819,7 @@
             (then
                (local.set $written
                   (call $caml_putblock (ref.cast (ref $channel) (local.get $ch))
-                     (ref.cast (ref $string) (local.get $s))
+                     (ref.cast (ref $bytes) (local.get $s))
                      (local.get $pos) (local.get $len)))
                (local.set $pos (i32.add (local.get $pos) (local.get $written)))
                (local.set $len (i32.sub (local.get $len) (local.get $written)))
diff --git a/runtime/wasm/jslib.wat b/runtime/wasm/jslib.wat
index b15f2809a7..b28a52c0de 100644
--- a/runtime/wasm/jslib.wat
+++ b/runtime/wasm/jslib.wat
@@ -16,7 +16,6 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "bindings" "log" (func $log_js (param anyref)))
    (import "bindings" "identity" (func $to_float (param anyref) (result f64)))
    (import "bindings" "identity" (func $from_float (param f64) (result anyref)))
    (import "bindings" "identity" (func $to_bool (param anyref) (result i32)))
@@ -85,13 +84,22 @@
       (func $caml_is_closure (param (ref eq)) (result i32)))
    (import "obj" "caml_is_last_arg"
       (func $caml_is_last_arg (param (ref eq)) (result i32)))
+   (import "jsstring" "jsstring_of_bytes"
+      (func $jsstring_of_bytes (param (ref $bytes)) (result anyref)))
+   (import "jsstring" "bytes_of_jsstring"
+      (func $bytes_of_jsstring (param anyref) (result (ref $bytes))))
    (import "jsstring" "jsstring_of_string"
-      (func $jsstring_of_string (param (ref $string)) (result anyref)))
+      (func $jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "jsstring" "string_of_jsstring"
-      (func $string_of_jsstring (param anyref) (result (ref $string))))
+      (func $string_of_jsstring (param (ref eq)) (result (ref eq))))
+   (import "jsstring" "jsbytes_of_bytes"
+      (func $jsbytes_of_bytes (param (ref $bytes)) (result anyref)))
+   (import "jsstring" "bytes_of_jsbytes"
+      (func $bytes_of_jsbytes (param anyref) (result (ref $bytes))))
+   (import "jsstring" "jsstring_of_subbytes"
+      (func $jsstring_of_subbytes (param (ref $bytes) i32 i32) (result anyref)))
    (import "jsstring" "jsstring_of_substring"
-      (func $jsstring_of_substring
-         (param (ref $string) i32 i32) (result anyref)))
+      (func $jsstring_of_substring (param (ref eq) i32 i32) (result (ref eq))))
    (import "int32" "caml_copy_int32"
       (func $caml_copy_int32 (param i32) (result (ref eq))))
    (import "int32" "Int32_val"
@@ -104,7 +112,8 @@
    (type $block (array (mut (ref eq))))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $js (struct (field anyref)))
    (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
    (type $closure (sub (struct (;(field i32);) (field (ref $function_1)))))
@@ -187,9 +196,9 @@
    (func (export "caml_js_meth_call")
       (param $o (ref eq)) (param $f (ref eq)) (param $args (ref eq))
       (result (ref eq))
-      (if (ref.test (ref $string) (local.get $f))
+      (if (ref.test (ref $bytes) (local.get $f))
          (then
-            (local.set $f (call $caml_jsbytes_of_string (local.get $f)))))
+            (local.set $f (call $caml_jsbytes_of_bytes (local.get $f)))))
       (return_call $wrap
          (call $meth_call (call $unwrap (local.get $o))
             (call $unwrap (local.get $f))
@@ -197,9 +206,9 @@
 
    (func (export "caml_js_get")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (if (ref.test (ref $string) (local.get 1))
+      (if (ref.test (ref $bytes) (local.get 1))
          (then
-            (local.set 1 (call $caml_jsbytes_of_string (local.get 1)))))
+            (local.set 1 (call $caml_jsbytes_of_bytes (local.get 1)))))
       (return_call $wrap
          (call $get
             (ref.as_non_null (extern.convert_any (call $unwrap (local.get 0))))
@@ -207,18 +216,18 @@
 
    (func (export "caml_js_set")
       (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (if (ref.test (ref $string) (local.get 1))
+      (if (ref.test (ref $bytes) (local.get 1))
          (then
-            (local.set 1 (call $caml_jsbytes_of_string (local.get 1)))))
+            (local.set 1 (call $caml_jsbytes_of_bytes (local.get 1)))))
       (call $set (call $unwrap (local.get 0)) (call $unwrap (local.get 1))
          (call $unwrap (local.get 2)))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_js_delete")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (if (ref.test (ref $string) (local.get 1))
+      (if (ref.test (ref $bytes) (local.get 1))
          (then
-            (local.set 1 (call $caml_jsbytes_of_string (local.get 1)))))
+            (local.set 1 (call $caml_jsbytes_of_bytes (local.get 1)))))
       (call $delete (call $unwrap (local.get 0)) (call $unwrap (local.get 1)))
       (ref.i31 (i32.const 0)))
 
@@ -455,126 +464,86 @@
                                  (local.get $acc)))))))))
       (return_call $unwrap (local.get $acc)))
 
-   (export "caml_js_from_string" (func $caml_jsstring_of_string))
+   (export "caml_js_from_string" (func $jsstring_of_string))
+
+(#if use-js-string
+(#then
+   (export "caml_jsstring_of_string" (func $jsstring_of_string))
+   (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))
+      (return_call $jsstring_of_string (local.get 0)))
+)
+(#else
    (func $caml_jsstring_of_string (export "caml_jsstring_of_string")
       (param (ref eq)) (result (ref eq))
-      (local $s (ref $string))
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
-      (return (struct.new $js (call $jsstring_of_string (local.get $s)))))
+      (local $s (ref $bytes))
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
+      (return (struct.new $js (call $jsstring_of_bytes (local.get $s)))))
+))
 
+(#if use-js-string
+(#then
+   (func (export "caml_jsstring_of_substring")
+      (param $s (ref eq)) (param $i (ref eq)) (param $l (ref eq))
+      (result (ref eq))
+      (return_call $jsstring_of_substring
+         (local.get $s)
+         (i31.get_u (ref.cast (ref i31) (local.get $i)))
+         (i31.get_u (ref.cast (ref i31) (local.get $l)))))
+)
+(#else
    (func (export "caml_jsstring_of_substring")
       (param $s (ref eq)) (param $i (ref eq)) (param $l (ref eq))
       (result (ref eq))
       (return
          (struct.new $js
-            (call $jsstring_of_substring
-               (ref.cast (ref $string) (local.get $s))
+            (call $jsstring_of_subbytes
+               (ref.cast (ref $bytes) (local.get $s))
                (i31.get_u (ref.cast (ref i31) (local.get $i)))
                (i31.get_u (ref.cast (ref i31) (local.get $l)))))))
+))
 
-   (func $caml_jsbytes_of_string (export "caml_jsbytes_of_string")
+(#if use-js-string
+(#then
+   (func (export "caml_jsbytes_of_string")
       (param (ref eq)) (result (ref eq))
-      (local $s (ref $string))
-      (local $s' (ref $string))
-      (local $l i32) (local $i i32) (local $n i32) (local $c i32)
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
-      (local.set $l (array.len (local.get $s)))
-      (local.set $i (i32.const 0))
-      (local.set $n (i32.const 0))
-      (loop $count
-         (if (i32.lt_u (local.get $i) (local.get $l))
-            (then
-               (if (i32.ge_u (array.get_u $string (local.get $s) (local.get $i))
-                      (i32.const 128))
-                  (then (local.set $n (i32.add (local.get $n) (i32.const 1)))))
-               (local.set $i (i32.add (local.get $i) (i32.const 1)))
-               (br $count))))
-      (if (i32.eqz (local.get $n))
-         (then
-            (return
-               (struct.new $js
-                  (call $jsstring_of_string (local.get $s))))))
-      (local.set $s'
-         (array.new $string (i32.const 0)
-            (i32.add (local.get $i) (local.get $n))))
-      (local.set $i (i32.const 0))
-      (local.set $n (i32.const 0))
-      (loop $fill
-         (if (i32.lt_u (local.get $i) (local.get $l))
-            (then
-               (local.set $c (array.get_u $string (local.get $s) (local.get $i)))
-               (if (i32.lt_u (local.get $c) (i32.const 128))
-                  (then
-                     (array.set $string
-                        (local.get $s') (local.get $n) (local.get $c))
-                     (local.set $n (i32.add (local.get $n) (i32.const 1))))
-                  (else
-                     (array.set $string (local.get $s')
-                        (local.get $n)
-                        (i32.or (i32.shr_u (local.get $c) (i32.const 6))
-                           (i32.const 0xC0)))
-                     (array.set $string (local.get $s')
-                        (i32.add (local.get $n) (i32.const 1))
-                        (i32.or (i32.const 0x80)
-                           (i32.and (local.get $c) (i32.const 0x3F))))
-                     (local.set $n (i32.add (local.get $n) (i32.const 2)))))
-               (local.set $i (i32.add (local.get $i) (i32.const 1)))
-               (br $fill))))
-      (return (struct.new $js (call $jsstring_of_string (local.get $s')))))
+      (local.get 0))
+)
+(#else
+   (export "caml_jsbytes_of_string" (func $caml_jsbytes_of_bytes))
+))
 
+   (func $caml_jsbytes_of_bytes
+      (param (ref eq)) (result (ref eq))
+      (local $s (ref $bytes))
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
+      (return (struct.new $js (call $jsbytes_of_bytes (local.get $s)))))
+
+(#if use-js-string
+(#then
+   (export "caml_js_to_string" (func $string_of_jsstring))
+   (export "caml_string_of_jsstring" (func $string_of_jsstring))
+   (func $caml_string_of_jsstring (param (ref eq)) (result (ref eq))
+      (return_call $string_of_jsstring (local.get 0)))
+)
+(#else
    (export "caml_js_to_string" (func $caml_string_of_jsstring))
    (func $caml_string_of_jsstring (export "caml_string_of_jsstring")
       (param $s (ref eq)) (result (ref eq))
-      (return_call $string_of_jsstring
+      (return_call $bytes_of_jsstring
          (struct.get $js 0 (ref.cast (ref $js) (local.get $s)))))
+))
 
+(#if use-js-string
+(#then
    (func (export "caml_string_of_jsbytes")
       (param $s (ref eq)) (result (ref eq))
-      (local $l i32) (local $i i32) (local $n i32) (local $c i32)
-      (local $s' (ref $string)) (local $s'' (ref $string))
-      (local.set $s'
-         (call $string_of_jsstring
-            (struct.get $js 0 (ref.cast (ref $js) (local.get $s)))))
-      (local.set $l (array.len (local.get $s')))
-      (local.set $i (i32.const 0))
-      (local.set $n (i32.const 0))
-      (loop $count
-         (if (i32.lt_u (local.get $i) (local.get $l))
-            (then
-               (if (i32.ge_u (array.get_u $string (local.get $s') (local.get $i))
-                      (i32.const 0xC0))
-                  (then (local.set $n (i32.add (local.get $n) (i32.const 1)))))
-               (local.set $i (i32.add (local.get $i) (i32.const 1)))
-               (br $count))))
-      (if (i32.eqz (local.get $n)) (then (return (local.get $s'))))
-      (local.set $s''
-         (array.new $string (i32.const 0)
-            (i32.sub (local.get $i) (local.get $n))))
-      (local.set $i (i32.const 0))
-      (local.set $n (i32.const 0))
-      (loop $fill
-         (if (i32.lt_u (local.get $i) (local.get $l))
-            (then
-               (local.set $c
-                  (array.get_u $string (local.get $s') (local.get $i)))
-               (if (i32.lt_u (local.get $c) (i32.const 0xC0))
-                  (then
-                     (array.set $string
-                        (local.get $s'') (local.get $n) (local.get $c))
-                     (local.set $i (i32.add (local.get $i) (i32.const 1))))
-                  (else
-                     (array.set $string (local.get $s'')
-                        (local.get $n)
-                        (i32.sub
-                           (i32.add
-                              (i32.shl (local.get $c) (i32.const 6))
-                              (array.get_u $string (local.get $s')
-                                 (i32.add (local.get $i) (i32.const 1))))
-                           (i32.const 0x3080)))
-                     (local.set $i (i32.add (local.get $i) (i32.const 2)))))
-               (local.set $n (i32.add (local.get $n) (i32.const 1)))
-               (br $fill))))
-      (local.get $s''))
+      (local.get 0))
+)
+(#else
+   (func (export "caml_string_of_jsbytes") (param $s (ref eq)) (result (ref eq))
+      (return_call $bytes_of_jsbytes
+         (struct.get $js 0 (ref.cast (ref $js) (local.get $s)))))
+))
 
    (func (export "caml_list_to_js_array")
       (param (ref eq)) (result (ref eq))
@@ -629,12 +598,9 @@
                (br $loop))))
       (local.get $l))
 
-   (global $jsError (ref $string)
-      (array.new_fixed $string 7 ;; 'jsError'
-         (i32.const 106) (i32.const 115) (i32.const 69) (i32.const 114)
-         (i32.const 114) (i32.const 111) (i32.const 114)))
+   (#string $jsError "jsError")
 
-   (data $toString "toString")
+   (#string $toString "toString")
 
    (func (export "caml_wrap_exception") (param externref) (result (ref eq))
       (local $exn anyref)
@@ -653,9 +619,7 @@
                (call $meth_call
                   (local.get $exn)
                   (call $unwrap
-                     (call $caml_jsstring_of_string
-                        (array.new_data $string $toString
-                           (i32.const 0) (i32.const 8))))
+                     (call $caml_jsstring_of_string (global.get $toString)))
                   (any.convert_extern (call $new_array (i32.const 0))))))))
 
    (func (export "caml_js_error_option_of_exception")
@@ -687,7 +651,15 @@
                      (array.get $block (local.get $exn) (i32.const 2)))))))
       (call $wrap (ref.null any)))
 
-   (func (export "log_str") (param $s (ref $string))
-      (call $log_js
-         (call $unwrap (call $caml_jsstring_of_string (local.get $s)))))
+   (func (export "caml_jsoo_flags_use_js_string")
+      (param (ref eq)) (result (ref eq))
+      (ref.i31
+(#if use-js-string
+(#then
+         (i32.const 1)
+)
+(#else
+         (i32.const 0)
+))
+      ))
 )
diff --git a/runtime/wasm/jslib_js_of_ocaml.wat b/runtime/wasm/jslib_js_of_ocaml.wat
index 3b26b553ce..7c1b014fc3 100644
--- a/runtime/wasm/jslib_js_of_ocaml.wat
+++ b/runtime/wasm/jslib_js_of_ocaml.wat
@@ -32,7 +32,8 @@
       (func $caml_js_html_entities (param anyref) (result anyref)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $js (struct (field anyref)))
 
    (func (export "caml_js_html_escape") (param (ref eq)) (result (ref eq))
       (return_call $wrap
@@ -42,19 +43,18 @@
       (return_call $wrap
          (call $caml_js_html_entities (call $unwrap (local.get 0)))))
 
-   (data $console "console")
+   (#jsstring $console "console")
 
    (func (export "caml_js_get_console") (param (ref eq)) (result (ref eq))
       (return_call $caml_js_get (call $caml_js_global (ref.i31 (i32.const 0)))
-         (array.new_data $string $console (i32.const 0) (i32.const 7))))
+         (global.get $console)))
 
-   (data $XMLHttpRequest "XMLHttpRequest")
+   (#jsstring $XMLHttpRequest "XMLHttpRequest")
 
    (func (export "caml_xmlhttprequest_create") (param (ref eq)) (result (ref eq))
       (return_call $caml_js_new
          (call $caml_js_get
             (call $caml_js_global (ref.i31 (i32.const 0)))
-            (array.new_data $string $XMLHttpRequest
-               (i32.const 0) (i32.const 14)))
+            (global.get $XMLHttpRequest))
          (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))
 )
diff --git a/runtime/wasm/jsstring.wat b/runtime/wasm/jsstring.wat
index 93956d3282..2f64b1b5fc 100644
--- a/runtime/wasm/jsstring.wat
+++ b/runtime/wasm/jsstring.wat
@@ -22,19 +22,23 @@
       (func $is_string (param externref) (result i32)))
    (import "wasm:js-string" "hash"
       (func $hash_string (param i32) (param anyref) (result i32)))
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "substring"
+      (func $string_sub (param externref i32 i32) (result (ref extern))))
    (import "wasm:js-string" "fromCharCodeArray"
       (func $fromCharCodeArray
          (param (ref null $wstring)) (param i32) (param i32)
          (result (ref extern))))
-
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
    (import "wasm:text-decoder" "decodeStringFromUTF8Array"
       (func $decodeStringFromUTF8Array
-         (param (ref null $string)) (param i32) (param i32)
+         (param (ref null $bytes)) (param i32) (param i32)
          (result (ref extern))))
    (import "wasm:text-encoder" "encodeStringToUTF8Array"
       (func $encodeStringToUTF8Array
-         (param externref) (result (ref $string))))
-
+         (param externref) (result (ref $bytes))))
    (import "bindings" "read_string"
       (func $read_string (param i32) (result anyref)))
    (import "bindings" "read_string_stream"
@@ -43,8 +47,13 @@
       (func $write_string (param anyref) (result i32)))
    (import "bindings" "append_string"
       (func $append_string (param anyref) (param anyref) (result anyref)))
+   (import "js" "caml_utf16_of_utf8"
+      (func $utf16_of_utf8 (param anyref) (result anyref)))
+   (import "js" "caml_utf8_of_utf16"
+      (func $utf8_of_utf16 (param anyref) (result anyref)))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $wstring (array (mut i16)))
 
    (global $text_converters_available (mut i32) (i32.const 0))
@@ -64,10 +73,10 @@
             (i32.const 0)
             (call $compare_strings
                (call $decodeStringFromUTF8Array
-                  (array.new_fixed $string 1 (i32.const 0))
+                  (array.new_fixed $bytes 1 (i32.const 0))
                   (i32.const 0) (i32.const 1))
                (call $decodeStringFromUTF8Array
-                  (array.new_fixed $string 1 (i32.const 1))
+                  (array.new_fixed $bytes 1 (i32.const 1))
                    (i32.const 0) (i32.const 1)))))
       (global.set $string_builtins_available
          (i32.ne
@@ -96,11 +105,13 @@
    (func (export "jsstring_test") (param $s anyref) (result i32)
       (return_call $is_string (extern.convert_any (local.get $s))))
 
+   (func (export "jsstring_length") (param $s anyref) (result i32)
+      (return_call $string_length (extern.convert_any (local.get $s))))
+
    (export "jsstring_hash" (func $hash_string))
 
-   ;; Used by package zarith_stubs_js
-   (func $jsstring_of_substring (export "jsstring_of_substring")
-      (param $s (ref $string)) (param $pos i32) (param $len i32)
+   (func $jsstring_of_subbytes (export "jsstring_of_subbytes")
+      (param $s (ref $bytes)) (param $pos i32) (param $len i32)
       (result anyref)
       (local $i i32) (local $c i32)
       (if (global.get $text_converters_available)
@@ -118,7 +129,7 @@
                (if (i32.lt_u (local.get $i) (local.get $len))
                   (then
                      (local.set $c
-                        (array.get $string (local.get $s)
+                        (array.get $bytes (local.get $s)
                            (i32.add (local.get $pos) (local.get $i))))
                      (br_if $continue
                         (i32.ge_u (local.get $c) (i32.const 128)))
@@ -130,19 +141,163 @@
                (any.convert_extern
                   (call $fromCharCodeArray (global.get $buffer)
                      (i32.const 0) (local.get $len))))))
-      (return_call $jsstring_of_substring_fallback
+      (return_call $jsstring_of_subbytes_fallback
          (local.get $s) (local.get $pos) (local.get $len)))
 
-   (func (export "jsstring_of_string") (param $s (ref $string)) (result anyref)
-      (return_call $jsstring_of_substring
+   (func $jsstring_of_bytes (export "jsstring_of_bytes")
+      (param $s (ref $bytes)) (result anyref)
+      (return_call $jsstring_of_subbytes
          (local.get $s) (i32.const 0) (array.len (local.get $s))))
 
-   (func (export "string_of_jsstring") (param $s anyref) (result (ref $string))
+   (func $bytes_of_jsstring (export "bytes_of_jsstring")
+      (param $s anyref) (result (ref $bytes))
       (if (global.get $text_converters_available)
          (then
             (return_call $encodeStringToUTF8Array
                (extern.convert_any (local.get $s)))))
-      (return_call $string_of_jsstring_fallback (local.get $s)))
+      (return_call $bytes_of_jsstring_fallback (local.get $s)))
+
+   (func $string_is_ascii (param $vs (ref eq)) (result i32)
+      (local $s externref) (local $len i32) (local $i i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0 (ref.cast (ref $string) (local.get $vs)))))
+      (local.set $len (call $string_length (local.get $s)))
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $len))
+            (then
+               (if (i32.ge_u (call $string_get (local.get $s) (local.get $i))
+                      (i32.const 128))
+                  (then (return (i32.const 0))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (i32.const 1))
+
+   (func $jsstring_of_string (export "jsstring_of_string")
+      (param $s (ref eq)) (result (ref eq))
+      (if (result (ref eq)) (call $string_is_ascii (local.get $s))
+         (then
+            (local.get $s))
+         (else
+            (return
+               (struct.new $string
+                  (call $utf16_of_utf8
+                     (struct.get $string 0
+                        (ref.cast (ref $string) (local.get $s)))))))))
+
+   (func (export "jsstring_of_substring")
+      (param $s (ref eq)) (param $i i32) (param $l i32) (result (ref eq))
+      (return_call $jsstring_of_string
+         (struct.new $string
+            (any.convert_extern
+               (call $string_sub
+                  (extern.convert_any
+                     (struct.get $string 0
+                        (ref.cast (ref $string) (local.get $s))))
+                  (local.get $i)
+                  (i32.add (local.get $i) (local.get $l)))))))
+
+   (func (export "string_of_jsstring")
+      (param $s (ref eq)) (result (ref eq))
+      (if (result (ref eq)) (call $string_is_ascii (local.get $s))
+         (then
+            (local.get $s))
+         (else
+            (return
+               (struct.new $string
+                  (call $utf8_of_utf16
+                     (struct.get $string 0
+                        (ref.cast (ref $string) (local.get $s)))))))))
+
+   (func (export "jsbytes_of_bytes") (param $s (ref $bytes)) (result anyref)
+      (local $s' (ref $bytes))
+      (local $l i32) (local $i i32) (local $n i32) (local $c i32)
+      (local.set $l (array.len (local.get $s)))
+      (local.set $i (i32.const 0))
+      (local.set $n (i32.const 0))
+      (loop $count
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (if (i32.ge_u (array.get_u $bytes (local.get $s) (local.get $i))
+                      (i32.const 128))
+                  (then (local.set $n (i32.add (local.get $n) (i32.const 1)))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $count))))
+      (if (i32.eqz (local.get $n))
+         (then
+            (return_call $jsstring_of_bytes (local.get $s))))
+      (local.set $s'
+         (array.new $bytes (i32.const 0)
+            (i32.add (local.get $i) (local.get $n))))
+      (local.set $i (i32.const 0))
+      (local.set $n (i32.const 0))
+      (loop $fill
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (local.set $c (array.get_u $bytes (local.get $s) (local.get $i)))
+               (if (i32.lt_u (local.get $c) (i32.const 128))
+                  (then
+                     (array.set $bytes
+                        (local.get $s') (local.get $n) (local.get $c))
+                     (local.set $n (i32.add (local.get $n) (i32.const 1))))
+                  (else
+                     (array.set $bytes (local.get $s')
+                        (local.get $n)
+                        (i32.or (i32.shr_u (local.get $c) (i32.const 6))
+                           (i32.const 0xC0)))
+                     (array.set $bytes (local.get $s')
+                        (i32.add (local.get $n) (i32.const 1))
+                        (i32.or (i32.const 0x80)
+                           (i32.and (local.get $c) (i32.const 0x3F))))
+                     (local.set $n (i32.add (local.get $n) (i32.const 2)))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $fill))))
+      (return_call $jsstring_of_bytes (local.get $s')))
+
+   (func (export "bytes_of_jsbytes") (param $s anyref) (result (ref $bytes))
+      (local $l i32) (local $i i32) (local $n i32) (local $c i32)
+      (local $s' (ref $bytes)) (local $s'' (ref $bytes))
+      (local.set $s' (call $bytes_of_jsstring (local.get $s)))
+      (local.set $l (array.len (local.get $s')))
+      (local.set $i (i32.const 0))
+      (local.set $n (i32.const 0))
+      (loop $count
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (if (i32.ge_u (array.get_u $bytes (local.get $s') (local.get $i))
+                      (i32.const 0xC0))
+                  (then (local.set $n (i32.add (local.get $n) (i32.const 1)))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $count))))
+      (if (i32.eqz (local.get $n)) (then (return (local.get $s'))))
+      (local.set $s''
+         (array.new $bytes (i32.const 0)
+            (i32.sub (local.get $i) (local.get $n))))
+      (local.set $i (i32.const 0))
+      (local.set $n (i32.const 0))
+      (loop $fill
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (local.set $c
+                  (array.get_u $bytes (local.get $s') (local.get $i)))
+               (if (i32.lt_u (local.get $c) (i32.const 0xC0))
+                  (then
+                     (array.set $bytes
+                        (local.get $s'') (local.get $n) (local.get $c))
+                     (local.set $i (i32.add (local.get $i) (i32.const 1))))
+                  (else
+                     (array.set $bytes (local.get $s'')
+                        (local.get $n)
+                        (i32.sub
+                           (i32.add
+                              (i32.shl (local.get $c) (i32.const 6))
+                              (array.get_u $bytes (local.get $s')
+                                 (i32.add (local.get $i) (i32.const 1))))
+                           (i32.const 0x3080)))
+                     (local.set $i (i32.add (local.get $i) (i32.const 2)))))
+               (local.set $n (i32.add (local.get $n) (i32.const 1)))
+               (br $fill))))
+      (local.get $s''))
 
    ;; Fallback implementation of string conversion functions
 
@@ -151,19 +306,19 @@
    (global $buffer_size i32 (i32.const 65536))
 
    (func $write_to_buffer
-      (param $s (ref $string)) (param $pos i32) (param $len i32)
+      (param $s (ref $bytes)) (param $pos i32) (param $len i32)
       (local $i i32)
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $len))
             (then
                (i32.store8 (local.get $i)
-                  (array.get_u $string (local.get $s)
+                  (array.get_u $bytes (local.get $s)
                      (i32.add (local.get $pos) (local.get $i))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop)))))
 
-   (func $jsstring_of_substring_fallback
-      (param $s (ref $string)) (param $pos i32) (param $len i32)
+   (func $jsstring_of_subbytes_fallback
+      (param $s (ref $bytes)) (param $pos i32) (param $len i32)
       (result anyref)
       (local $s' anyref)
       (local $continued i32)
@@ -195,30 +350,30 @@
       (local.get $s'))
 
    (func $read_from_buffer
-      (param $s (ref $string)) (param $pos i32) (param $len i32)
+      (param $s (ref $bytes)) (param $pos i32) (param $len i32)
       (local $i i32)
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $len))
             (then
-               (array.set $string (local.get $s)
+               (array.set $bytes (local.get $s)
                   (i32.add (local.get $pos) (local.get $i))
                   (i32.load8_u (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop)))))
 
    (type $stack
-      (struct (field $s (ref $string)) (field $next (ref null $stack))))
+      (struct (field $s (ref $bytes)) (field $next (ref null $stack))))
    (global $stack (mut (ref null $stack)) (ref.null $stack))
 
-   (func $string_of_jsstring_fallback (param $s anyref) (result (ref $string))
+   (func $bytes_of_jsstring_fallback (param $s anyref) (result (ref $bytes))
       (local $ofs i32) (local $len i32)
-      (local $s' (ref $string)) (local $s'' (ref $string))
+      (local $s' (ref $bytes)) (local $s'' (ref $bytes))
       (local $item (ref $stack))
       (local.set $len (call $write_string (local.get $s)))
       (if (ref.is_null (global.get $stack))
          (then
             (local.set $s'
-               (array.new $string (i32.const 0) (local.get $len)))
+               (array.new $bytes (i32.const 0) (local.get $len)))
             (call $read_from_buffer
                (local.get $s') (i32.const 0) (local.get $len))
             (return (local.get $s'))))
@@ -232,7 +387,7 @@
                (br_on_null $done (struct.get $stack $next (local.get $item))))
             (br $loop)))
       (local.set $s'
-         (array.new $string (i32.const 0)
+         (array.new $bytes (i32.const 0)
             (i32.add (local.get $len) (local.get $ofs))))
       (call $read_from_buffer
          (local.get $s') (local.get $ofs) (local.get $len))
@@ -243,7 +398,7 @@
             (local.set $s'' (struct.get $stack $s (local.get $item)))
             (local.set $len (array.len (local.get $s'')))
             (local.set $ofs (i32.sub (local.get $ofs) (local.get $len)))
-            (array.copy $string $string
+            (array.copy $bytes $bytes
                (local.get $s') (local.get $ofs)
                (local.get $s'') (i32.const 0)
                (local.get $len))
@@ -253,8 +408,110 @@
       (local.get $s'))
 
    (func (export "caml_extract_string") (param $len i32)
-      (local $s (ref $string))
-      (local.set $s (array.new $string (i32.const 0) (local.get $len)))
+      (local $s (ref $bytes))
+      (local.set $s (array.new $bytes (i32.const 0) (local.get $len)))
       (call $read_from_buffer (local.get $s) (i32.const 0) (local.get $len))
       (global.set $stack (struct.new $stack (local.get $s) (global.get $stack))))
+
+   (func $utf16_to_utf8
+      (param $s externref) (param $l i32) (param $b (ref $wstring)) (result i32)
+      (local $i i32) (local $j i32) (local $c i32) (local $d i32)
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (local.set $c (call $string_get (local.get $s) (local.get $i)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (if (i32.lt_u (local.get $c) (i32.const 0x80))
+                  (then
+                     (array.set $wstring
+                        (local.get $b) (local.get $j) (local.get $c))
+                     (local.set $j (i32.add (local.get $j) (i32.const 1)))
+                     (br $loop)))
+               (if (i32.lt_u (local.get $c) (i32.const 0x800))
+                  (then
+                     (array.set $wstring
+                        (local.get $b) (local.get $j)
+                        (i32.or (i32.const 0xC0)
+                           (i32.shr_u (local.get $c) (i32.const 6))))
+                     (array.set $wstring
+                        (local.get $b)
+                        (i32.add (local.get $j) (i32.const 1))
+                        (i32.or (i32.const 0x80)
+                           (i32.and (local.get $c) (i32.const 0x3F))))
+                     (local.set $j (i32.add (local.get $j) (i32.const 2)))
+                     (br $loop)))
+               (if (i32.and
+                      (i32.ge_u (local.get $c) (i32.const 0xD800))
+                      (i32.lt_u (local.get $c) (i32.const 0xE000)))
+                  (then
+                     (if (i32.and
+                           (i32.lt_u (local.get $c) (i32.const 0xDC00))
+                           (i32.lt_u (local.get $i) (local.get $l)))
+                        (then
+                           (local.set $d
+                              (call $string_get (local.get $s) (local.get $i)))
+                           (if (i32.and
+                                  (i32.ge_u (local.get $c) (i32.const 0xDC00))
+                                  (i32.lt_u (local.get $c) (i32.const 0xE000)))
+                              (then
+                                 (local.set $i
+                                    (i32.add (local.get $i) (i32.const 1)))
+                                 (local.set $c
+                                    (i32.sub
+                                       (i32.add
+                                          (i32.shl
+                                             (local.get $c)
+                                             (i32.const 10))
+                                          (local.get $d))
+                                       (i32.const 0x35fdc00)))
+                                 (array.set $wstring
+                                    (local.get $b) (local.get $j)
+                                    (i32.or (i32.const 0xE0)
+                                       (i32.shr_u (local.get $c)
+                                          (i32.const 18))))
+                                 (array.set $wstring
+                                    (local.get $b)
+                                    (i32.add (local.get $j) (i32.const 1))
+                                    (i32.or (i32.const 0x80)
+                                       (i32.and
+                                          (i32.shr_u (local.get $c)
+                                             (i32.const 12))
+                                          (i32.const 0x3F))))
+                                 (array.set $wstring
+                                    (local.get $b)
+                                    (i32.add (local.get $j) (i32.const 2))
+                                    (i32.or (i32.const 0x80)
+                                       (i32.and
+                                          (i32.shr_u (local.get $c)
+                                             (i32.const 6))
+                                          (i32.const 0x3F))))
+                                 (array.set $wstring
+                                    (local.get $b)
+                                    (i32.add (local.get $j) (i32.const 3))
+                                    (i32.or (i32.const 0x80)
+                                       (i32.and (local.get $c)
+                                          (i32.const 0x3F))))
+                                 (local.set $j
+                                    (i32.add (local.get $j) (i32.const 4)))
+                                 (br $loop)))))
+                     ;; replacement character
+                     (local.set $c (i32.const 0xFFFD))))
+               (array.set $wstring
+                  (local.get $b) (local.get $j)
+                  (i32.or (i32.const 0xE0)
+                     (i32.shr_u (local.get $c) (i32.const 12))))
+               (array.set $wstring
+                  (local.get $b)
+                  (i32.add (local.get $j) (i32.const 1))
+                  (i32.or (i32.const 0x80)
+                     (i32.and (i32.shr_u (local.get $c) (i32.const 6))
+                        (i32.const 0x3F))))
+               (array.set $wstring
+                  (local.get $b)
+                  (i32.add (local.get $j) (i32.const 2))
+                  (i32.or (i32.const 0x80)
+                     (i32.and (local.get $c) (i32.const 0x3F))))
+               (local.set $j (i32.add (local.get $j) (i32.const 3)))
+               (br $loop))))
+      (local.get $j))
 )
diff --git a/runtime/wasm/lexing.wat b/runtime/wasm/lexing.wat
index eb9b94b1ad..8fc76703a4 100644
--- a/runtime/wasm/lexing.wat
+++ b/runtime/wasm/lexing.wat
@@ -19,16 +19,39 @@
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
-   (func $get (param $a (ref eq)) (param $i i32) (result i32)
-      (local $s (ref $string))
-      (local.set $s (ref.cast (ref $string) (local.get $a)))
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
+
+   (func $get
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (result i32)
       (local.set $i (i32.add (local.get $i) (local.get $i)))
       (i32.extend16_s
-         (i32.or (array.get_u $string (local.get $s) (local.get $i))
+         (i32.or (call $string_get (local.get $s) (local.get $i))
             (i32.shl
-               (array.get_u $string (local.get $s)
+               (call $string_get (local.get $s)
                   (i32.add (local.get $i) (i32.const 1)))
                (i32.const 8)))))
 
@@ -52,7 +75,7 @@
    (global $lex_check_code i32 (i32.const 10))
    (global $lex_code i32 (i32.const 11))
 
-   (data $lexing_empty_token "lexing: empty token")
+   (#string $lexing_empty_token "lexing: empty token")
 
    (func (export "caml_lex_engine")
       (param $vtbl (ref eq)) (param $start_state (ref eq))
@@ -62,21 +85,32 @@
       (local $lexbuf (ref $block))
       (local $c i32)
       (local $state i32)
-      (local $buffer (ref $string))
+      (local $buffer (ref $bytes))
       (local $vpos (ref eq)) (local $action (ref eq))
       (local $pos i32) (local $base i32) (local $backtrk i32)
-      (local $lex_base (ref $string))
-      (local $lex_backtrk (ref $string))
-      (local $lex_check (ref $string))
-      (local $lex_check_code (ref $string))
-      (local $lex_trans (ref $string))
-      (local $lex_default (ref $string))
+(#if use-js-string
+(#then
+      (local $lex_base externref)
+      (local $lex_backtrk externref)
+      (local $lex_check externref)
+      (local $lex_check_code externref)
+      (local $lex_trans externref)
+      (local $lex_default externref)
+)
+(#else
+      (local $lex_base (ref $bytes))
+      (local $lex_backtrk (ref $bytes))
+      (local $lex_check (ref $bytes))
+      (local $lex_check_code (ref $bytes))
+      (local $lex_trans (ref $bytes))
+      (local $lex_default (ref $bytes))
+))
       (local.set $tbl (ref.cast (ref $block) (local.get $vtbl)))
       (local.set $lexbuf (ref.cast (ref $block) (local.get $vlexbuf)))
       (local.set $state
          (i31.get_s (ref.cast (ref i31) (local.get $start_state))))
       (local.set $buffer
-         (ref.cast (ref $string)
+         (ref.cast (ref $bytes)
             (array.get $block (local.get $lexbuf) (global.get $lex_buffer))))
       (if (i32.ge_s (local.get $state) (i32.const 0))
          (then
@@ -91,22 +125,22 @@
          (else
             (local.set $state (i32.sub (i32.const -1) (local.get $state)))))
       (local.set $lex_base
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base))))
       (local.set $lex_backtrk
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk))))
       (local.set $lex_check
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check))))
       (local.set $lex_check_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check_code))))
       (local.set $lex_trans
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans))))
       (local.set $lex_default
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_default))))
       (loop $loop
          (local.set $base (call $get (local.get $lex_base) (local.get $state)))
@@ -149,7 +183,7 @@
                         (array.get $block (local.get $lexbuf)
                            (global.get $lex_curr_pos)))))
                (local.set $c
-                  (array.get_u $string (local.get $buffer) (local.get $pos)))
+                  (array.get_u $bytes (local.get $buffer) (local.get $pos)))
                (array.set $block (local.get $lexbuf) (global.get $lex_curr_pos)
                   (ref.i31 (i32.add (local.get $pos) (i32.const 1))))))
          (if (i32.eq
@@ -172,10 +206,7 @@
                   (array.get $block (local.get $lexbuf)
                      (global.get $lex_last_action)))
                (if (ref.eq (local.get $action) (ref.i31 (i32.const -1)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $lexing_empty_token
-                           (i32.const 0) (i32.const 19)))))
+                  (then (call $caml_failwith (global.get $lexing_empty_token))))
                (return (local.get $action))))
          (if (i32.eq (local.get $c) (i32.const 256))
             (then
@@ -185,7 +216,14 @@
          (br $loop)))
 
    (func $run_mem
-      (param $s (ref $string)) (param $i i32) (param $lexbuf (ref $block))
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (param $lexbuf (ref $block))
       (param $curr_pos (ref eq))
       (local $dst i32) (local $src i32)
       (local $mem (ref $block))
@@ -193,11 +231,11 @@
          (ref.cast (ref $block)
             (array.get $block (local.get $lexbuf) (global.get $lex_mem))))
       (loop $loop
-         (local.set $dst (array.get_u $string (local.get $s) (local.get $i)))
+         (local.set $dst (call $string_get (local.get $s) (local.get $i)))
          (if (i32.eq (local.get $dst) (i32.const 0xff))
             (then (return)))
          (local.set $src
-            (array.get_u $string (local.get $s)
+            (call $string_get (local.get $s)
               (i32.add (local.get $i) (i32.const 1))))
          (local.set $i (i32.add (local.get $i) (i32.const 2)))
          (array.set $block (local.get $mem)
@@ -211,7 +249,14 @@
          (br $loop)))
 
    (func $run_tag
-      (param $s (ref $string)) (param $i i32) (param $lexbuf (ref $block))
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (param $lexbuf (ref $block))
       (return_call $run_mem (local.get $s) (local.get $i) (local.get $lexbuf)
          (ref.i31 (i32.const -1))))
 
@@ -223,27 +268,43 @@
       (local $lexbuf (ref $block))
       (local $c i32)
       (local $state i32) (local $pstate i32)
-      (local $buffer (ref $string))
+      (local $buffer (ref $bytes))
       (local $vpos (ref eq)) (local $action (ref eq))
       (local $pos i32) (local $base i32) (local $backtrk i32)
       (local $pc_off i32) (local $base_code i32)
-      (local $lex_code (ref $string))
-      (local $lex_base (ref $string))
-      (local $lex_base_code (ref $string))
-      (local $lex_backtrk (ref $string))
-      (local $lex_backtrk_code (ref $string))
-      (local $lex_check (ref $string))
-      (local $lex_check_code (ref $string))
-      (local $lex_trans (ref $string))
-      (local $lex_trans_code (ref $string))
-      (local $lex_default (ref $string))
-      (local $lex_default_code (ref $string))
+(#if use-js-string
+(#then
+      (local $lex_code externref)
+      (local $lex_base externref)
+      (local $lex_base_code externref)
+      (local $lex_backtrk externref)
+      (local $lex_backtrk_code externref)
+      (local $lex_check externref)
+      (local $lex_check_code externref)
+      (local $lex_trans externref)
+      (local $lex_trans_code externref)
+      (local $lex_default externref)
+      (local $lex_default_code externref)
+)
+(#else
+      (local $lex_code (ref $bytes))
+      (local $lex_base (ref $bytes))
+      (local $lex_base_code (ref $bytes))
+      (local $lex_backtrk (ref $bytes))
+      (local $lex_backtrk_code (ref $bytes))
+      (local $lex_check (ref $bytes))
+      (local $lex_check_code (ref $bytes))
+      (local $lex_trans (ref $bytes))
+      (local $lex_trans_code (ref $bytes))
+      (local $lex_default (ref $bytes))
+      (local $lex_default_code (ref $bytes))
+))
       (local.set $tbl (ref.cast (ref $block) (local.get $vtbl)))
       (local.set $lexbuf (ref.cast (ref $block) (local.get $vlexbuf)))
       (local.set $state
          (i31.get_s (ref.cast (ref i31) (local.get $start_state))))
       (local.set $buffer
-         (ref.cast (ref $string)
+         (ref.cast (ref $bytes)
             (array.get $block (local.get $lexbuf) (global.get $lex_buffer))))
       (if (i32.ge_s (local.get $state) (i32.const 0))
          (then
@@ -258,37 +319,37 @@
          (else
             (local.set $state (i32.sub (i32.const -1) (local.get $state)))))
       (local.set $lex_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_code))))
       (local.set $lex_base
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base))))
       (local.set $lex_base_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base_code))))
       (local.set $lex_backtrk
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk))))
       (local.set $lex_backtrk_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk_code))))
       (local.set $lex_check
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check))))
       (local.set $lex_check_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check_code))))
       (local.set $lex_trans
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans))))
       (local.set $lex_trans_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans_code))))
       (local.set $lex_default
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_default))))
       (local.set $lex_default_code
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_default_code))))
       (loop $loop
          (local.set $base (call $get (local.get $lex_base) (local.get $state)))
@@ -339,7 +400,7 @@
                         (array.get $block (local.get $lexbuf)
                            (global.get $lex_curr_pos)))))
                (local.set $c
-                  (array.get_u $string (local.get $buffer) (local.get $pos)))
+                  (array.get_u $bytes (local.get $buffer) (local.get $pos)))
                (array.set $block (local.get $lexbuf) (global.get $lex_curr_pos)
                   (ref.i31 (i32.add (local.get $pos) (i32.const 1))))))
          (local.set $pstate (local.get $state))
@@ -363,10 +424,7 @@
                   (array.get $block (local.get $lexbuf)
                      (global.get $lex_last_action)))
                (if (ref.eq (local.get $action) (ref.i31 (i32.const -1)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $lexing_empty_token
-                           (i32.const 0) (i32.const 19)))))
+                  (then (call $caml_failwith (global.get $lexing_empty_token))))
                (return (local.get $action))))
          (local.set $base_code
             (call $get (local.get $lex_base_code) (local.get $pstate)))
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 46373b2bf7..526f606df3 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -20,7 +20,6 @@
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
    (import "fail" "caml_raise_end_of_file" (func $caml_raise_end_of_file))
-   (import "obj" "double_array_tag" (global $double_array_tag i32))
    (import "string" "caml_string_concat"
       (func $caml_string_concat
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
@@ -35,10 +34,10 @@
       (func $map_set (param (ref any)) (param (ref eq)) (param (ref i31))))
    (import "io" "caml_really_putblock"
       (func $caml_really_putblock
-         (param (ref eq)) (param (ref $string)) (param i32) (param i32)))
+         (param (ref eq)) (param (ref $bytes)) (param i32) (param i32)))
    (import "io" "caml_really_getblock"
       (func $caml_really_getblock
-         (param (ref eq)) (param (ref $string)) (param i32) (param i32)
+         (param (ref eq)) (param (ref $bytes)) (param i32) (param i32)
          (result i32)))
    (import "io" "caml_flush_if_unbuffered"
       (func $caml_flush_if_unbuffered (param (ref eq))))
@@ -46,28 +45,44 @@
       (func $caml_init_custom_operations))
    (import "custom" "caml_find_custom_operations"
       (func $caml_find_custom_operations
-         (param (ref $string)) (result (ref null $custom_operations))))
+         (param (ref $bytes)) (result (ref null $custom_operations))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_bytes_of_string"
+      (func $caml_bytes_of_string (param (ref eq)) (result (ref eq))))
+   (import "jsstring" "jsstring_test"
+      (func $jsstring_test (param anyref) (result i32)))
+   (import "jsstring" "jsstring_length"
+      (func $jsstring_length (param anyref) (result i32)))
+   (import "string" "caml_blit_string"
+      (func $caml_blit_string
+         (param (ref eq) (ref eq) (ref eq) (ref eq) (ref eq))
+         (result (ref eq))))
 
    (import "version-dependent" "caml_marshal_header_size"
       (global $caml_marshal_header_size i32))
 
-   (global $input_val_from_string (ref $string)
-      (array.new_fixed $string 21
-         (i32.const 105) (i32.const 110) (i32.const 112) (i32.const 117)
-         (i32.const 116) (i32.const 95) (i32.const 118) (i32.const 97)
-         (i32.const 108) (i32.const 95) (i32.const 102) (i32.const 114)
-         (i32.const 111) (i32.const 109) (i32.const 95) (i32.const 115)
-         (i32.const 116) (i32.const 114) (i32.const 105) (i32.const 110)
-         (i32.const 103)))
+   (#string $input_val_from_string "input_val_from_string")
 
+(#if use-js-string
+(#then
+   (func (export "caml_input_value_from_string")
+      (param $vstr (ref eq)) (param $vofs (ref eq)) (result (ref eq))
+      ;; It would be better to parse the header and extract just the
+      ;; relevant substring
+      (return_call $caml_input_value_from_bytes
+         (call $caml_bytes_of_string (local.get $vstr)) (local.get $vofs)))
+)
+(#else
    (export "caml_input_value_from_string" (func $caml_input_value_from_bytes))
+))
    (func $caml_input_value_from_bytes (export "caml_input_value_from_bytes")
       (param $vstr (ref eq)) (param $vofs (ref eq)) (result (ref eq))
-      (local $str (ref $string))
+      (local $str (ref $bytes))
       (local $ofs i32)
       (local $s (ref $intern_state))
       (local $h (ref $marshal_header))
-      (local.set $str (ref.cast (ref $string) (local.get $vstr)))
+      (local.set $str (ref.cast (ref $bytes) (local.get $vstr)))
       (local.set $ofs (i31.get_u (ref.cast (ref i31) (local.get $vofs))))
       (local.set $s
          (call $get_intern_state (local.get $str) (local.get $ofs)))
@@ -82,49 +97,39 @@
             (call $bad_length (global.get $input_val_from_string))))
       (return_call $intern_rec (local.get $s) (local.get $h)))
 
-   (data $truncated_obj "input_value: truncated object")
-
-   (global $input_value (ref $string)
-      (array.new_fixed $string 11
-         (i32.const 105) (i32.const 110) (i32.const 112) (i32.const 117)
-         (i32.const 116) (i32.const 95) (i32.const 118) (i32.const 97)
-         (i32.const 108) (i32.const 117) (i32.const 101)))
+   (#string $truncated_obj "input_value: truncated object")
+   (#string $input_value "input_value")
 
    (func (export "caml_input_value") (param $ch (ref eq)) (result (ref eq))
       ;; ZZZ check binary channel?
       (local $r i32) (local $len i32)
-      (local $header (ref $string)) (local $buf (ref $string))
+      (local $header (ref $bytes)) (local $buf (ref $bytes))
       (local $s (ref $intern_state)) (local $h (ref $marshal_header))
-      (local.set $header (array.new $string (i32.const 0) (i32.const 20)))
+      (local.set $header (array.new $bytes (i32.const 0) (i32.const 20)))
       (local.set $r
          (call $caml_really_getblock
             (local.get $ch) (local.get $header) (i32.const 0) (i32.const 20)))
       (if (i32.eqz (local.get $r))
          (then (call $caml_raise_end_of_file)))
       (if (i32.lt_u (local.get $r) (i32.const 20))
-         (then
-            (call $caml_failwith
-               (array.new_data $string $truncated_obj
-                  (i32.const 0) (i32.const 29)))))
+         (then (call $caml_failwith (global.get $truncated_obj))))
       (local.set $s
          (call $get_intern_state (local.get $header) (i32.const 0)))
       (local.set $h
          (call $parse_header (local.get $s) (global.get $input_value)))
       (local.set $len (struct.get $marshal_header $data_len (local.get $h)))
-      (local.set $buf (array.new $string (i32.const 0) (local.get $len)))
+      (local.set $buf (array.new $bytes (i32.const 0) (local.get $len)))
       (if (i32.lt_u
              (call $caml_really_getblock (local.get $ch)
                 (local.get $buf) (i32.const 0) (local.get $len))
              (local.get $len))
-         (then
-            (call $caml_failwith
-               (array.new_data $string $truncated_obj
-                  (i32.const 0) (i32.const 29)))))
+         (then (call $caml_failwith (global.get $truncated_obj))))
       (local.set $s (call $get_intern_state (local.get $buf) (i32.const 0)))
       (return_call $intern_rec (local.get $s) (local.get $h)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
    (type $js (struct (field anyref)))
@@ -140,7 +145,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -181,13 +186,13 @@
 
    (type $intern_state
       (struct
-         (field $src (ref $string))
+         (field $src (ref $bytes))
          (field $pos (mut i32))
          (field $obj_table (mut (ref null $block)))
          (field $obj_counter (mut i32))))
 
    (func $get_intern_state
-      (param $src (ref $string)) (param $pos i32) (result (ref $intern_state))
+      (param $src (ref $bytes)) (param $pos i32) (result (ref $intern_state))
       (struct.new $intern_state
          (local.get $src) (local.get $pos) (ref.null $block) (i32.const 0)))
 
@@ -195,7 +200,7 @@
       (local $pos i32) (local $res i32)
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (local.set $res
-         (array.get_u $string
+         (array.get_u $bytes
             (struct.get $intern_state $src (local.get $s))
             (local.get $pos)))
       (struct.set $intern_state $pos (local.get $s)
@@ -206,7 +211,7 @@
       (local $pos i32) (local $res i32)
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (local.set $res
-         (array.get_s $string
+         (array.get_s $bytes
             (struct.get $intern_state $src (local.get $s))
             (local.get $pos)))
       (struct.set $intern_state $pos (local.get $s)
@@ -214,84 +219,84 @@
       (local.get $res))
 
    (func $read16u (param $s (ref $intern_state)) (result i32)
-      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local $src (ref $bytes)) (local $pos i32) (local $res i32)
       (local.set $src (struct.get $intern_state $src (local.get $s)))
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (local.set $res
          (i32.or
             (i32.shl
-               (array.get_u $string (local.get $src) (local.get $pos))
+               (array.get_u $bytes (local.get $src) (local.get $pos))
                (i32.const 8))
-            (array.get_u $string (local.get $src)
+            (array.get_u $bytes (local.get $src)
                (i32.add (local.get $pos) (i32.const 1)))))
       (struct.set $intern_state $pos (local.get $s)
          (i32.add (local.get $pos) (i32.const 2)))
       (local.get $res))
 
    (func $read16s (param $s (ref $intern_state)) (result i32)
-      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local $src (ref $bytes)) (local $pos i32) (local $res i32)
       (local.set $src (struct.get $intern_state $src (local.get $s)))
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (local.set $res
          (i32.or
             (i32.shl
-               (array.get_s $string (local.get $src) (local.get $pos))
+               (array.get_s $bytes (local.get $src) (local.get $pos))
                (i32.const 8))
-            (array.get_u $string (local.get $src)
+            (array.get_u $bytes (local.get $src)
                (i32.add (local.get $pos) (i32.const 1)))))
       (struct.set $intern_state $pos (local.get $s)
          (i32.add (local.get $pos) (i32.const 2)))
       (local.get $res))
 
    (func $read32 (param $s (ref $intern_state)) (result i32)
-      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local $src (ref $bytes)) (local $pos i32) (local $res i32)
       (local.set $src (struct.get $intern_state $src (local.get $s)))
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (local.set $res
          (i32.or
             (i32.or
                (i32.shl
-                  (array.get_u $string (local.get $src) (local.get $pos))
+                  (array.get_u $bytes (local.get $src) (local.get $pos))
                   (i32.const 24))
                (i32.shl
-                  (array.get_u $string (local.get $src)
+                  (array.get_u $bytes (local.get $src)
                      (i32.add (local.get $pos) (i32.const 1)))
                   (i32.const 16)))
             (i32.or
                (i32.shl
-                  (array.get_u $string (local.get $src)
+                  (array.get_u $bytes (local.get $src)
                      (i32.add (local.get $pos) (i32.const 2)))
                   (i32.const 8))
-               (array.get_u $string (local.get $src)
+               (array.get_u $bytes (local.get $src)
                   (i32.add (local.get $pos) (i32.const 3))))))
       (struct.set $intern_state $pos (local.get $s)
          (i32.add (local.get $pos) (i32.const 4)))
       (local.get $res))
 
-   (func $readblock (param $s (ref $intern_state)) (param $str (ref $string))
+   (func $readblock (param $s (ref $intern_state)) (param $str (ref $bytes))
       (local $len i32) (local $pos i32)
       (local.set $len (array.len (local.get $str)))
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (local.get $str) (i32.const 0)
          (struct.get $intern_state $src (local.get $s)) (local.get $pos)
          (local.get $len))
       (struct.set $intern_state $pos (local.get $s)
          (i32.add (local.get $pos) (local.get $len))))
 
-   (func $readstr (param $s (ref $intern_state)) (result (ref $string))
-      (local $len i32) (local $pos i32) (local $res (ref $string))
-      (local $src (ref $string))
+   (func $readstr (param $s (ref $intern_state)) (result (ref $bytes))
+      (local $len i32) (local $pos i32) (local $res (ref $bytes))
+      (local $src (ref $bytes))
       (local.set $src (struct.get $intern_state $src (local.get $s)))
       (local.set $pos (struct.get $intern_state $pos (local.get $s)))
       (loop $loop
-         (if (array.get_u $string (local.get $src)
+         (if (array.get_u $bytes (local.get $src)
                 (i32.add (local.get $pos) (local.get $len)))
             (then
                (local.set $len (i32.add (local.get $len) (i32.const 1)))
                (br $loop))))
-      (local.set $res (array.new $string (i32.const 0) (local.get $len)))
-      (array.copy $string $string
+      (local.set $res (array.new $bytes (i32.const 0) (local.get $len)))
+      (array.copy $bytes $bytes
          (local.get $res) (i32.const 0)
          (local.get $src) (local.get $pos)
          (local.get $len))
@@ -301,7 +306,7 @@
 
    (func $readfloat
       (param $s (ref $intern_state)) (param $code i32) (result f64)
-      (local $src (ref $string)) (local $pos i32) (local $res i32)
+      (local $src (ref $bytes)) (local $pos i32) (local $res i32)
       (local $d i64)
       (local $i i32)
       (local $v (ref eq))
@@ -316,7 +321,7 @@
                   (i64.or
                      (i64.shl (local.get $d) (i64.const 8))
                      (i64.extend_i32_u
-                        (array.get_u $string (local.get $src)
+                        (array.get_u $bytes (local.get $src)
                           (i32.add (local.get $pos) (local.get $i))))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br_if $loop (i32.lt_u (local.get $i) (i32.const 8)))))
@@ -326,7 +331,7 @@
                   (i64.rotr
                      (i64.or (local.get $d)
                         (i64.extend_i32_u
-                           (array.get_u $string (local.get $src)
+                           (array.get_u $bytes (local.get $src)
                              (i32.add (local.get $pos) (local.get $i)))))
                      (i64.const 8)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
@@ -394,13 +399,13 @@
          (field $pos (mut i32))
          (field $next (ref null $stack_item))))
 
-   (data $integer_too_large "input_value: integer too large")
-   (data $code_pointer "input_value: code pointer")
-   (data $ill_formed "input_value: ill-formed message")
+   (#string $integer_too_large "input_value: integer too large")
+   (#string $code_pointer "input_value: code pointer")
+   (#string $ill_formed "input_value: ill-formed message")
 
-   (data $unknown_custom "input_value: unknown custom block identifier")
-   (data $expected_size "input_value: expected a fixed-size custom block")
-   (data $incorrect_size
+   (#string $unknown_custom "input_value: unknown custom block identifier")
+   (#string $expected_size "input_value: expected a fixed-size custom block")
+   (#string $incorrect_size
       "input_value: incorrect length of serialized custom block")
 
    (func $intern_custom
@@ -437,17 +442,10 @@
                   (i32.ne (tuple.extract 2 1 (local.get $r))
                      (local.get $expected_size))
                   (i32.ne (local.get $code) (global.get $CODE_CUSTOM)))
-               (then
-                  (call $caml_failwith
-                     (array.new_data $string $incorrect_size
-                        (i32.const 0) (i32.const 56)))))
+               (then (call $caml_failwith (global.get $incorrect_size))))
             (return (tuple.extract 2 0 (local.get $r))))
-         (call $caml_failwith
-            (array.new_data $string $expected_size
-               (i32.const 0) (i32.const 47))))
-      (call $caml_failwith
-         (array.new_data $string $unknown_custom
-            (i32.const 0) (i32.const 44)))
+         (call $caml_failwith (global.get $expected_size)))
+      (call $caml_failwith (global.get $unknown_custom))
       (ref.i31 (i32.const 0)))
 
    (func $intern_rec
@@ -460,7 +458,7 @@
       (local $header i32) (local $tag i32) (local $size i32)
       (local $len i32) (local $pos i32) (local $pos' i32) (local $ofs i32)
       (local $b (ref $block))
-      (local $str (ref $string))
+      (local $str (ref $bytes))
       (local $v (ref eq))
       (call $caml_init_custom_operations)
       (local.set $res (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
@@ -543,8 +541,7 @@
                                             (local.get $code)))
                                         ;; default
                                         (call $caml_failwith
-                                           (array.new_data $string $ill_formed
-                                              (i32.const 0) (i32.const 31)))
+                                           (global.get $ill_formed))
                                         (br $done))
                                        ;; CUSTOM
                                        (local.set $v
@@ -555,8 +552,7 @@
                                        (br $done))
                                       ;; CODEPOINTER
                                       (call $caml_failwith
-                                        (array.new_data $string $code_pointer
-                                           (i32.const 0) (i32.const 25)))
+                                        (global.get $code_pointer))
                                       (br $done))
                                      ;; DOUBLE_ARRAY32
                                      (local.set $len
@@ -600,8 +596,7 @@
                              (br $read_shared))
                             ;; INT64
                             (call $caml_failwith
-                               (array.new_data $string $integer_too_large
-                                  (i32.const 0) (i32.const 30)))
+                               (global.get $integer_too_large))
                             (br $done))
                            ;; INT32
                            (local.set $v (ref.i31 (call $read32 (local.get $s))))
@@ -632,9 +627,9 @@
            (call $register_object (local.get $s) (local.get $v))
            (br $done))
           ;; read_string
-          (local.set $str (array.new $string (i32.const 0) (local.get $len)))
+          (local.set $str (array.new $bytes (i32.const 0) (local.get $len)))
           (call $readblock (local.get $s) (local.get $str))
-          (local.set $v (local.get $str))
+          (local.set $v (call $caml_string_of_bytes (local.get $str)))
           (call $register_object (local.get $s) (local.get $v))
           (br $done))
          ;; read_block
@@ -656,26 +651,23 @@
         (br $loop)))
       (array.get $block (local.get $res) (i32.const 0)))
 
-   (data $too_large ": object too large to be read back on a 32-bit platform")
+   (#string $too_large ": object too large to be read back on a 32-bit platform")
 
-   (func $too_large (param $prim (ref $string))
+   (func $too_large (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $string $too_large (i32.const 0) (i32.const 55)))))
+         (call $caml_string_concat (local.get $prim) (global.get $too_large))))
 
-   (data $bad_object ": bad object")
+   (#string $bad_object ": bad object")
 
-   (func $bad_object (param $prim (ref $string))
+   (func $bad_object (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $string $bad_object (i32.const 0) (i32.const 12)))))
+         (call $caml_string_concat (local.get $prim) (global.get $bad_object))))
 
-   (data $bad_length ": bad length")
+   (#string $bad_length ": bad length")
 
-   (func $bad_length (param $prim (ref $string))
+   (func $bad_length (param $prim (ref eq))
       (call $caml_failwith
-         (call $caml_string_concat (local.get $prim)
-            (array.new_data $string $bad_length (i32.const 0) (i32.const 12)))))
+         (call $caml_string_concat (local.get $prim) (global.get $bad_length))))
 
    (type $marshal_header
       (struct
@@ -683,17 +675,15 @@
          (field $num_objects i32)))
 
    (func $parse_header
-      (param $s (ref $intern_state)) (param $prim (ref $string))
+      (param $s (ref $intern_state)) (param $prim (ref eq))
       (result (ref $marshal_header))
       (local $magic i32)
       (local $data_len i32) (local $num_objects i32) (local $whsize i32)
       (local.set $magic (call $read32 (local.get $s)))
       (if (i32.eq (local.get $magic) (global.get $Intext_magic_number_big))
-         (then
-            (call $too_large (local.get $prim))))
+         (then (call $too_large (local.get $prim))))
       (if (i32.ne (local.get $magic) (global.get $Intext_magic_number_small))
-         (then
-            (call $bad_object (local.get $prim))))
+         (then (call $bad_object (local.get $prim))))
       (local.set $data_len (call $read32 (local.get $s)))
       (local.set $num_objects (call $read32 (local.get $s)))
       (drop (call $read32 (local.get $s)))
@@ -702,7 +692,7 @@
          (local.get $data_len)
          (local.get $num_objects)))
 
-   (data $marshal_data_size "Marshal.data_size")
+   (#string $marshal_data_size "Marshal.data_size")
 
    (func (export "caml_marshal_data_size")
       (param $buf (ref eq)) (param $ofs (ref eq)) (result (ref eq))
@@ -710,19 +700,13 @@
       (local $magic i32)
       (local.set $s
          (call $get_intern_state
-            (ref.cast (ref $string) (local.get $buf))
+            (ref.cast (ref $bytes) (local.get $buf))
             (i31.get_u (ref.cast (ref i31) (local.get $ofs)))))
       (local.set $magic (call $read32 (local.get $s)))
       (if (i32.eq (local.get $magic) (global.get $Intext_magic_number_big))
-         (then
-            (call $too_large
-               (array.new_data $string $marshal_data_size
-                  (i32.const 0) (i32.const 17)))))
+         (then (call $too_large (global.get $marshal_data_size))))
       (if (i32.ne (local.get $magic) (global.get $Intext_magic_number_small))
-         (then
-            (call $bad_object
-               (array.new_data $string $marshal_data_size
-                  (i32.const 0) (i32.const 17)))))
+         (then (call $bad_object (global.get $marshal_data_size))))
       (ref.i31
          (i32.add
             (i32.sub (i32.const 20)
@@ -733,7 +717,7 @@
       (struct
          (field $next (mut (ref null $output_block)))
          (field $end (mut i32))
-         (field $data (ref $string))))
+         (field $data (ref $bytes))))
 
    (type $extern_state
       (struct
@@ -747,7 +731,7 @@
          ;; Position of already marshalled objects
          (field $pos_table (ref any))
          ;; Buffers
-         (field $buf (mut (ref $string)))
+         (field $buf (mut (ref $bytes)))
          (field $pos (mut i32))
          (field $limit (mut i32))
          (field $output_first (ref $output_block))
@@ -781,7 +765,7 @@
          (local.get $output)
          (local.get $output)))
 
-   (data $buffer_overflow "Marshal.to_buffer: buffer overflow")
+   (#string $buffer_overflow "Marshal.to_buffer: buffer overflow")
 
    (global $SIZE_EXTERN_OUTPUT_BLOCK i32 (i32.const 8100))
 
@@ -789,7 +773,7 @@
       (param $s (ref $extern_state)) (param $required i32) (result i32)
       (local $last (ref $output_block)) (local $blk (ref $output_block))
       (local $pos i32) (local $extra i32)
-      (local $buf (ref $string))
+      (local $buf (ref $bytes))
       (local.set $pos (struct.get $extern_state $pos (local.get $s)))
       (if (i32.le_u (i32.add (local.get $pos) (local.get $required))
              (struct.get $extern_state $limit (local.get $s)))
@@ -798,10 +782,7 @@
                (i32.add (local.get $pos) (local.get $required)))
             (return (local.get $pos))))
       (if (struct.get $extern_state $user_provided_output (local.get $s))
-         (then
-            (call $caml_failwith
-               (array.new_data $string $buffer_overflow
-                  (i32.const 0) (i32.const 34)))))
+         (then (call $caml_failwith (global.get $buffer_overflow))))
       (local.set $last (struct.get $extern_state $output_last (local.get $s)))
       (struct.set $output_block $end (local.get $last)
          (struct.get $extern_state $pos (local.get $s)))
@@ -810,7 +791,7 @@
          (then
             (local.set $extra (local.get $required))))
       (local.set $buf
-         (array.new $string (i32.const 0)
+         (array.new $bytes (i32.const 0)
             (i32.add (global.get $SIZE_EXTERN_OUTPUT_BLOCK) (local.get $extra))))
       (local.set $blk
          (struct.new $output_block
@@ -825,25 +806,25 @@
          (array.len (local.get $buf)))
       (i32.const 0))
 
-   (func $store16 (param $s (ref $string)) (param $pos i32) (param $n i32)
-      (array.set $string (local.get $s) (local.get $pos)
+   (func $store16 (param $s (ref $bytes)) (param $pos i32) (param $n i32)
+      (array.set $bytes (local.get $s) (local.get $pos)
          (i32.shr_u (local.get $n) (i32.const 8)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $pos) (i32.const 1)) (local.get $n)))
 
-   (func $store32 (param $s (ref $string)) (param $pos i32) (param $n i32)
-      (array.set $string (local.get $s) (local.get $pos)
+   (func $store32 (param $s (ref $bytes)) (param $pos i32) (param $n i32)
+      (array.set $bytes (local.get $s) (local.get $pos)
          (i32.shr_u (local.get $n) (i32.const 24)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $pos) (i32.const 1))
          (i32.shr_u (local.get $n) (i32.const 16)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $pos) (i32.const 2))
          (i32.shr_u (local.get $n) (i32.const 8)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $pos) (i32.const 3)) (local.get $n)))
 
-   (func $store64 (param $s (ref $string)) (param $pos i32) (param $n i64)
+   (func $store64 (param $s (ref $bytes)) (param $pos i32) (param $n i64)
       (call $store32 (local.get $s) (local.get $pos)
          (i32.wrap_i64 (i64.shr_u (local.get $n) (i64.const 32))))
       (call $store32 (local.get $s) (i32.add (local.get $pos) (i32.const 4))
@@ -853,58 +834,71 @@
       (local $pos i32)
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 1)))
-      (array.set $string (struct.get $extern_state $buf (local.get $s))
+      (array.set $bytes (struct.get $extern_state $buf (local.get $s))
          (local.get $pos) (local.get $c)))
 
    (func $writecode8
       (param $s (ref $extern_state)) (param $c i32) (param $v i32)
-      (local $pos i32) (local $buf (ref $string))
+      (local $pos i32) (local $buf (ref $bytes))
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 2)))
       (local.set $buf (struct.get $extern_state $buf (local.get $s)))
-      (array.set $string (local.get $buf) (local.get $pos) (local.get $c))
-      (array.set $string (local.get $buf)
+      (array.set $bytes (local.get $buf) (local.get $pos) (local.get $c))
+      (array.set $bytes (local.get $buf)
          (i32.add (local.get $pos) (i32.const 1)) (local.get $v)))
 
    (func $writecode16
       (param $s (ref $extern_state)) (param $c i32) (param $v i32)
-      (local $pos i32) (local $buf (ref $string))
+      (local $pos i32) (local $buf (ref $bytes))
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 3)))
       (local.set $buf (struct.get $extern_state $buf (local.get $s)))
-      (array.set $string (local.get $buf) (local.get $pos) (local.get $c))
+      (array.set $bytes (local.get $buf) (local.get $pos) (local.get $c))
       (call $store16 (local.get $buf) (i32.add (local.get $pos) (i32.const 1))
          (local.get $v)))
 
    (func $writecode32
       (param $s (ref $extern_state)) (param $c i32) (param $v i32)
-      (local $pos i32) (local $buf (ref $string))
+      (local $pos i32) (local $buf (ref $bytes))
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 5)))
       (local.set $buf (struct.get $extern_state $buf (local.get $s)))
-      (array.set $string (local.get $buf) (local.get $pos) (local.get $c))
+      (array.set $bytes (local.get $buf) (local.get $pos) (local.get $c))
       (call $store32 (local.get $buf) (i32.add (local.get $pos) (i32.const 1))
          (local.get $v)))
 
    (func $writeblock
-      (param $s (ref $extern_state)) (param $str (ref $string))
+      (param $s (ref $extern_state)) (param $str (ref $bytes))
       (local $len i32) (local $pos i32)
       (local.set $len (array.len (local.get $str)))
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (local.get $len)))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (struct.get $extern_state $buf (local.get $s)) (local.get $pos)
          (local.get $str) (i32.const 0) (local.get $len)))
 
+   (func $writestring
+      (param $s (ref $extern_state)) (param $str anyref) (param $len i32)
+      (local $pos i32)
+      (local.set $len (call $jsstring_length (local.get $str)))
+      (local.set $pos
+         (call $reserve_extern_output (local.get $s) (local.get $len)))
+      (drop
+         (call $caml_blit_string ;; ZZZ lower level func?
+            (struct.new $js (local.get $str)) (ref.i31 (i32.const 0))
+            (struct.get $extern_state $buf (local.get $s))
+            (ref.i31 (local.get $pos))
+            (ref.i31 (local.get $len)))))
+
    (func $writefloat
       (param $s (ref $extern_state)) (param $f f64)
-      (local $pos i32) (local $buf (ref $string)) (local $d i64) (local $i i32)
+      (local $pos i32) (local $buf (ref $bytes)) (local $d i64) (local $i i32)
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 8)))
       (local.set $buf (struct.get $extern_state $buf (local.get $s)))
       (local.set $d (i64.reinterpret_f64 (local.get $f)))
       (loop $loop
-         (array.set $string (local.get $buf)
+         (array.set $bytes (local.get $buf)
             (i32.add (local.get $pos) (local.get $i))
             (i32.wrap_i64
                (i64.shr_u (local.get $d)
@@ -914,7 +908,7 @@
 
    (func $writefloats
       (param $s (ref $extern_state)) (param $b (ref $float_array))
-      (local $pos i32) (local $sz i32) (local $buf (ref $string)) (local $d i64)
+      (local $pos i32) (local $sz i32) (local $buf (ref $bytes)) (local $d i64)
       (local $i i32) (local $j i32)
       (local.set $sz (array.len (local.get $b)))
       (local.set $pos
@@ -930,7 +924,7 @@
                      (array.get $float_array (local.get $b) (local.get $j))))
                (local.set $i (i32.const 0))
                (loop $loop
-                  (array.set $string (local.get $buf)
+                  (array.set $bytes (local.get $buf)
                      (i32.add (local.get $pos) (local.get $i))
                      (i32.wrap_i64
                         (i64.shr_u (local.get $d)
@@ -1022,7 +1016,7 @@
                (i32.or (local.get $tag)
                   (i32.shl (local.get $sz) (i32.const 10)))))))
 
-   (func $extern_string (param $s (ref $extern_state)) (param $v (ref $string))
+   (func $extern_bytes (param $s (ref $extern_state)) (param $v (ref $bytes))
       (local $len i32)
       (local.set $len (array.len (local.get $v)))
       (if (i32.lt_u (local.get $len) (i32.const 0x20))
@@ -1038,6 +1032,22 @@
             (local.get $len))))))
       (call $writeblock (local.get $s) (local.get $v)))
 
+   (func $extern_string (param $s (ref $extern_state)) (param $v anyref)
+      (local $len i32)
+      (local.set $len (call $jsstring_length (local.get $v)))
+      (if (i32.lt_u (local.get $len) (i32.const 0x20))
+         (then
+            (call $write (local.get $s)
+               (i32.add (global.get $PREFIX_SMALL_STRING) (local.get $len))))
+      (else (if (i32.lt_u (local.get $len) (i32.const 0x100))
+         (then
+            (call $writecode8 (local.get $s) (global.get $CODE_STRING8)
+               (local.get $len)))
+      (else
+         (call $writecode32 (local.get $s) (global.get $CODE_STRING32)
+            (local.get $len))))))
+      (call $writestring (local.get $s) (local.get $v) (local.get $len)))
+
    (func $extern_float (param $s (ref $extern_state)) (param $v f64)
       (call $write (local.get $s) (global.get $CODE_DOUBLE_LITTLE))
       (call $writefloat (local.get $s) (local.get $v)))
@@ -1055,14 +1065,14 @@
                (global.get $CODE_DOUBLE_ARRAY32_LITTLE) (local.get $nfloats))))
       (call $writefloats (local.get $s) (local.get $v)))
 
-   (data $incorrect_sizes "output_value: incorrect fixed sizes specified by ")
+   (#string $incorrect_sizes "output_value: incorrect fixed sizes specified by ")
 
    (func $extern_custom
       (param $s (ref $extern_state)) (param $v (ref $custom)) (result i32 i32)
       (local $ops (ref $custom_operations))
       (local $serialize (ref $serialize))
       (local $fixed_length (ref $fixed_length))
-      (local $pos i32) (local $buf (ref $string))
+      (local $pos i32) (local $buf (ref $bytes))
       (local $r (tuple i32 i32))
       (local.set $ops (struct.get $custom 0 (local.get $v)))
       (block $abstract
@@ -1091,8 +1101,7 @@
                 (then
                    (call $caml_failwith
                       (call $caml_string_concat
-                         (array.new_data $string $incorrect_sizes
-                            (i32.const 0) (i32.const 49))
+                         (global.get $incorrect_sizes)
                          (struct.get $custom_operations $id
                             (local.get $ops))))))
             (return (local.get $r)))
@@ -1112,24 +1121,24 @@
          (call $store32 (local.get $buf) (i32.add (local.get $pos) (i32.const 8))
             (tuple.extract 2 1 (local.get $r)))
          (return (local.get $r)))
-      (call $caml_invalid_argument
-         (array.new_data $string $cust_value (i32.const 0) (i32.const 37)))
+      (call $caml_invalid_argument (global.get $cust_value))
       (return (tuple.make 2 (i32.const 0) (i32.const 0))))
 
-   (data $func_value "output_value: functional value")
-   (data $cont_value "output_value: continuation value")
-   (data $js_value "output_value: abstract value (JavaScript value)")
-   (data $abstract_value "output_value: abstract value")
-   (data $cust_value "output_value: abstract value (Custom)")
+   (#string $func_value "output_value: functional value")
+   (#string $cont_value "output_value: continuation value")
+   (#string $js_value "output_value: abstract value (JavaScript value)")
+   (#string $abstract_value "output_value: abstract value")
+   (#string $cust_value "output_value: abstract value (Custom)")
 
    (func $extern_rec (param $s (ref $extern_state)) (param $v (ref eq))
       (local $sp (ref null $stack_item))
       (local $item (ref $stack_item))
-      (local $b (ref $block)) (local $str (ref $string))
+      (local $b (ref $block)) (local $str (ref $bytes))
       (local $fa (ref $float_array))
       (local $hd i32) (local $tag i32) (local $sz i32)
       (local $pos i32)
       (local $r (tuple i32 i32))
+      (local $js anyref)
       (loop $loop
          (block $next_item
             (drop (block $not_int (result (ref eq))
@@ -1187,9 +1196,9 @@
             (call $extern_record_location (local.get $s) (local.get $v))
             (drop (block $not_string (result (ref eq))
                (local.set $str
-                  (br_on_cast_fail $not_string (ref eq) (ref $string)
+                  (br_on_cast_fail $not_string (ref eq) (ref $bytes)
                      (local.get $v)))
-               (call $extern_string (local.get $s) (local.get $str))
+               (call $extern_bytes (local.get $s) (local.get $str))
                (local.set $sz (array.len (local.get $str)))
                (call $extern_size (local.get $s)
                   (i32.add (i32.const 1)
@@ -1228,23 +1237,27 @@
                         (i32.const 3)))
                (br $next_item)))
             (if (call $caml_is_closure (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $string $func_value
-                        (i32.const 0) (i32.const 30)))))
+               (then (call $caml_invalid_argument (global.get $func_value))))
             (if (call $caml_is_continuation (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $string $cont_value
-                        (i32.const 0) (i32.const 32)))))
-            (if (ref.test (ref $js) (local.get $v))
-               (then
-                  (call $caml_invalid_argument
-                     (array.new_data $string $js_value
-                        (i32.const 0) (i32.const 47)))))
-            (call $caml_invalid_argument
-               (array.new_data $string $abstract_value
-                  (i32.const 0) (i32.const 28)))
+               (then (call $caml_invalid_argument (global.get $cont_value))))
+            (drop (block $not_js (result (ref eq))
+               (local.set $js
+                  (struct.get $js 0
+                     (br_on_cast_fail $not_js (ref eq) (ref $js)
+                        (local.get $v))))
+               (if (call $jsstring_test (local.get $js))
+                  (then
+                     (call $extern_string (local.get $s) (local.get $js))
+                     (local.set $sz (call $jsstring_length (local.get $js)))
+                     (call $extern_size (local.get $s)
+                        (i32.add (i32.const 1)
+                           (i32.shr_u (local.get $sz) (i32.const 2)))
+                        (i32.add (i32.const 1)
+                           (i32.shr_u (local.get $sz) (i32.const 3))))
+                     (br $next_item)))
+               (call $caml_invalid_argument (global.get $js_value))
+               (ref.i31 (i32.const 0))))
+            (call $caml_invalid_argument (global.get $abstract_value))
          )
          ;; next_item
          (block $done
@@ -1291,9 +1304,9 @@
    (func $extern_value
       (param $flags (ref eq)) (param $output (ref $output_block))
       (param $pos i32) (param $user_provided_output i32) (param $v (ref eq))
-      (result i32 (ref $string) (ref $extern_state))
+      (result i32 (ref $bytes) (ref $extern_state))
       (local $s (ref $extern_state)) (local $len i32)
-      (local $header (ref $string))
+      (local $header (ref $bytes))
       (local.set $s
          (call $init_extern_state
             (local.get $flags) (local.get $output) (local.get $pos)
@@ -1301,7 +1314,7 @@
       (call $extern_rec (local.get $s) (local.get $v))
       (local.set $len
          (call $extern_output_length (local.get $s) (local.get $pos)))
-      (local.set $header (array.new $string (i32.const 0) (i32.const 20)))
+      (local.set $header (array.new $bytes (i32.const 0) (i32.const 20)))
       (call $store32 (local.get $header) (i32.const 0)
          (global.get $Intext_magic_number_small))
       (call $store32 (local.get $header) (i32.const 4) (local.get $len))
@@ -1313,32 +1326,43 @@
          (struct.get $extern_state $size_64 (local.get $s)))
       (tuple.make 3 (local.get $len) (local.get $header) (local.get $s)))
 
+(#if use-js-string
+(#then
    (func (export "caml_output_value_to_string")
       (param $v (ref eq)) (param $flags (ref eq)) (result (ref eq))
-      (local $r (tuple i32 (ref $string) (ref $extern_state)))
+      (return_call $caml_string_of_bytes
+         (call $caml_output_value_to_bytes (local.get $v) (local.get $flags))))
+)
+(#else
+   (export "caml_output_value_to_string" (func $caml_output_value_to_bytes))
+))
+
+   (func $caml_output_value_to_bytes (export "caml_output_value_to_bytes")
+      (param $v (ref eq)) (param $flags (ref eq)) (result (ref eq))
+      (local $r (tuple i32 (ref $bytes) (ref $extern_state)))
       (local $blk (ref $output_block)) (local $pos i32) (local $len i32)
-      (local $res (ref $string))
+      (local $res (ref $bytes))
       (local.set $blk
          (struct.new $output_block
             (ref.null $output_block)
             (global.get $SIZE_EXTERN_OUTPUT_BLOCK)
-            (array.new $string (i32.const 0)
+            (array.new $bytes (i32.const 0)
                (global.get $SIZE_EXTERN_OUTPUT_BLOCK))))
       (local.set $r
          (call $extern_value
             (local.get $flags) (local.get $blk)
             (i32.const 0) (i32.const 0) (local.get $v)))
       (local.set $res
-         (array.new $string (i32.const 0)
+         (array.new $bytes (i32.const 0)
             (i32.add (tuple.extract 3 0 (local.get $r)) (i32.const 20))))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (local.get $res) (i32.const 0)
          (tuple.extract 3 1 (local.get $r)) (i32.const 0) (i32.const 20))
       (local.set $pos (i32.const 20))
       (loop $loop
          (block $done
             (local.set $len (struct.get $output_block $end (local.get $blk)))
-            (array.copy $string $string
+            (array.copy $bytes $bytes
                (local.get $res) (local.get $pos)
                (struct.get $output_block $data (local.get $blk)) (i32.const 0)
                (local.get $len))
@@ -1352,10 +1376,10 @@
    (func (export "caml_output_value_to_buffer")
       (param $vbuf (ref eq)) (param $vpos (ref eq)) (param $vlen (ref eq))
       (param $v (ref eq)) (param $flags (ref eq)) (result (ref eq))
-      (local $buf (ref $string)) (local $pos i32) (local $len i32)
-      (local $r (tuple i32 (ref $string) (ref $extern_state)))
+      (local $buf (ref $bytes)) (local $pos i32) (local $len i32)
+      (local $r (tuple i32 (ref $bytes) (ref $extern_state)))
       (local $blk (ref $output_block))
-      (local.set $buf (ref.cast (ref $string) (local.get $vbuf)))
+      (local.set $buf (ref.cast (ref $bytes) (local.get $vbuf)))
       (local.set $pos (i31.get_u (ref.cast (ref i31) (local.get $vpos))))
       (local.set $len (i31.get_u (ref.cast (ref i31) (local.get $vlen))))
       (local.set $blk
@@ -1370,7 +1394,7 @@
             (i32.add (local.get $pos) (i32.const 20))
             (i32.const 1)
             (local.get $v)))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (local.get $buf) (local.get $pos)
          (tuple.extract 3 1 (local.get $r)) (i32.const 0) (i32.const 20))
       (ref.i31 (i32.const 0)))
@@ -1378,15 +1402,15 @@
    (func (export "caml_output_value")
       (param $ch (ref eq)) (param $v (ref eq)) (param $flags (ref eq))
       (result (ref eq))
-      (local $r (tuple i32 (ref $string) (ref $extern_state)))
+      (local $r (tuple i32 (ref $bytes) (ref $extern_state)))
       (local $blk (ref $output_block)) (local $len i32)
-      (local $res (ref $string))
+      (local $res (ref $bytes))
       ;; ZZZ check if binary channel?
       (local.set $blk
          (struct.new $output_block
             (ref.null $output_block)
             (global.get $SIZE_EXTERN_OUTPUT_BLOCK)
-            (array.new $string (i32.const 0)
+            (array.new $bytes (i32.const 0)
                (global.get $SIZE_EXTERN_OUTPUT_BLOCK))))
       (local.set $r
          (call $extern_value
@@ -1414,7 +1438,7 @@
       (local.set $s (ref.cast (ref $extern_state) (local.get $vs)))
       (local.set $pos
          (call $reserve_extern_output (local.get $s) (i32.const 1)))
-      (array.set $string (struct.get $extern_state $buf (local.get $s))
+      (array.set $bytes (struct.get $extern_state $buf (local.get $s))
          (local.get $pos) (local.get $i)))
 
    (func (export "caml_serialize_int_2") (param $vs (ref eq)) (param $i i32)
diff --git a/runtime/wasm/md5.wat b/runtime/wasm/md5.wat
index c8149eca6b..ada3645788 100644
--- a/runtime/wasm/md5.wat
+++ b/runtime/wasm/md5.wat
@@ -18,11 +18,15 @@
 (module
    (import "io" "caml_getblock"
       (func $caml_getblock
-         (param (ref eq)) (param (ref $string)) (param i32) (param i32)
+         (param (ref eq)) (param (ref $bytes)) (param i32) (param i32)
          (result i32)))
    (import "fail" "caml_raise_end_of_file" (func $caml_raise_end_of_file))
+   (import "string" "caml_bytes_of_string"
+      (func $caml_bytes_of_string (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
    (type $int_array (array (mut i32)))
 
    (type $context
@@ -30,13 +34,25 @@
          (field (ref $int_array)) ;; w
          (field (mut i64))        ;; len
          (field (ref $int_array)) ;; buffer
-         (field (ref $string))))  ;; intermediate buffer
+         (field (ref $bytes))))  ;; intermediate buffer
 
-   (func (export "caml_md5_string") (export "caml_md5_bytes")
+(#if use-js-string
+(#then
+   (func (export "caml_md5_string")
+      (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
+      (return_call $caml_md5_bytes
+         (call $caml_bytes_of_string (local.get 0))
+         (local.get 1) (local.get 2)))
+)
+(#else
+   (export "caml_md5_string" (func $caml_md5_bytes))
+))
+
+   (func $caml_md5_bytes (export "caml_md5_bytes")
       (param (ref eq)) (param (ref eq)) (param (ref eq)) (result (ref eq))
       (local $ctx (ref $context))
       (local.set $ctx (call $MD5Init))
-      (call $MD5Update (local.get $ctx) (ref.cast (ref $string) (local.get 0))
+      (call $MD5Update (local.get $ctx) (ref.cast (ref $bytes) (local.get 0))
          (i31.get_u (ref.cast (ref i31) (local.get 1)))
          (i31.get_u (ref.cast (ref i31) (local.get 2))))
       (return_call $MD5Final (local.get $ctx)))
@@ -44,10 +60,10 @@
    (func (export "caml_md5_chan")
       (param $ch (ref eq)) (param $vlen (ref eq)) (result (ref eq))
       (local $len i32) (local $read i32)
-      (local $buf (ref $string))
+      (local $buf (ref $bytes))
       (local $ctx (ref $context))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
-      (local.set $buf (array.new $string (i32.const 0) (i32.const 4096)))
+      (local.set $buf (array.new $bytes (i32.const 0) (i32.const 4096)))
       (local.set $ctx (call $MD5Init))
       (if (i32.lt_s (local.get $len) (i32.const 0))
          (then
@@ -124,24 +140,24 @@
          (local.get $a) (local.get $b)
          (local.get $x) (local.get $s) (local.get $t)))
 
-   (func $get_32 (param $s (ref $string)) (param $p i32) (result i32)
+   (func $get_32 (param $s (ref $bytes)) (param $p i32) (result i32)
        (i32.or
           (i32.or
-             (array.get_u $string (local.get $s) (local.get $p))
-             (i32.shl (array.get_u $string (local.get $s)
+             (array.get_u $bytes (local.get $s) (local.get $p))
+             (i32.shl (array.get_u $bytes (local.get $s)
                          (i32.add (local.get $p) (i32.const 1)))
                       (i32.const 8)))
           (i32.or
-             (i32.shl (array.get_u $string (local.get $s)
+             (i32.shl (array.get_u $bytes (local.get $s)
                          (i32.add (local.get $p) (i32.const 2)))
                       (i32.const 16))
-             (i32.shl (array.get_u $string (local.get $s)
+             (i32.shl (array.get_u $bytes (local.get $s)
                          (i32.add (local.get $p) (i32.const 3)))
                         (i32.const 24)))))
 
    (func $MD5Transform
       (param $w (ref $int_array)) (param $buffer (ref $int_array))
-      (param $buffer' (ref $string)) (param $p i32)
+      (param $buffer' (ref $bytes)) (param $p i32)
       (local $i i32)
       (local $a i32) (local $b i32) (local $c i32) (local $d i32)
       (loop $loop
@@ -430,10 +446,10 @@
              (i32.const 0x98BADCFE) (i32.const 0x10325476))
           (i64.const 0)
           (array.new $int_array (i32.const 0) (i32.const 16))
-          (array.new $string (i32.const 0) (i32.const 64))))
+          (array.new $bytes (i32.const 0) (i32.const 64))))
 
    (func $MD5Update
-      (param $ctx (ref $context)) (param $input (ref $string))
+      (param $ctx (ref $context)) (param $input (ref $bytes))
       (param $input_pos i32) (param $input_len i32)
       (local $in_buf i32) (local $len i64)
       (local $missing i32)
@@ -447,13 +463,13 @@
             (local.set $missing (i32.sub (i32.const 64) (local.get $in_buf)))
             (if (i32.lt_u (local.get $input_len) (local.get $missing))
                (then
-                  (array.copy $string $string
+                  (array.copy $bytes $bytes
                      (struct.get $context 3 (local.get $ctx))
                      (local.get $missing)
                      (local.get $input) (local.get $input_pos)
                      (local.get $input_len))
                   (return)))
-            (array.copy $string $string
+            (array.copy $bytes $bytes
                (struct.get $context 3 (local.get $ctx))
                (local.get $missing)
                (local.get $input) (local.get $input_pos) (local.get $missing))
@@ -479,20 +495,20 @@
                 (br $loop))))
       (if (local.get $input_len)
          (then
-            (array.copy $string $string
+            (array.copy $bytes $bytes
                (struct.get $context 3 (local.get $ctx)) (i32.const 0)
                (local.get $input) (local.get $input_pos)
                (local.get $input_len)))))
 
-   (func $MD5Final (param $ctx (ref $context)) (result (ref $string))
+   (func $MD5Final (param $ctx (ref $context)) (result (ref eq))
       (local $in_buf i32) (local $i i32) (local $len i64)
       (local $w (ref $int_array))
-      (local $buffer (ref $string)) (local $res (ref $string))
+      (local $buffer (ref $bytes)) (local $res (ref $bytes))
       (local.set $len (struct.get $context 1 (local.get $ctx)))
       (local.set $in_buf
          (i32.and (i32.wrap_i64 (local.get $len)) (i32.const 0x3f)))
       (local.set $buffer (struct.get $context 3 (local.get $ctx)))
-      (array.set $string (local.get $buffer) (local.get $in_buf)
+      (array.set $bytes (local.get $buffer) (local.get $in_buf)
          (i32.const 0x80))
       (local.set $in_buf (i32.add (local.get $in_buf) (i32.const 1)))
       (if (i32.gt_u (local.get $in_buf) (i32.const 56))
@@ -501,7 +517,7 @@
             (loop $loop
                (if (i32.lt_u (local.get $i) (i32.const 64))
                   (then
-                     (array.set $string
+                     (array.set $bytes
                         (local.get $buffer) (local.get $i) (i32.const 0))
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (br $loop))))
@@ -512,40 +528,40 @@
             (local.set $in_buf (i32.const 0))))
       (local.set $i (local.get $in_buf))
       (loop $loop
-         (array.set $string (local.get $buffer) (local.get $i) (i32.const 0))
+         (array.set $bytes (local.get $buffer) (local.get $i) (i32.const 0))
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (br_if $loop (i32.lt_u (local.get $i) (i32.const 56))))
       (local.set $len (i64.shl (local.get $len) (i64.const 3)))
-      (array.set $string (local.get $buffer) (i32.const 56)
+      (array.set $bytes (local.get $buffer) (i32.const 56)
          (i32.wrap_i64 (local.get $len)))
-      (array.set $string (local.get $buffer) (i32.const 57)
+      (array.set $bytes (local.get $buffer) (i32.const 57)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 8))))
-      (array.set $string (local.get $buffer) (i32.const 58)
+      (array.set $bytes (local.get $buffer) (i32.const 58)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 16))))
-      (array.set $string (local.get $buffer) (i32.const 59)
+      (array.set $bytes (local.get $buffer) (i32.const 59)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 24))))
-      (array.set $string (local.get $buffer) (i32.const 60)
+      (array.set $bytes (local.get $buffer) (i32.const 60)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 32))))
-      (array.set $string (local.get $buffer) (i32.const 61)
+      (array.set $bytes (local.get $buffer) (i32.const 61)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 40))))
-      (array.set $string (local.get $buffer) (i32.const 62)
+      (array.set $bytes (local.get $buffer) (i32.const 62)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 48))))
-      (array.set $string (local.get $buffer) (i32.const 63)
+      (array.set $bytes (local.get $buffer) (i32.const 63)
          (i32.wrap_i64 (i64.shr_u (local.get $len) (i64.const 56))))
       (call $MD5Transform (struct.get $context 0 (local.get $ctx))
          (struct.get $context 2 (local.get $ctx))
          (local.get $buffer)
          (i32.const 0))
-      (local.set $res (array.new $string (i32.const 0) (i32.const 16)))
+      (local.set $res (array.new $bytes (i32.const 0) (i32.const 16)))
       (local.set $i (i32.const 0))
       (local.set $w (struct.get $context 0 (local.get $ctx)))
       (loop $loop
-         (array.set $string (local.get $res) (local.get $i)
+         (array.set $bytes (local.get $res) (local.get $i)
             (i32.shr_u
                (array.get $int_array (local.get $w)
                   (i32.shr_u (local.get $i) (i32.const 2)))
                (i32.shl (local.get $i) (i32.const 3))))
          (local.set $i (i32.add (local.get $i) (i32.const 1)))
          (br_if $loop (i32.lt_u (local.get $i) (i32.const 16))))
-      (local.get $res))
+      (return_call $caml_string_of_bytes (local.get $res)))
 )
diff --git a/runtime/wasm/obj.wat b/runtime/wasm/obj.wat
index f0057eee2a..908cfcae58 100644
--- a/runtime/wasm/obj.wat
+++ b/runtime/wasm/obj.wat
@@ -25,9 +25,13 @@
       (func $caml_is_continuation (param (ref eq)) (result i32)))
    (import "effect" "caml_trampoline_ref"
       (global $caml_trampoline_ref (mut (ref null $function_1))))
+   (import "jsstring" "jsstring_test"
+      (func $jsstring_test (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
+   (type $js (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
    (type $function_1 (func (param (ref eq) (ref eq)) (result (ref eq))))
@@ -177,7 +181,7 @@
       (param (ref eq)) (result (ref eq))
       (local $orig (ref $block)) (local $res (ref $block))
       (local $forig (ref $float_array)) (local $fres (ref $float_array))
-      (local $s (ref $string)) (local $s' (ref $string))
+      (local $s (ref $bytes)) (local $s' (ref $bytes))
       (local $len i32)
       (drop (block $not_block (result (ref eq))
          (local.set $orig (br_on_cast_fail $not_block (ref eq) (ref $block)
@@ -202,11 +206,11 @@
             (local.get $len))
          (return (local.get $fres))))
       (drop (block $not_string (result (ref eq))
-         (local.set $s (br_on_cast_fail $not_string (ref eq) (ref $string)
+         (local.set $s (br_on_cast_fail $not_string (ref eq) (ref $bytes)
             (local.get 0)))
          (local.set $len (array.len (local.get $s)))
-         (local.set $s' (array.new $string (i32.const 0) (local.get $len)))
-         (array.copy $string $string
+         (local.set $s' (array.new $bytes (i32.const 0) (local.get $len)))
+         (array.copy $bytes $bytes
             (local.get $s') (i32.const 0) (local.get $s) (i32.const 0)
             (local.get $len))
          (return (local.get $s'))))
@@ -216,6 +220,8 @@
                (struct.get $float 0
                   (br_on_cast_fail $not_float (ref eq) (ref $float)
                      (local.get 0)))))))
+      (if (ref.test (ref $js) (local.get 0))
+         (then (return (local.get 0))))
       (call $caml_dup_custom (local.get 0)))
 
    (func (export "caml_obj_with_tag")
@@ -246,7 +252,7 @@
             (array.get $block
               (br_on_cast_fail $not_block (ref eq) (ref $block) (local.get $v))
               (i32.const 0)))))
-      (if (ref.test (ref $string) (local.get $v))
+      (if (ref.test (ref $bytes) (local.get $v))
          (then (return (ref.i31 (global.get $string_tag)))))
       (if (ref.test (ref $float) (local.get $v))
          (then (return (ref.i31 (global.get $float_tag)))))
@@ -258,6 +264,13 @@
          (then (return (ref.i31 (global.get $closure_tag)))))
       (if (call $caml_is_continuation (local.get $v))
          (then (return (ref.i31 (global.get $cont_tag)))))
+      (drop (block $not_string (result (ref eq))
+         (if (call $jsstring_test
+                (struct.get $js 0
+                   (br_on_cast_fail $not_string (ref eq) (ref $js)
+                      (local.get $v))))
+            (then (return (ref.i31 (global.get $string_tag)))))
+         (ref.i31 (i32.const 0))))
       (ref.i31 (global.get $abstract_tag)))
 
    (func (export "caml_obj_make_forward")
@@ -340,21 +353,18 @@
          (local.get $v))
       (ref.i31 (i32.const 0)))
 
-   (data $not_implemented "Obj.add_offset is not supported")
+   (#string $not_implemented "Obj.add_offset is not supported")
 
    (func (export "caml_obj_add_offset")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $string $not_implemented (i32.const 0) (i32.const 31)))
+      (call $caml_failwith (global.get $not_implemented))
       (ref.i31 (i32.const 0)))
 
-   (data $truncate_not_implemented "Obj.truncate is not supported")
+   (#string $truncate_not_implemented "Obj.truncate is not supported")
 
    (func (export "caml_obj_truncate")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $string $truncate_not_implemented
-            (i32.const 0) (i32.const 29)))
+      (call $caml_failwith (global.get $truncate_not_implemented))
       (ref.i31 (i32.const 0)))
 
    (global $method_cache (mut (ref $int_array))
diff --git a/runtime/wasm/parsing.wat b/runtime/wasm/parsing.wat
index 4e4a0bb29b..804ff8e196 100644
--- a/runtime/wasm/parsing.wat
+++ b/runtime/wasm/parsing.wat
@@ -16,16 +16,15 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
-   (import "jslib" "caml_string_of_jsstring"
-      (func $caml_string_of_jsstring (param (ref eq)) (result (ref eq))))
    (import "io" "caml_stderr" (global $caml_stderr (mut (ref eq))))
-   (import "io" "caml_ml_open_descriptor_out"
-      (func $caml_ml_open_descriptor_out (param (ref eq)) (result (ref eq))))
    (import "io" "caml_ml_output"
       (func $caml_ml_output
          (param (ref eq)) (param (ref eq)) (param (ref eq)) (param (ref eq))
          (result (ref eq))))
+   (import "io" "caml_ml_output_bytes"
+      (func $caml_ml_output_bytes
+         (param (ref eq)) (param (ref eq)) (param (ref eq)) (param (ref eq))
+         (result (ref eq))))
    (import "io" "caml_ml_flush"
       (func $caml_ml_flush (param (ref eq)) (result (ref eq))))
    (import "ints" "caml_format_int"
@@ -34,19 +33,64 @@
    (import "float" "caml_format_float"
       (func $caml_format_float
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_length"
+      (func $caml_string_length (param (ref eq)) (result i32)))
 
    (type $float (struct (field f64)))
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
+
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "substring"
+      (func $string_substring (param externref i32 i32) (result (ref extern))))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_sub
+      (param $s externref) (param $i i32) (param $l i32) (result (ref eq))
+      (struct.new $string
+         (any.convert_extern
+            (call $string_substring
+               (local.get $s)
+               (local.get $i)
+               (i32.add (local.get $i) (local.get $l))))))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_sub
+      (param $s (ref $bytes)) (param $i i32) (param $l i32) (result (ref eq))
+      (local $s' (ref $bytes))
+      (local.set $s' (array.new $bytes (i32.const 0) (local.get $l)))
+      (array.copy $bytes $bytes
+         (local.get $s') (i32.const 0)
+         (local.get $s) (local.get $i)
+         (local.get $l))
+      (local.get $s'))
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
 
-   (func $get (param $a (ref eq)) (param $i i32) (result i32)
-      (local $s (ref $string))
-      (local.set $s (ref.cast (ref $string) (local.get $a)))
+   (func $get
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $i i32) (result i32)
       (local.set $i (i32.add (local.get $i) (local.get $i)))
       (i32.extend16_s
-         (i32.or (array.get_u $string (local.get $s) (local.get $i))
+         (i32.or (call $string_get (local.get $s) (local.get $i))
             (i32.shl
-               (array.get_u $string (local.get $s)
+               (call $string_get (local.get $s)
                   (i32.add (local.get $i) (i32.const 1)))
                (i32.const 8)))))
 
@@ -105,29 +149,41 @@
    (global $tbl_names_const i32 (i32.const 15))
    (global $tbl_names_block i32 (i32.const 16))
 
-   (func $strlen (param $s (ref $string)) (param $p i32) (result i32)
+   (func $strlen
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $p i32) (result i32)
       (local $i i32)
       (local.set $i (local.get $p))
       (loop $loop
-         (if (i32.ne (array.get_u $string (local.get $s) (local.get $i))
+         (if (i32.ne (call $string_get (local.get $s) (local.get $i))
                (i32.const 0))
             (then
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (i32.sub (local.get $i) (local.get $p)))
 
-   (data $unknown_token "<unknown token>")
+   (#string $unknown_token "<unknown token>")
    (func $token_name
       (param $vnames (ref eq)) (param $number i32) (result (ref eq))
-      (local $names (ref $string)) (local $i i32) (local $len i32)
-      (local $name (ref $string))
-      (local.set $names (ref.cast (ref $string) (local.get $vnames)))
+(#if use-js-string
+(#then
+      (local $names externref)
+)
+(#else
+      (local $names (ref $bytes))
+))
+      (local $i i32) (local $len i32)
+      (local.set $names (call $string_val (local.get $vnames)))
       (loop $loop
-         (if (i32.eqz (array.get_u $string (local.get $names) (local.get $i)))
+         (if (i32.eqz (call $string_get (local.get $names) (local.get $i)))
             (then
-               (return
-                  (array.new_data $string $unknown_token
-                     (i32.const 0) (i32.const 15)))))
+               (return (global.get $unknown_token))))
          (if (i32.ne (local.get $number) (i32.const 0))
             (then
                (local.set $i
@@ -137,35 +193,40 @@
                (local.set $number (i32.sub (local.get $number) (i32.const 1)))
                (br $loop))))
       (local.set $len (call $strlen (local.get $names) (local.get $i)))
-      (local.set $name (array.new $string (i32.const 0) (local.get $len)))
-      (array.copy $string $string
-         (local.get $name) (i32.const 0)
-         (local.get $names) (local.get $i) (local.get $len))
-      (local.get $name))
+      (return_call $string_sub
+         (local.get $names) (local.get $i) (local.get $len)))
 
-   (func $output (param (ref eq))
-      (local $s (ref $string))
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
+   (func $output (param $s (ref eq))
       (drop
          (call $caml_ml_output (global.get $caml_stderr)
+            (local.get $s) (ref.i31 (i32.const 0))
+            (ref.i31 (call $caml_string_length (local.get $s))))))
+
+   (func $output_bytes (param (ref eq))
+      (local $s (ref $bytes))
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
+      (drop
+         (call $caml_ml_output_bytes (global.get $caml_stderr)
             (local.get $s) (ref.i31 (i32.const 0))
             (ref.i31 (array.len (local.get $s))))))
 
    (func $output_nl
       (drop
-         (call $caml_ml_output (global.get $caml_stderr)
-            (array.new_fixed $string 1 (i32.const 10))
+         (call $caml_ml_output_bytes (global.get $caml_stderr)
+            (array.new_fixed $bytes 1 (i32.const 10))
             (ref.i31 (i32.const 0)) (ref.i31 (i32.const 1))))
       (drop (call $caml_ml_flush (global.get $caml_stderr))))
 
+   (#string $int_format "%d")
+
    (func $output_int (param i32)
       (call $output
-         (call $caml_format_int
-            (array.new_fixed $string 2 (i32.const 37) (i32.const 100))
+         (call $caml_format_int (global.get $int_format)
             (ref.i31 (local.get 0)))))
 
-   (data $State "State ")
-   (data $read_token ": read token ")
+   (#string $State "State ")
+   (#string $read_token ": read token ")
+   (#string $float_format "%g")
 
    (func $print_token
       (param $tables (ref $block)) (param $state i32) (param $tok (ref eq))
@@ -173,11 +234,9 @@
       (local $v (ref eq))
       (if (ref.test (ref i31) (local.get $tok))
          (then
-            (call $output
-               (array.new_data $string $State (i32.const 0) (i32.const 6)))
+            (call $output (global.get $State))
             (call $output_int (local.get $state))
-            (call $output
-               (array.new_data $string $read_token (i32.const 0) (i32.const 13)))
+            (call $output (global.get $read_token))
             (call $output
                (call $token_name
                   (array.get $block (local.get $tables)
@@ -185,11 +244,9 @@
                   (i31.get_u (ref.cast (ref i31) (local.get $tok)))))
             (call $output_nl))
          (else
-            (call $output
-               (array.new_data $string $State (i32.const 0) (i32.const 6)))
+            (call $output (global.get $State))
             (call $output_int (local.get $state))
-            (call $output
-               (array.new_data $string $read_token (i32.const 0) (i32.const 13)))
+            (call $output (global.get $read_token))
             (local.set $b (ref.cast (ref $block) (local.get $tok)))
             (call $output
                (call $token_name
@@ -198,33 +255,31 @@
                   (i31.get_u
                      (ref.cast (ref i31)
                         (array.get $block (local.get $b) (i32.const 0))))))
-            (call $output (array.new_fixed $string 1 (i32.const 40))) ;; "("
+            (call $output_bytes (array.new_fixed $bytes 1 (i32.const 40))) ;; "("
             (local.set $v (array.get $block (local.get $b) (i32.const 1)))
             (if (ref.test (ref i31) (local.get $v))
                (then
                   (call $output_int
                      (i31.get_s (ref.cast (ref i31) (local.get $v)))))
-            (else (if (ref.test (ref $string) (local.get $v))
+            (else (if (ref.test (ref $bytes) (local.get $v))
                (then (call $output (local.get $v)))
             (else (if (ref.test (ref $float) (local.get $v))
                (then
                   (call $output
-                     (call $caml_format_float
-                        (array.new_fixed $string 2
-                           (i32.const 37) (i32.const 103))
+                     (call $caml_format_float (global.get $float_format)
                         (local.get $v))))
             (else
-               (call $output
-                  (array.new_fixed $string 1 (i32.const 95))))))))) ;; '_'
-            (call $output (array.new_fixed $string 1 (i32.const 41))) ;; ")"
+               (call $output_bytes
+                  (array.new_fixed $bytes 1 (i32.const 95))))))))) ;; '_'
+            (call $output_bytes (array.new_fixed $bytes 1 (i32.const 41))) ;; ")"
             (call $output_nl))))
 
-   (data $recovering_in_state "Recovering in state ")
-   (data $discarding_state "Discarding state ")
-   (data $no_more_states_to_discard "No more states to discard")
-   (data $discarding_last_token_read "Discarding last token read")
-   (data $shift_to_state ": shift to state ")
-   (data $reduce_by_rule ": reduce by rule ")
+   (#string $recovering_in_state "Recovering in state ")
+   (#string $discarding_state "Discarding state ")
+   (#string $no_more_states_to_discard "No more states to discard")
+   (#string $discarding_last_token_read "Discarding last token read")
+   (#string $shift_to_state ": shift to state ")
+   (#string $reduce_by_rule ": reduce by rule ")
 
    (func (export "caml_parse_engine")
       (param $vtables (ref eq)) (param $venv (ref eq)) (param $vcmd (ref eq))
@@ -235,42 +290,56 @@
       (local $errflag i32)
       (local $tables (ref $block)) (local $env (ref $block)) (local $cmd i32)
       (local $arg (ref $block))
-      (local $tbl_defred (ref $string))
-      (local $tbl_sindex (ref $string))
-      (local $tbl_check (ref $string))
-      (local $tbl_rindex (ref $string))
-      (local $tbl_table (ref $string))
-      (local $tbl_len (ref $string))
-      (local $tbl_lhs (ref $string))
-      (local $tbl_gindex (ref $string))
-      (local $tbl_dgoto (ref $string))
+(#if use-js-string
+(#then
+      (local $tbl_defred externref)
+      (local $tbl_sindex externref)
+      (local $tbl_check externref)
+      (local $tbl_rindex externref)
+      (local $tbl_table externref)
+      (local $tbl_len externref)
+      (local $tbl_lhs externref)
+      (local $tbl_gindex externref)
+      (local $tbl_dgoto externref)
+)
+(#else
+      (local $tbl_defred (ref $bytes))
+      (local $tbl_sindex (ref $bytes))
+      (local $tbl_check (ref $bytes))
+      (local $tbl_rindex (ref $bytes))
+      (local $tbl_table (ref $bytes))
+      (local $tbl_len (ref $bytes))
+      (local $tbl_lhs (ref $bytes))
+      (local $tbl_gindex (ref $bytes))
+      (local $tbl_dgoto (ref $bytes))
+))
       (local.set $tables (ref.cast (ref $block) (local.get $vtables)))
       (local.set $tbl_defred
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_defred))))
       (local.set $tbl_sindex
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_sindex))))
       (local.set $tbl_check
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_check))))
       (local.set $tbl_rindex
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_rindex))))
       (local.set $tbl_table
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_table))))
       (local.set $tbl_len
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_len))))
       (local.set $tbl_lhs
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_lhs))))
       (local.set $tbl_gindex
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_gindex))))
       (local.set $tbl_dgoto
-         (ref.cast (ref $string)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_dgoto))))
       (local.set $env (ref.cast (ref $block) (local.get $venv)))
       (local.set $cmd (i31.get_s (ref.cast (ref i31) (local.get $vcmd))))
@@ -461,10 +530,8 @@
                                          (if (global.get $caml_parser_trace)
                                             (then
                                                (call $output
-                                                  (array.new_data $string
-                                                     $recovering_in_state
-                                                     (i32.const 0)
-                                                     (i32.const 20)))
+                                                  (global.get
+                                                     $recovering_in_state))
                                                (call $output_int
                                                   (local.get $state1))
                                                (call $output_nl)))
@@ -473,9 +540,7 @@
                                          (br $next)))))))
                        (if (global.get $caml_parser_trace)
                           (then
-                             (call $output
-                                (array.new_data $string $discarding_state
-                                   (i32.const 0) (i32.const 17)))
+                             (call $output (global.get $discarding_state))
                              (call $output_int (local.get $state1))
                              (call $output_nl)))
                        (if (i32.le_s (local.get $sp)
@@ -487,9 +552,7 @@
                              (if (global.get $caml_parser_trace)
                                 (then
                                    (call $output
-                                      (array.new_data $string
-                                          $no_more_states_to_discard
-                                          (i32.const 0) (i32.const 25)))
+                                      (global.get $no_more_states_to_discard))
                                    (call $output_nl)))
                              (return (ref.i31 (global.get $RAISE_PARSE_ERROR)))))
                        (local.set $sp (i32.sub (local.get $sp) (i32.const 1)))
@@ -504,8 +567,7 @@
                      (if (global.get $caml_parser_trace)
                         (then
                            (call $output
-                              (array.new_data $string $discarding_last_token_read
-                                 (i32.const 0) (i32.const 26)))
+                              (global.get $discarding_last_token_read))
                            (call $output_nl)))
                      (array.set $block (local.get $env)
                         (global.get $env_curr_char)
@@ -523,13 +585,9 @@
              ;; shift_recover:
              (if (global.get $caml_parser_trace)
                 (then
-                   (call $output
-                      (array.new_data $string $State
-                         (i32.const 0) (i32.const 6)))
+                   (call $output (global.get $State))
                    (call $output_int (local.get $state))
-                   (call $output
-                      (array.new_data $string $shift_to_state
-                         (i32.const 0) (i32.const 17)))
+                   (call $output (global.get $shift_to_state))
                    (call $output_int
                       (call $get (local.get $tbl_table) (local.get $n2)))
                    (call $output_nl)))
@@ -573,12 +631,9 @@
            ;; reduce:
            (if (global.get $caml_parser_trace)
               (then
-                 (call $output
-                    (array.new_data $string $State (i32.const 0) (i32.const 6)))
+                 (call $output (global.get $State))
                  (call $output_int (local.get $state))
-                 (call $output
-                    (array.new_data $string $reduce_by_rule
-                       (i32.const 0) (i32.const 17)))
+                 (call $output (global.get $reduce_by_rule))
                  (call $output_int (local.get $n))
                  (call $output_nl)))
            (local.set $m (call $get (local.get $tbl_len) (local.get $n)))
diff --git a/runtime/wasm/printexc.wat b/runtime/wasm/printexc.wat
index 84b616829d..e175f6d437 100644
--- a/runtime/wasm/printexc.wat
+++ b/runtime/wasm/printexc.wat
@@ -16,61 +16,84 @@
 ;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 (module
-   (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
-   (import "jslib" "caml_jsstring_of_string"
-      (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
    (import "fail" "caml_is_special_exception"
       (func $caml_is_special_exception (param (ref eq)) (result i32)))
    (import "ints" "caml_format_int"
       (func $caml_format_int
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_length"
+      (func $caml_string_length (param (ref eq)) (result i32)))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_blit_string"
+      (func $caml_blit_string
+         (param (ref eq) (ref eq) (ref eq) (ref eq) (ref eq))
+         (result (ref eq))))
+   (import "jsstring" "jsstring_test"
+      (func $jsstring_test (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $string (struct (field anyref)))
+   (type $bytes (array (mut i8)))
 
    (type $buffer
       (struct
          (field (mut i32))
-         (field (ref $string))))
+         (field (ref $bytes))))
 
    (func $add_char (param $buf (ref $buffer)) (param $c i32)
       (local $pos i32)
-      (local $data (ref $string))
+      (local $data (ref $bytes))
       (local.set $pos (struct.get $buffer 0 (local.get $buf)))
       (local.set $data (struct.get $buffer 1 (local.get $buf)))
       (if (i32.lt_u (local.get $pos) (array.len (local.get $data)))
          (then
-            (array.set $string (local.get $data) (local.get $pos) (local.get $c))
+            (array.set $bytes (local.get $data) (local.get $pos) (local.get $c))
             (struct.set $buffer 0 (local.get $buf)
                (i32.add (local.get $pos) (i32.const 1))))))
 
-   (func $add_string (param $buf (ref $buffer)) (param $v (ref eq))
+   (func $add_string (param $buf (ref $buffer)) (param $s (ref eq))
       (local $pos i32) (local $len i32)
-      (local $data (ref $string))
-      (local $s (ref $string))
+      (local $data (ref $bytes))
       (local.set $pos (struct.get $buffer 0 (local.get $buf)))
       (local.set $data (struct.get $buffer 1 (local.get $buf)))
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $caml_string_length (local.get $s)))
       (if (i32.gt_u (i32.add (local.get $pos) (local.get $len))
                     (array.len (local.get $data)))
          (then
             (local.set $len
                 (i32.sub (array.len (local.get $data)) (local.get $pos)))))
-      (array.copy $string $string
-         (local.get $data) (local.get $pos)
-         (local.get $s) (i32.const 0)
-         (local.get $len))
+      (drop (call $caml_blit_string
+         (local.get $s) (ref.i31 (i32.const 0))
+         (local.get $data) (ref.i31 (local.get $pos))
+         (ref.i31 (local.get $len))))
       (struct.set $buffer 0 (local.get $buf)
          (i32.add (local.get $pos) (local.get $len))))
 
+(#if use-js-string
+(#then
+   (func $is_string (param $v (ref eq)) (result i32)
+      (drop (block $not_string (result (ref eq))
+         (return_call $jsstring_test
+            (struct.get $string 0
+               (br_on_cast_fail $not_string (ref eq) (ref $string)
+                  (local.get $v))))))
+      (i32.const 0))
+)
+(#else
+   (func $is_string (param $v (ref eq)) (result i32)
+      (ref.test (ref $bytes) (local.get $v)))
+))
+
+   (#string $int_format "%d")
+
    (func (export "caml_format_exception") (param (ref eq)) (result (ref eq))
       (local $exn (ref $block))
       (local $buf (ref $buffer))
       (local $v (ref eq))
       (local $bucket (ref $block))
       (local $i i32) (local $len i32)
-      (local $s (ref $string))
+      (local $s (ref $bytes))
       (local.set $exn (ref.cast (ref $block) (local.get 0)))
       (if (result (ref eq))
           (ref.eq (array.get $block (local.get $exn) (i32.const 0))
@@ -79,7 +102,7 @@
             (local.set $buf
                (struct.new $buffer
                   (i32.const 0)
-                  (array.new $string (i32.const 0) (i32.const 256))))
+                  (array.new $bytes (i32.const 0) (i32.const 256))))
             (call $add_string
                (local.get $buf)
                (array.get $block
@@ -120,10 +143,9 @@
                        (then
                            (call $add_string (local.get $buf)
                               (call $caml_format_int
-                                  (array.new_fixed $string 2
-                                     (i32.const 37) (i32.const 100)) ;; %d
+                                  (global.get $int_format)
                                   (ref.cast (ref i31) (local.get $v)))))
-                    (else (if (ref.test (ref $string) (local.get $v))
+                    (else (if (call $is_string (local.get $v))
                        (then
                           (call $add_char (local.get $buf)
                              (i32.const 34)) ;; '\"'
@@ -143,13 +165,13 @@
                           (br $loop))))
                  (call $add_char (local.get $buf) (i32.const 41)))) ;; '\)'
             (local.set $s
-               (array.new $string (i32.const 0)
+               (array.new $bytes (i32.const 0)
                   (struct.get $buffer 0 (local.get $buf))))
-            (array.copy $string $string
+            (array.copy $bytes $bytes
                (local.get $s) (i32.const 0)
                (struct.get $buffer 1 (local.get $buf)) (i32.const 0)
                (struct.get $buffer 0 (local.get $buf)))
-            (local.get $s))
+            (call $caml_string_of_bytes (local.get $s)))
          (else
             (array.get $block (local.get $exn) (i32.const 1)))))
 )
diff --git a/runtime/wasm/runtime.js b/runtime/wasm/runtime.js
index 00adc8f4a6..34241c72dc 100644
--- a/runtime/wasm/runtime.js
+++ b/runtime/wasm/runtime.js
@@ -130,7 +130,27 @@
     return (((h + (h << 2)) | 0) + (0xe6546b64 | 0)) | 0;
   }
   function hash_string(h, s) {
-    for (var i = 0; i < s.length; i++) h = hash_int(h, s.charCodeAt(i));
+    const len = s.length;
+    for (var i = 0; i + 4 <= len; i += 4) {
+      var w =
+        s.charCodeAt(i) |
+        (s.charCodeAt(i + 1) << 8) |
+        (s.charCodeAt(i + 2) << 16) |
+        (s.charCodeAt(i + 3) << 24);
+      h = hash_int(h, w);
+    }
+    w = 0;
+    switch (len & 3) {
+      case 3:
+        // biome-ignore lint/suspicious/noFallthroughSwitchClause:
+        w = s.charCodeAt(i + 2) << 16;
+      case 2:
+        // biome-ignore lint/suspicious/noFallthroughSwitchClause:
+        w |= s.charCodeAt(i + 1) << 8;
+      case 1:
+        w |= s.charCodeAt(i);
+        h = hash_int(h, w);
+    }
     return h ^ s.length;
   }
 
@@ -221,12 +241,23 @@
     ta_copy: (ta, t, s, n) => ta.copyWithin(t, s, n),
     ta_bytes: (a) =>
       new Uint8Array(a.buffer, a.byteOffset, a.length * a.BYTES_PER_ELEMENT),
-    ta_blit_from_string: (s, p1, a, p2, l) => {
+    ta_blit_from_bytes: (s, p1, a, p2, l) => {
       for (let i = 0; i < l; i++) a[p2 + i] = string_get(s, p1 + i);
     },
-    ta_blit_to_string: (a, p1, s, p2, l) => {
+    ta_blit_to_bytes: (a, p1, s, p2, l) => {
       for (let i = 0; i < l; i++) string_set(s, p2 + i, a[p1 + i]);
     },
+    ta_blit_from_string: (s, p1, a, p2, l) => {
+      for (let i = 0; i < l; i++) a[p2 + i] = s.charCodeAt(p1 + i);
+    },
+    ta_to_string: (a) => {
+      let len = a.length;
+      if (len <= 4096) return String.fromCharCode(...a);
+      var s = "";
+      for (let i = 0; 0 < len; i += 1024, len -= 1024)
+        s += String.fromCharCode(...a.subarray(i, i + Math.min(len, 1024)));
+      return s;
+    },
     wrap_callback: (f) =>
       function (...args) {
         if (args.length === 0) {
@@ -438,11 +469,16 @@
   };
   const string_ops = {
     test: (v) => +(typeof v === "string"),
-    compare: (s1, s2) => (s1 < s2 ? -1 : +(s1 > s2)),
+    compare: (s1, s2) => (s1 === s2 ? 0 : s1 < s2 ? -1 : 1),
     hash: hash_string,
     decodeStringFromUTF8Array: () => "",
     encodeStringToUTF8Array: () => 0,
     fromCharCodeArray: () => "",
+    length: (s) => s.length,
+    charCodeAt: (s, i) => s.charCodeAt(i),
+    concat: (s1, s2) => s1.concat(s2),
+    equals: (s1, s2) => +(s1 === s2),
+    substring: (s, i, j) => s.substring(i, j),
   };
   const imports = Object.assign(
     {
@@ -456,7 +492,10 @@
     },
     generated,
   );
-  const options = { builtins: ["js-string", "text-decoder", "text-encoder"] };
+  const options = {
+    builtins: ["js-string", "text-decoder", "text-encoder"],
+    importedStringConstants: "",
+  };
 
   function loadRelative(src) {
     const path = require("node:path");
diff --git a/runtime/wasm/stdlib.wat b/runtime/wasm/stdlib.wat
index 5176402152..63a445453b 100644
--- a/runtime/wasm/stdlib.wat
+++ b/runtime/wasm/stdlib.wat
@@ -22,11 +22,8 @@
    (import "string" "caml_string_equal"
       (func $caml_string_equal
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
-   (import "jslib" "caml_string_of_jsstring"
-      (func $caml_string_of_jsstring (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_jsstring_of_string"
       (func $caml_jsstring_of_string (param (ref eq)) (result (ref eq))))
-   (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "obj" "caml_callback_1"
       (func $caml_callback_1
@@ -38,6 +35,8 @@
    (import "string" "caml_string_concat"
       (func $caml_string_concat
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
    (import "printexc" "caml_format_exception"
       (func $caml_format_exception (param (ref eq)) (result (ref eq))))
    (import "sys" "ocaml_exit" (tag $ocaml_exit (param i32)))
@@ -46,11 +45,12 @@
    (import "bindings" "throw" (func $throw (param externref)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (type $assoc
       (struct
-         (field (ref $string))
+         (field (ref eq))
          (field (mut (ref eq)))
          (field (mut (ref null $assoc)))))
 
@@ -117,9 +117,7 @@
          (return (ref.i31 (i32.const 0))))
       (array.set $assoc_array
          (global.get $named_value_table) (local.get $h)
-         (struct.new $assoc
-            (ref.cast (ref $string) (local.get 0))
-            (local.get 1) (local.get $r)))
+         (struct.new $assoc (local.get 0) (local.get 1) (local.get $r)))
       (ref.i31 (i32.const 0)))
 
    ;; Used only for testing (tests-jsoo/bin), but inconvenient to pull out
@@ -183,9 +181,9 @@
 
    (type $func (func (result (ref eq))))
 
-   (data $fatal_error "Fatal error: exception ")
-   (data $handle_uncaught_exception "Printexc.handle_uncaught_exception")
-   (data $do_at_exit "Pervasives.do_at_exit")
+   (#string $fatal_error "Fatal error: exception ")
+   (#string $handle_uncaught_exception "Printexc.handle_uncaught_exception")
+   (#string $do_at_exit "Pervasives.do_at_exit")
 
    (global $uncaught_exception (mut externref) (ref.null extern))
 
@@ -212,9 +210,7 @@
                      (call $caml_callback_2
                         (br_on_null $not_registered
                            (call $caml_named_value
-                               (array.new_data $string
-                                  $handle_uncaught_exception
-                                  (i32.const 0) (i32.const 34))))
+                              (global.get $handle_uncaught_exception)))
                         (local.get $exn)
                         (ref.i31 (i32.const 0))))
                   (br $exit))
@@ -222,19 +218,17 @@
                   (drop
                      (call $caml_callback_1
                         (br_on_null $null
-                           (call $caml_named_value
-                              (array.new_data $string $do_at_exit
-                                 (i32.const 0) (i32.const 21))))
+                           (call $caml_named_value (global.get $do_at_exit)))
                         (ref.i31 (i32.const 0)))))
                (call $write (i32.const 2)
                   (call $unwrap
                      (call $caml_jsstring_of_string
                         (call $caml_string_concat
-                           (array.new_data $string $fatal_error
-                              (i32.const 0) (i32.const 23))
+                           (global.get $fatal_error)
                            (call $caml_string_concat
                               (call $caml_format_exception (local.get $exn))
-                              (array.new_fixed $string 1
-                                 (i32.const 10)))))))) ;; `\n`
-               (call $exit (i32.const 2)))))
+                              (call $caml_string_of_bytes
+                                 (array.new_fixed $bytes 1
+                                    (i32.const 10))))))))) ;; `\n`
+            (call $exit (i32.const 2)))))
 )
diff --git a/runtime/wasm/str.wat b/runtime/wasm/str.wat
index 4fa9409ee7..200547a4bd 100644
--- a/runtime/wasm/str.wat
+++ b/runtime/wasm/str.wat
@@ -19,8 +19,35 @@
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param (ref eq))))
    (import "fail" "caml_failwith" (func $caml_failwith (param (ref eq))))
+   (import "string" "caml_string_of_bytes"
+      (func $caml_string_of_bytes (param (ref eq)) (result (ref eq))))
+   (import "string" "caml_blit_string"
+      (func $caml_blit_string
+         (param (ref eq) (ref eq) (ref eq) (ref eq) (ref eq))
+         (result (ref eq))))
 
-   (type $string (array (mut i8)))
+(#if use-js-string
+(#then
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+
+   (func $string_val (param $s (ref eq)) (result externref)
+      (extern.convert_any
+         (struct.get $string 0 (ref.cast (ref $string) (local.get $s)))))
+)
+(#else
+   (func $string_length (param $s (ref $bytes)) (result i32)
+      (array.len (local.get $s)))
+   (func $string_get (param $s (ref $bytes)) (param $i i32) (result i32)
+      (array.get $bytes (local.get $s) (local.get $i)))
+   (func $string_val (param $s (ref eq)) (result (ref $bytes))
+      (ref.cast (ref $bytes) (local.get $s)))
+))
+
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $block (array (mut (ref eq))))
 
    (type $char_table (array i8))
@@ -67,24 +94,48 @@
                (i32.shr_u (local.get $c) (i32.const 3)))
             (i32.and (local.get $c) (i32.const 7)))))
 
-   (func $in_bitset (param $s (ref $string)) (param $c i32) (result i32)
+   (func $in_bitset
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $c i32) (result i32)
       (i32.and (i32.const 1)
          (i32.shr_u
-            (array.get_u $string (local.get $s)
+            (call $string_get (local.get $s)
                (i32.shr_u (local.get $c) (i32.const 3)))
             (i32.and (local.get $c) (i32.const 7)))))
 
    (func $re_match
-      (param $vre (ref eq)) (param $s (ref $string)) (param $pos i32)
-      (param $accept_partial_match i32) (result (ref eq))
+      (param $vre (ref eq))
+(#if use-js-string
+(#then
+      (param $s externref)
+)
+(#else
+      (param $s (ref $bytes))
+))
+      (param $pos i32) (param $accept_partial_match i32) (result (ref eq))
       (local $res (ref $block))
-      (local $s' (ref $string)) (local $set (ref $string))
+(#if use-js-string
+(#then
+      (local $s' externref)
+      (local $set externref)
+      (local $normtable externref)
+)
+(#else
+      (local $s' (ref $bytes))
+      (local $set (ref $bytes))
+      (local $normtable (ref $bytes))
+))
       (local $len i32) (local $instr i32) (local $arg i32) (local $i i32)
       (local $j i32) (local $l i32)
       (local $re (ref $block))
       (local $prog (ref $block))
       (local $cpool (ref $block))
-      (local $normtable (ref $string))
       (local $numgroups i32)
       (local $numregisters i32)
       (local $group_start (ref $int_array))
@@ -94,7 +145,7 @@
       (local $stack (ref null $stack))
       (local $u (ref $undo))
       (local $p (ref $pos))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (local.set $re (ref.cast (ref $block) (local.get $vre)))
       (local.set $prog
          (ref.cast (ref $block)
@@ -103,8 +154,7 @@
          (ref.cast (ref $block)
             (array.get $block (local.get $re) (i32.const 2))))
       (local.set $normtable
-         (ref.cast (ref $string)
-            (array.get $block (local.get $re) (i32.const 3))))
+         (call $string_val (array.get $block (local.get $re) (i32.const 3))))
       (local.set $numgroups
          (i31.get_s
             (ref.cast (ref i31)
@@ -166,7 +216,7 @@
                                (i32.shr_u (local.get $instr) (i32.const 8)))
                             (br_if $backtrack
                                (i32.ne (local.get $arg)
-                                  (array.get_u $string
+                                  (call $string_get
                                      (local.get $s) (local.get $pos))))
                             (local.set $pos
                                (i32.add (local.get $pos) (i32.const 1)))
@@ -178,9 +228,9 @@
                               (i32.shr_u (local.get $instr) (i32.const 8)))
                            (br_if $backtrack
                               (i32.ne (local.get $arg)
-                                 (array.get_u $string
+                                 (call $string_get
                                     (local.get $normtable)
-                                    (array.get_u $string
+                                    (call $string_get
                                        (local.get $s) (local.get $pos)))))
                            (local.set $pos
                               (i32.add (local.get $pos) (i32.const 1)))
@@ -189,11 +239,12 @@
                           (local.set $arg
                              (i32.shr_u (local.get $instr) (i32.const 8)))
                           (local.set $s'
-                             (ref.cast (ref $string)
+                             (call $string_val
                                 (array.get $block (local.get $cpool)
-                                   (i32.add (local.get $arg) (i32.const 1)))))
+                                   (i32.add (local.get $arg)
+                                      (i32.const 1)))))
                           (local.set $i (i32.const 0))
-                          (local.set $l (array.len (local.get $s')))
+                          (local.set $l (call $string_length (local.get $s')))
                           (loop $loop
                              (if (i32.lt_u (local.get $i) (local.get $l))
                                 (then
@@ -202,9 +253,9 @@
                                          (local.get $pos) (local.get $len)))
                                    (br_if $backtrack
                                       (i32.ne
-                                         (array.get_u $string (local.get $s')
+                                         (call $string_get (local.get $s')
                                             (local.get $i))
-                                         (array.get_u $string (local.get $s)
+                                         (call $string_get (local.get $s)
                                             (local.get $pos))))
                                    (local.set $pos
                                       (i32.add (local.get $pos) (i32.const 1)))
@@ -216,11 +267,12 @@
                          (local.set $arg
                             (i32.shr_u (local.get $instr) (i32.const 8)))
                          (local.set $s'
-                            (ref.cast (ref $string)
+                            (call $string_val
                                (array.get $block (local.get $cpool)
-                                  (i32.add (local.get $arg) (i32.const 1)))))
+                                  (i32.add (local.get $arg)
+                                      (i32.const 1)))))
                          (local.set $i (i32.const 0))
-                         (local.set $l (array.len (local.get $s')))
+                         (local.set $l (call $string_length (local.get $s')))
                          (loop $loop
                             (if (i32.lt_u (local.get $i) (local.get $l))
                                (then
@@ -229,11 +281,11 @@
                                         (local.get $pos) (local.get $len)))
                                   (br_if $backtrack
                                      (i32.ne
-                                        (array.get_u $string (local.get $s')
+                                        (call $string_get (local.get $s')
                                            (local.get $i))
-                                        (array.get_u $string
+                                        (call $string_get
                                            (local.get $normtable)
-                                           (array.get_u $string (local.get $s)
+                                           (call $string_get (local.get $s)
                                               (local.get $pos)))))
                                   (local.set $pos
                                      (i32.add (local.get $pos) (i32.const 1)))
@@ -249,11 +301,11 @@
                         (br_if $backtrack
                            (i32.eqz
                               (call $in_bitset
-                                 (ref.cast (ref $string)
+                                 (call $string_val
                                     (array.get $block (local.get $cpool)
                                        (i32.add (local.get $arg)
                                           (i32.const 1))))
-                                 (array.get_u $string (local.get $s)
+                                 (call $string_get (local.get $s)
                                     (local.get $pos)))))
                         (local.set $pos
                            (i32.add (local.get $pos) (i32.const 1)))
@@ -262,7 +314,7 @@
                        (br_if $continue (i32.eqz (local.get $pos)))
                        (br_if $continue
                           (i32.eq (i32.const 10) ;; '\n'
-                             (array.get_u $string (local.get $s)
+                             (call $string_get (local.get $s)
                                 (i32.sub (local.get $pos) (i32.const 1)))))
                        (br $backtrack))
                       ;; EOL
@@ -270,7 +322,7 @@
                          (i32.eq (local.get $pos) (local.get $len)))
                       (br_if $continue
                          (i32.eq (i32.const 10) ;; '\n'
-                            (array.get_u $string (local.get $s)
+                            (call $string_get (local.get $s)
                                (local.get $pos))))
                       (br $backtrack))
                      ;; WORDBOUNDARY
@@ -280,7 +332,7 @@
                               (i32.eq (local.get $pos) (local.get $len)))
                            (br_if $continue
                               (call $is_word_letter
-                                 (array.get_u $string (local.get $s)
+                                 (call $string_get (local.get $s)
                                     (local.get $pos))))
                            (br $backtrack))
                         (else
@@ -288,7 +340,7 @@
                               (then
                                  (br_if $continue
                                     (call $is_word_letter
-                                       (array.get_u $string (local.get $s)
+                                       (call $string_get (local.get $s)
                                           (i32.sub (local.get $pos)
                                              (i32.const 1)))))
                                  (br $backtrack))
@@ -296,11 +348,11 @@
                                  (br_if $continue
                                     (i32.ne
                                        (call $is_word_letter
-                                          (array.get_u $string (local.get $s)
+                                          (call $string_get (local.get $s)
                                              (i32.sub (local.get $pos)
                                                 (i32.const 1))))
                                        (call $is_word_letter
-                                          (array.get_u $string (local.get $s)
+                                          (call $string_get (local.get $s)
                                              (local.get $pos)))))
                                  (br $backtrack))))))
                     ;; BEGGROUP
@@ -348,9 +400,9 @@
                               (i32.eq (local.get $pos) (local.get $len)))
                            (br_if $backtrack
                               (i32.ne
-                                 (array.get_u $string (local.get $s)
+                                 (call $string_get (local.get $s)
                                     (local.get $i))
-                                 (array.get_u $string (local.get $s)
+                                 (call $string_get (local.get $s)
                                     (local.get $pos))))
                            (local.set $pos
                               (i32.add (local.get $pos) (i32.const 1)))
@@ -363,10 +415,11 @@
                  (if (i32.lt_u (local.get $pos) (local.get $len))
                     (then
                        (if (call $in_bitset
-                              (ref.cast (ref $string)
+                              (call $string_val
                                  (array.get $block (local.get $cpool)
-                                    (i32.add (local.get $arg) (i32.const 1))))
-                              (array.get_u $string (local.get $s)
+                                    (i32.add (local.get $arg)
+                                       (i32.const 1))))
+                              (call $string_get (local.get $s)
                                  (local.get $pos)))
                           (then
                              (local.set $pos
@@ -375,14 +428,14 @@
                 ;; SIMPLESTAR
                 (local.set $arg (i32.shr_u (local.get $instr) (i32.const 8)))
                 (local.set $set
-                   (ref.cast (ref $string)
+                   (call $string_val
                       (array.get $block (local.get $cpool)
-                      (i32.add (local.get $arg) (i32.const 1)))))
+                         (i32.add (local.get $arg) (i32.const 1)))))
                 (loop $loop
                    (if (i32.lt_u (local.get $pos) (local.get $len))
                       (then
                          (if (call $in_bitset (local.get $set)
-                                (array.get_u $string (local.get $s)
+                                (call $string_get (local.get $s)
                                    (local.get $pos)))
                             (then
                                (local.set $pos
@@ -393,20 +446,20 @@
                (br_if $prefix_match (i32.eq (local.get $pos) (local.get $len)))
                (local.set $arg (i32.shr_u (local.get $instr) (i32.const 8)))
                (local.set $set
-                  (ref.cast (ref $string)
+                  (call $string_val
                      (array.get $block (local.get $cpool)
-                     (i32.add (local.get $arg) (i32.const 1)))))
+                        (i32.add (local.get $arg) (i32.const 1)))))
                (br_if $backtrack
                   (i32.eqz
                      (call $in_bitset (local.get $set)
-                        (array.get_u $string (local.get $s) (local.get $pos)))))
+                        (call $string_get (local.get $s) (local.get $pos)))))
                (loop $loop
                   (local.set $pos (i32.add (local.get $pos) (i32.const 1)))
                   (if (i32.lt_u (local.get $pos) (local.get $len))
                      (then
                         (br_if $loop
                            (call $in_bitset (local.get $set)
-                              (array.get_u $string (local.get $s)
+                              (call $string_get (local.get $s)
                                  (local.get $pos)))))))
                (br $continue))
               ;; GOTO
@@ -507,23 +560,26 @@
       ;; reject
       (ref.i31 (i32.const 0)))
 
-   (data $search_forward "Str.search_forward")
+   (#string $search_forward "Str.search_forward")
 
    (func (export "re_search_forward")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $pos i32) (local $len i32)
       (local $res (ref eq))
-      (local.set $s (ref.cast (ref $string) (local.get $vs)))
+      (local.set $s (call $string_val (local.get $vs)))
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $search_forward
-                  (i32.const 0) (i32.const 18)))))
+         (then (call $caml_invalid_argument (global.get $search_forward))))
       (loop $loop
          (local.set $res
             (call $re_match
@@ -535,23 +591,26 @@
          (br_if $loop (i32.le_u (local.get $pos) (local.get $len))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $search_backward "Str.search_backward")
+   (#string $search_backward "Str.search_backward")
 
    (func (export "re_search_backward")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $pos i32) (local $len i32)
       (local $res (ref eq))
-      (local.set $s (ref.cast (ref $string) (local.get $vs)))
+      (local.set $s (call $string_val (local.get $vs)))
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $search_backward
-                  (i32.const 0) (i32.const 19)))))
+         (then (call $caml_invalid_argument (global.get $search_backward))))
       (loop $loop
          (local.set $res
             (call $re_match
@@ -563,23 +622,26 @@
          (br_if $loop (i32.ge_s (local.get $pos) (i32.const 0))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $string_match "Str.string_match")
+   (#string $string_match "Str.string_match")
 
    (func (export "re_string_match")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $pos i32) (local $len i32)
       (local $res (ref eq))
-      (local.set $s (ref.cast (ref $string) (local.get $vs)))
+      (local.set $s (call $string_val (local.get $vs)))
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $string_match
-                  (i32.const 0) (i32.const 16)))))
+         (then (call $caml_invalid_argument (global.get $string_match))))
       (local.set $res
          (call $re_match
             (local.get $re) (local.get $s) (local.get $pos) (i32.const 0)))
@@ -588,23 +650,26 @@
             (return (local.get $res))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $string_partial_match "Str.string_partial_match")
+   (#string $string_partial_match "Str.string_partial_match")
 
    (func (export "re_partial_match")
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
-      (local $s (ref $string))
+(#if use-js-string
+(#then
+      (local $s externref)
+)
+(#else
+      (local $s (ref $bytes))
+))
       (local $pos i32) (local $len i32)
       (local $res (ref eq))
-      (local.set $s (ref.cast (ref $string) (local.get $vs)))
+      (local.set $s (call $string_val (local.get $vs)))
       (local.set $pos (i31.get_s (ref.cast (ref i31) (local.get $vpos))))
-      (local.set $len (array.len (local.get $s)))
+      (local.set $len (call $string_length (local.get $s)))
       (if (i32.gt_u (local.get $pos) (local.get $len))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $string_partial_match
-                  (i32.const 0) (i32.const 24)))))
+         (then (call $caml_invalid_argument (global.get $string_partial_match))))
       (local.set $res
          (call $re_match
             (local.get $re) (local.get $s) (local.get $pos) (i32.const 1)))
@@ -613,39 +678,40 @@
             (return (local.get $res))))
       (array.new_fixed $block 1 (ref.i31 (i32.const 0))))
 
-   (data $illegal_backslash "Str.replace: illegal backslash sequence")
-   (data $unmatched_group "Str.replace: reference to unmatched group")
+   (#string $illegal_backslash "Str.replace: illegal backslash sequence")
+   (#string $unmatched_group "Str.replace: reference to unmatched group")
 
    (func (export "re_replacement_text")
-      (param $vrepl (ref eq)) (param $vgroups (ref eq)) (param $vorig (ref eq))
+      (param $vrepl (ref eq)) (param $vgroups (ref eq)) (param $orig (ref eq))
       (result (ref eq))
-      (local $repl (ref $string))
+(#if use-js-string
+(#then
+      (local $repl externref)
+)
+(#else
+      (local $repl (ref $bytes))
+))
       (local $groups (ref $block))
-      (local $orig (ref $string))
-      (local $res (ref $string))
+      (local $res (ref $bytes))
       (local $i i32) (local $j i32) (local $l i32) (local $len i32)
       (local $c i32) (local $start i32) (local $end i32)
-      (local.set $repl (ref.cast (ref $string) (local.get $vrepl)))
-      (local.set $l (array.len (local.get $repl)))
+      (local.set $repl (call $string_val (local.get $vrepl)))
+      (local.set $l (call $string_length (local.get $repl)))
       (local.set $groups (ref.cast (ref $block) (local.get $vgroups)))
-      (local.set $orig (ref.cast (ref $string) (local.get $vorig)))
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $l))
             (then
                (local.set $c
-                  (array.get_u $string (local.get $repl) (local.get $i)))
+                  (call $string_get (local.get $repl) (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (if (i32.ne (local.get $c) (i32.const 92)) ;; '\\'
                   (then
                      (local.set $len (i32.add (local.get $len) (i32.const 1)))
                      (br $loop)))
                (if (i32.eq (local.get $i) (local.get $l))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $illegal_backslash
-                           (i32.const 0) (i32.const 39)))))
+                  (then (call $caml_failwith (global.get $illegal_backslash))))
                (local.set $c
-                  (array.get_u $string (local.get $repl) (local.get $i)))
+                  (call $string_get (local.get $repl) (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (if (i32.eq (local.get $c) (i32.const 92)) ;; '\\'
                   (then
@@ -659,10 +725,7 @@
                (local.set $c (i32.shl (local.get $c) (i32.const 1)))
                (if (i32.gt_u (i32.add (local.get $c) (i32.const 1))
                       (array.len (local.get $groups)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $start
                   (i31.get_s
                      (ref.cast (ref i31)
@@ -674,43 +737,40 @@
                         (array.get $block (local.get $groups)
                            (i32.add (local.get $c) (i32.const 2))))))
                (if (i32.eq (local.get $start) (i32.const -1))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $len
                    (i32.add (local.get $len)
                       (i32.sub (local.get $end) (local.get $start))))
                (br $loop))))
-      (local.set $res (array.new $string (i32.const 0) (local.get $len)))
+      (local.set $res (array.new $bytes (i32.const 0) (local.get $len)))
       (local.set $i (i32.const 0))
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $l))
             (then
                (local.set $c
-                  (array.get_u $string (local.get $repl) (local.get $i)))
+                  (call $string_get (local.get $repl) (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (if (i32.ne (local.get $c) (i32.const 92)) ;; '\\'
                   (then
-                     (array.set $string (local.get $res) (local.get $j)
+                     (array.set $bytes (local.get $res) (local.get $j)
                         (local.get $c))
                      (local.set $j (i32.add (local.get $j) (i32.const 1)))
                      (br $loop)))
                (local.set $c
-                  (array.get_u $string (local.get $repl) (local.get $i)))
+                  (call $string_get (local.get $repl) (local.get $i)))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (if (i32.eq (local.get $c) (i32.const 92)) ;; '\\'
                   (then
-                     (array.set $string (local.get $res) (local.get $j)
+                     (array.set $bytes (local.get $res) (local.get $j)
                         (local.get $c))
                      (local.set $j (i32.add (local.get $j) (i32.const 1)))
                      (br $loop)))
                (local.set $c (i32.sub (local.get $c) (i32.const 48))) ;; '0'
                (if (i32.gt_u (local.get $c) (i32.const 9))
                   (then
-                     (array.set $string (local.get $res) (local.get $j)
+                     (array.set $bytes (local.get $res) (local.get $j)
                         (i32.const 92))
-                     (array.set $string (local.get $res)
+                     (array.set $bytes (local.get $res)
                         (i32.add (local.get $j) (i32.const 1))
                         (i32.add (local.get $c) (i32.const 48)))
                      (local.set $j (i32.add (local.get $j) (i32.const 2)))
@@ -718,10 +778,7 @@
                (local.set $c (i32.shl (local.get $c) (i32.const 1)))
                (if (i32.gt_u (i32.add (local.get $c) (i32.const 1))
                       (array.len (local.get $groups)))
-                  (then
-                     (call $caml_failwith
-                        (array.new_data $string $unmatched_group
-                           (i32.const 0) (i32.const 41)))))
+                  (then (call $caml_failwith (global.get $unmatched_group))))
                (local.set $start
                   (i31.get_s
                      (ref.cast (ref i31)
@@ -733,11 +790,12 @@
                         (array.get $block (local.get $groups)
                            (i32.add (local.get $c) (i32.const 2))))))
                (local.set $len (i32.sub (local.get $end) (local.get $start)))
-               (array.copy $string $string
-                  (local.get $res) (local.get $j)
-                  (local.get $orig) (local.get $start)
-                  (local.get $len))
+               (drop
+                  (call $caml_blit_string
+                     (local.get $orig) (ref.i31 (local.get $start))
+                     (local.get $res) (ref.i31 (local.get $j))
+                     (ref.i31 (local.get $len))))
                (local.set $j (i32.add (local.get $j) (local.get $len)))
                (br $loop))))
-      (local.get $res))
+      (call $caml_string_of_bytes (local.get $res)))
 )
diff --git a/runtime/wasm/string.wat b/runtime/wasm/string.wat
index bf43b2f9e4..e242c7026a 100644
--- a/runtime/wasm/string.wat
+++ b/runtime/wasm/string.wat
@@ -19,18 +19,122 @@
    (import "fail" "caml_bound_error" (func $caml_bound_error))
    (import "fail" "caml_invalid_argument"
       (func $caml_invalid_argument (param $arg (ref eq))))
+   (import "wasm:js-string" "equals"
+      (func $string_equals (param externref externref) (result i32)))
+   (import "wasm:js-string" "compare"
+      (func $string_compare (param externref externref) (result i32)))
+   (import "wasm:js-string" "length"
+      (func $string_length (param externref) (result i32)))
+   (import "wasm:js-string" "charCodeAt"
+      (func $string_get (param externref i32) (result i32)))
+   (import "wasm:js-string" "concat"
+      (func $string_concat (param externref externref) (result (ref extern))))
+   (import "jsstring" "jsbytes_of_bytes"
+      (func $jsbytes_of_bytes (param (ref $bytes)) (result anyref)))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
-   (export "caml_bytes_equal" (func $caml_string_equal))
+(#if use-js-string
+(#then
    (func $caml_string_equal (export "caml_string_equal")
       (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
-      (local $s1 (ref $string)) (local $s2 (ref $string))
+      (ref.i31
+         (call $string_equals
+            (extern.convert_any
+               (struct.get $string 0
+                  (ref.cast (ref $string) (local.get $p1))))
+            (extern.convert_any
+               (struct.get $string 0
+                  (ref.cast (ref $string) (local.get $p2)))))))
+
+   (func (export "caml_string_notequal")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (return
+         (ref.i31 (i32.eqz (i31.get_u (ref.cast (ref i31)
+            (call $caml_string_equal (local.get $p1) (local.get $p2))))))))
+
+   (func (export "caml_string_compare")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (ref.i31
+         (call $string_compare
+            (extern.convert_any
+              (struct.get $string 0
+                 (ref.cast (ref $string) (local.get $p1))))
+            (extern.convert_any
+              (struct.get $string 0
+                 (ref.cast (ref $string) (local.get $p2)))))))
+
+   (func (export "caml_string_lessequal")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.le_s
+            (call $string_compare
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p1))))
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p2)))))
+            (i32.const 0))))
+
+   (func (export "caml_string_lessthan")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.lt_s
+            (call $string_compare
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p1))))
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p2)))))
+            (i32.const 0))))
+
+   (func (export "caml_string_greaterequal")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.ge_s
+            (call $string_compare
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p1))))
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p2)))))
+            (i32.const 0))))
+
+   (func (export "caml_string_greaterthan")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (ref.i31
+         (i32.gt_s
+            (call $string_compare
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p1))))
+               (extern.convert_any
+                 (struct.get $string 0
+                    (ref.cast (ref $string) (local.get $p2)))))
+            (i32.const 0))))
+)
+(#else
+   (export "caml_string_equal" (func $caml_bytes_equal))
+   (export "caml_string_notequal" (func $caml_bytes_notequal))
+   (export "caml_string_compare" (func $caml_bytes_compare))
+   (export "caml_string_lessequal" (func $caml_bytes_lessequal))
+   (export "caml_string_lessthan" (func $caml_bytes_lessthan))
+   (export "caml_string_greaterequal" (func $caml_bytes_greaterequal))
+   (export "caml_string_greaterthan" (func $caml_bytes_greaterthan))
+))
+
+   (func $caml_bytes_equal (export "caml_bytes_equal")
+      (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
+      (local $s1 (ref $bytes)) (local $s2 (ref $bytes))
       (local $len i32) (local $i i32)
       (if (ref.eq (local.get $p1) (local.get $p2))
          (then (return (ref.i31 (i32.const 1)))))
-      (local.set $s1 (ref.cast (ref $string) (local.get $p1)))
-      (local.set $s2 (ref.cast (ref $string) (local.get $p2)))
+      (local.set $s1 (ref.cast (ref $bytes) (local.get $p1)))
+      (local.set $s2 (ref.cast (ref $bytes) (local.get $p2)))
       (local.set $len (array.len (local.get $s1)))
       (if (i32.ne (local.get $len) (array.len (local.get $s2)))
          (then (return (ref.i31 (i32.const 0)))))
@@ -38,29 +142,28 @@
       (loop $loop
          (if (i32.lt_s (local.get $i) (local.get $len))
             (then
-               (if (i32.ne (array.get_u $string (local.get $s1) (local.get $i))
-                           (array.get_u $string (local.get $s2) (local.get $i)))
+               (if (i32.ne (array.get_u $bytes (local.get $s1) (local.get $i))
+                           (array.get_u $bytes (local.get $s2) (local.get $i)))
                   (then (return (ref.i31 (i32.const 0)))))
                (local.set $i (i32.add (local.get $i) (i32.const 1)))
                (br $loop))))
       (ref.i31 (i32.const 1)))
 
-   (export "caml_bytes_notequal" (func $caml_string_notequal))
-   (func $caml_string_notequal (export "caml_string_notequal")
+   (func $caml_bytes_notequal (export "caml_bytes_notequal")
       (param $p1 (ref eq)) (param $p2 (ref eq)) (result (ref eq))
       (return
          (ref.i31 (i32.eqz (i31.get_u (ref.cast (ref i31)
-            (call $caml_string_equal (local.get $p1) (local.get $p2))))))))
+            (call $caml_bytes_equal (local.get $p1) (local.get $p2))))))))
 
-   (func $string_compare
+   (func $bytes_compare
       (param $p1 (ref eq)) (param $p2 (ref eq)) (result i32)
-      (local $s1 (ref $string)) (local $s2 (ref $string))
+      (local $s1 (ref $bytes)) (local $s2 (ref $bytes))
       (local $l1 i32) (local $l2 i32) (local $len i32) (local $i i32)
       (local $c1 i32) (local $c2 i32)
       (if (ref.eq (local.get $p1) (local.get $p2))
          (then (return (i32.const 0))))
-      (local.set $s1 (ref.cast (ref $string) (local.get $p1)))
-      (local.set $s2 (ref.cast (ref $string) (local.get $p2)))
+      (local.set $s1 (ref.cast (ref $bytes) (local.get $p1)))
+      (local.set $s2 (ref.cast (ref $bytes) (local.get $p2)))
       (local.set $l1 (array.len (local.get $s1)))
       (local.set $l2 (array.len (local.get $s2)))
       (local.set $len (select (local.get $l1) (local.get $l2)
@@ -70,9 +173,9 @@
          (if (i32.lt_s (local.get $i) (local.get $len))
             (then
                (local.set $c1
-                  (array.get_u $string (local.get $s1) (local.get $i)))
+                  (array.get_u $bytes (local.get $s1) (local.get $i)))
                (local.set $c2
-                  (array.get_u $string (local.get $s2) (local.get $i)))
+                  (array.get_u $bytes (local.get $s2) (local.get $i)))
                (if (i32.lt_u (local.get $c1) (local.get $c2))
                   (then (return (i32.const -1))))
                (if (i32.gt_u (local.get $c1) (local.get $c2))
@@ -85,62 +188,112 @@
          (then (return (i32.const 1))))
       (i32.const 0))
 
-   (export "caml_bytes_compare" (func $caml_string_compare))
-   (func $caml_string_compare (export "caml_string_compare")
+   (func $caml_bytes_compare (export "caml_bytes_compare")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (call $string_compare (local.get 0) (local.get 1))))
+      (ref.i31 (call $bytes_compare (local.get 0) (local.get 1))))
 
-   (export "caml_bytes_lessequal" (func $caml_string_lessequal))
-   (func $caml_string_lessequal (export "caml_string_lessequal")
+   (func $caml_bytes_lessequal (export "caml_bytes_lessequal")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.le_s (call $string_compare (local.get 0) (local.get 1))
+      (ref.i31 (i32.le_s (call $bytes_compare (local.get 0) (local.get 1))
                          (i32.const 0))))
 
-   (export "caml_bytes_lessthan" (func $caml_string_lessthan))
-   (func $caml_string_lessthan (export "caml_string_lessthan")
+   (func $caml_bytes_lessthan (export "caml_bytes_lessthan")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.lt_s (call $string_compare (local.get 0) (local.get 1))
+      (ref.i31 (i32.lt_s (call $bytes_compare (local.get 0) (local.get 1))
                          (i32.const 0))))
 
-   (export "caml_bytes_greaterequal" (func $caml_string_greaterequal))
-   (func $caml_string_greaterequal (export "caml_string_greaterequal")
+   (func $caml_bytes_greaterequal (export "caml_bytes_greaterequal")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.ge_s (call $string_compare (local.get 0) (local.get 1))
+      (ref.i31 (i32.ge_s (call $bytes_compare (local.get 0) (local.get 1))
                          (i32.const 0))))
 
-   (export "caml_bytes_greaterthan" (func $caml_string_greaterthan))
-   (func $caml_string_greaterthan (export "caml_string_greaterthan")
+   (func $caml_bytes_greaterthan (export "caml_bytes_greaterthan")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.gt_s (call $string_compare (local.get 0) (local.get 1))
+      (ref.i31 (i32.gt_s (call $bytes_compare (local.get 0) (local.get 1))
                          (i32.const 0))))
 
+(#if use-js-string
+(#then
+   (func (export "caml_bytes_of_string") (param $v (ref eq)) (result (ref eq))
+      (local $s externref) (local $b (ref $bytes)) (local $l i32) (local $i i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $v)))))
+      (local.set $l (call $string_length (local.get $s)))
+      (local.set $b (array.new $bytes (i32.const 0) (local.get $l)))
+      ;; loop from JS ?
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $l))
+            (then
+               (array.set $bytes (local.get $b) (local.get $i)
+                  (call $string_get (local.get $s) (local.get $i)))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (local.get $b))
+
+   (func (export "caml_string_of_bytes") (param $b (ref eq)) (result (ref eq))
+      (return
+         (struct.new $string
+            (call $jsbytes_of_bytes (ref.cast (ref $bytes) (local.get $b))))))
+)
+(#else
    (export "caml_bytes_of_string" (func $caml_string_of_bytes))
    (func $caml_string_of_bytes (export "caml_string_of_bytes")
       (param $v (ref eq)) (result (ref eq))
       (local.get $v))
+))
 
-   (data $Bytes_create "Bytes.create")
+   (#string $string_create "Bytes.create")
 
    (func (export "caml_create_bytes")
       (param $len (ref eq)) (result (ref eq))
       (local $l i32)
       (local.set $l (i31.get_s (ref.cast (ref i31) (local.get $len))))
       (if (i32.lt_s (local.get $l) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $Bytes_create
-                               (i32.const 0) (i32.const 12)))))
-      (array.new $string (i32.const 0) (local.get $l)))
-
-   (export "caml_blit_bytes" (func $caml_blit_string))
-   (func $caml_blit_string (export "caml_blit_string")
+         (then (call $caml_invalid_argument (global.get $string_create))))
+      (array.new $bytes (i32.const 0) (local.get $l)))
+
+(#if use-js-string
+(#then
+   (func (export "caml_blit_string")
+      (param $v1 (ref eq)) (param $vi1 (ref eq))
+      (param $v2 (ref eq)) (param $vi2 (ref eq))
+      (param $vn (ref eq)) (result (ref eq))
+      (local $s externref) (local $b (ref $bytes))
+      (local $i1 i32) (local $i2 i32) (local $n i32) (local $i i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $v1)))))
+      (local.set $i1 (i31.get_s (ref.cast (ref i31) (local.get $vi1))))
+      (local.set $b (ref.cast (ref $bytes) (local.get $v2)))
+      (local.set $i2 (i31.get_s (ref.cast (ref i31) (local.get $vi2))))
+      (local.set $n (i31.get_s (ref.cast (ref i31) (local.get $vn))))
+      ;; loop from JS??
+      (loop $loop
+         (if (i32.lt_u (local.get $i) (local.get $n))
+            (then
+               (array.set $bytes (local.get $b)
+                  (i32.add (local.get $i2) (local.get $i))
+                  (call $string_get
+                     (local.get $s) (i32.add (local.get $i1) (local.get $i))))
+               (local.set $i (i32.add (local.get $i) (i32.const 1)))
+               (br $loop))))
+      (ref.i31 (i32.const 0)))
+)
+(#else
+   (export "caml_blit_string" (func $caml_blit_bytes))
+))
+
+   (func $caml_blit_bytes (export "caml_blit_bytes")
       (param $v1 (ref eq)) (param $i1 (ref eq))
       (param $v2 (ref eq)) (param $i2 (ref eq))
       (param $n (ref eq)) (result (ref eq))
-      (array.copy $string $string
-         (ref.cast (ref $string) (local.get $v2))
+      (array.copy $bytes $bytes
+         (ref.cast (ref $bytes) (local.get $v2))
          (i31.get_s (ref.cast (ref i31) (local.get $i2)))
-         (ref.cast (ref $string) (local.get $v1))
+         (ref.cast (ref $bytes) (local.get $v1))
          (i31.get_s (ref.cast (ref i31) (local.get $i1)))
          (i31.get_s (ref.cast (ref i31) (local.get $n))))
       (ref.i31 (i32.const 0)))
@@ -149,17 +302,121 @@
       (param $v (ref eq)) (param $offset (ref eq))
       (param $len (ref eq)) (param $init (ref eq))
       (result (ref eq))
-      (array.fill $string (ref.cast (ref $string) (local.get $v))
+      (array.fill $bytes (ref.cast (ref $bytes) (local.get $v))
          (i31.get_u (ref.cast (ref i31) (local.get $offset)))
          (i31.get_u (ref.cast (ref i31) (local.get $init)))
          (i31.get_u (ref.cast (ref i31) (local.get $len))))
       (ref.i31 (i32.const 0)))
 
+(#if use-js-string
+(#then
+   (func (export "caml_string_get16")
+      (param $v (ref eq)) (param $i (ref eq)) (result (ref eq))
+      (local $s externref) (local $p i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $v)))))
+      (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
+      (if (i32.lt_s (local.get $p) (i32.const 0))
+         (then (call $caml_bound_error)))
+      (if (i32.ge_u (i32.add (local.get $p) (i32.const 1))
+                    (call $string_length (local.get $s)))
+         (then (call $caml_bound_error)))
+      (ref.i31 (i32.or
+                  (call $string_get (local.get $s) (local.get $p))
+                  (i32.shl (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 1)))
+                           (i32.const 8)))))
+
+   (func (export "caml_string_get32")
+      (param $v (ref eq)) (param $i (ref eq)) (result i32)
+      (local $s externref) (local $p i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $v)))))
+      (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
+      (if (i32.lt_s (local.get $p) (i32.const 0))
+         (then (call $caml_bound_error)))
+      (if (i32.ge_u (i32.add (local.get $p) (i32.const 3))
+                    (call $string_length (local.get $s)))
+         (then (call $caml_bound_error)))
+      (i32.or
+         (i32.or
+            (call $string_get (local.get $s) (local.get $p))
+            (i32.shl (call $string_get (local.get $s)
+                        (i32.add (local.get $p) (i32.const 1)))
+                     (i32.const 8)))
+         (i32.or
+            (i32.shl (call $string_get (local.get $s)
+                        (i32.add (local.get $p) (i32.const 2)))
+                     (i32.const 16))
+            (i32.shl (call $string_get (local.get $s)
+                        (i32.add (local.get $p) (i32.const 3)))
+                     (i32.const 24)))))
+
+   (func (export "caml_string_get64")
+      (param $v (ref eq)) (param $i (ref eq)) (result i64)
+      (local $s externref) (local $p i32)
+      (local.set $s
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $v)))))
+      (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
+      (if (i32.lt_s (local.get $p) (i32.const 0))
+         (then (call $caml_bound_error)))
+      (if (i32.ge_u (i32.add (local.get $p) (i32.const 7))
+                    (call $string_length (local.get $s)))
+         (then (call $caml_bound_error)))
+      (i64.or
+         (i64.or
+            (i64.or
+               (i64.extend_i32_u
+                  (call $string_get (local.get $s) (local.get $p)))
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 1))))
+                        (i64.const 8)))
+            (i64.or
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 2))))
+                        (i64.const 16))
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 3))))
+                        (i64.const 24))))
+         (i64.or
+            (i64.or
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 4))))
+                        (i64.const 32))
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 5))))
+                        (i64.const 40)))
+            (i64.or
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 6))))
+                        (i64.const 48))
+               (i64.shl (i64.extend_i32_u
+                           (call $string_get (local.get $s)
+                              (i32.add (local.get $p) (i32.const 7))))
+                        (i64.const 56))))))
+)
+(#else
    (export "caml_string_get16" (func $caml_bytes_get16))
+   (export "caml_string_get32" (func $caml_bytes_get32))
+   (export "caml_string_get64" (func $caml_bytes_get64))
+))
+
    (func $caml_bytes_get16 (export "caml_bytes_get16")
       (param $v (ref eq)) (param $i (ref eq)) (result (ref eq))
-      (local $s (ref $string)) (local $p i32)
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
+      (local $s (ref $bytes)) (local $p i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get $v)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
          (then (call $caml_bound_error)))
@@ -167,16 +424,15 @@
                     (array.len (local.get $s)))
          (then (call $caml_bound_error)))
       (ref.i31 (i32.or
-                  (array.get_u $string (local.get $s) (local.get $p))
-                  (i32.shl (array.get_u $string (local.get $s)
+                  (array.get_u $bytes (local.get $s) (local.get $p))
+                  (i32.shl (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 1)))
                            (i32.const 8)))))
 
-   (export "caml_string_get32" (func $caml_bytes_get32))
    (func $caml_bytes_get32 (export "caml_bytes_get32")
       (param $v (ref eq)) (param $i (ref eq)) (result i32)
-      (local $s (ref $string)) (local $p i32)
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
+      (local $s (ref $bytes)) (local $p i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get $v)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
          (then (call $caml_bound_error)))
@@ -185,23 +441,22 @@
          (then (call $caml_bound_error)))
       (i32.or
          (i32.or
-            (array.get_u $string (local.get $s) (local.get $p))
-            (i32.shl (array.get_u $string (local.get $s)
+            (array.get_u $bytes (local.get $s) (local.get $p))
+            (i32.shl (array.get_u $bytes (local.get $s)
                         (i32.add (local.get $p) (i32.const 1)))
                      (i32.const 8)))
          (i32.or
-            (i32.shl (array.get_u $string (local.get $s)
+            (i32.shl (array.get_u $bytes (local.get $s)
                         (i32.add (local.get $p) (i32.const 2)))
                      (i32.const 16))
-            (i32.shl (array.get_u $string (local.get $s)
+            (i32.shl (array.get_u $bytes (local.get $s)
                         (i32.add (local.get $p) (i32.const 3)))
                      (i32.const 24)))))
 
-   (export "caml_string_get64" (func $caml_bytes_get64))
    (func $caml_bytes_get64 (export "caml_bytes_get64")
       (param $v (ref eq)) (param $i (ref eq)) (result i64)
-      (local $s (ref $string)) (local $p i32)
-      (local.set $s (ref.cast (ref $string) (local.get $v)))
+      (local $s (ref $bytes)) (local $p i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get $v)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get $i))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
          (then (call $caml_bound_error)))
@@ -212,44 +467,44 @@
          (i64.or
             (i64.or
                (i64.extend_i32_u
-                  (array.get_u $string (local.get $s) (local.get $p)))
+                  (array.get_u $bytes (local.get $s) (local.get $p)))
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 1))))
                         (i64.const 8)))
             (i64.or
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 2))))
                         (i64.const 16))
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 3))))
                         (i64.const 24))))
          (i64.or
             (i64.or
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 4))))
                         (i64.const 32))
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 5))))
                         (i64.const 40)))
             (i64.or
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 6))))
                         (i64.const 48))
                (i64.shl (i64.extend_i32_u
-                           (array.get_u $string (local.get $s)
+                           (array.get_u $bytes (local.get $s)
                               (i32.add (local.get $p) (i32.const 7))))
                         (i64.const 56))))))
 
    (func (export "caml_bytes_set16")
       (param (ref eq) (ref eq) (ref eq)) (result (ref eq))
-      (local $s (ref $string)) (local $p i32) (local $v i32)
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
+      (local $s (ref $bytes)) (local $p i32) (local $v i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (local.set $v (i31.get_s (ref.cast (ref i31) (local.get 2))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
@@ -257,86 +512,117 @@
       (if (i32.ge_u (i32.add (local.get $p) (i32.const 1))
                     (array.len (local.get $s)))
          (then (call $caml_bound_error)))
-      (array.set $string (local.get $s) (local.get $p) (local.get $v))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s) (local.get $p) (local.get $v))
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 1))
          (i32.shr_u (local.get $v) (i32.const 8)))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_bytes_set32")
       (param (ref eq)) (param (ref eq)) (param $v i32) (result (ref eq))
-      (local $s (ref $string)) (local $p i32)
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
+      (local $s (ref $bytes)) (local $p i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
          (then (call $caml_bound_error)))
       (if (i32.ge_u (i32.add (local.get $p) (i32.const 3))
                     (array.len (local.get $s)))
          (then (call $caml_bound_error)))
-      (array.set $string (local.get $s) (local.get $p) (local.get $v))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s) (local.get $p) (local.get $v))
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 1))
          (i32.shr_u (local.get $v) (i32.const 8)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 2))
          (i32.shr_u (local.get $v) (i32.const 16)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 3))
          (i32.shr_u (local.get $v) (i32.const 24)))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_bytes_set64")
       (param (ref eq)) (param (ref eq)) (param $v i64) (result (ref eq))
-      (local $s (ref $string)) (local $p i32)
-      (local.set $s (ref.cast (ref $string) (local.get 0)))
+      (local $s (ref $bytes)) (local $p i32)
+      (local.set $s (ref.cast (ref $bytes) (local.get 0)))
       (local.set $p (i31.get_s (ref.cast (ref i31) (local.get 1))))
       (if (i32.lt_s (local.get $p) (i32.const 0))
          (then (call $caml_bound_error)))
       (if (i32.ge_u (i32.add (local.get $p) (i32.const 7))
                     (array.len (local.get $s)))
          (then (call $caml_bound_error)))
-      (array.set $string (local.get $s) (local.get $p)
+      (array.set $bytes (local.get $s) (local.get $p)
          (i32.wrap_i64 (local.get $v)))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 1))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 8))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 2))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 16))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 3))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 24))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 4))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 32))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 5))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 40))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 6))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 48))))
-      (array.set $string (local.get $s)
+      (array.set $bytes (local.get $s)
          (i32.add (local.get $p) (i32.const 7))
          (i32.wrap_i64 (i64.shr_u (local.get $v) (i64.const 56))))
       (ref.i31 (i32.const 0)))
 
+(#if use-js-string
+(#then
+   (func (export "caml_string_concat")
+      (param $s1 (ref eq)) (param $s2 (ref eq)) (result (ref eq))
+      (return
+         (struct.new $string
+            (any.convert_extern
+               (call $string_concat
+                  (extern.convert_any
+                    (struct.get $string 0
+                       (ref.cast (ref $string) (local.get $s1))))
+                  (extern.convert_any
+                    (struct.get $string 0
+                       (ref.cast (ref $string) (local.get $s2)))))))))
+)
+(#else
    (func (export "caml_string_concat")
       (param $vs1 (ref eq)) (param $vs2 (ref eq)) (result (ref eq))
-      (local $s1 (ref $string)) (local $s2 (ref $string))
-      (local $s (ref $string))
+      (local $s1 (ref $bytes)) (local $s2 (ref $bytes))
+      (local $s (ref $bytes))
       (local $l1 i32) (local $l2 i32)
-      (local.set $s1 (ref.cast (ref $string) (local.get $vs1)))
-      (local.set $s2 (ref.cast (ref $string) (local.get $vs2)))
+      (local.set $s1 (ref.cast (ref $bytes) (local.get $vs1)))
+      (local.set $s2 (ref.cast (ref $bytes) (local.get $vs2)))
       (local.set $l1 (array.len (local.get $s1)))
       (local.set $l2 (array.len (local.get $s2)))
       (local.set $s
-         (array.new $string (i32.const 0)
+         (array.new $bytes (i32.const 0)
             (i32.add (local.get $l1) (local.get $l2))))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (local.get $s) (i32.const 0) (local.get $s1) (i32.const 0)
          (local.get $l1))
-      (array.copy $string $string
+      (array.copy $bytes $bytes
          (local.get $s) (local.get $l1) (local.get $s2) (i32.const 0)
          (local.get $l2))
       (local.get $s))
+))
+
+(#if use-js-string
+(#then
+   (func (export "caml_string_length") (param $s (ref eq)) (result i32)
+      (return_call $string_length
+         (extern.convert_any
+            (struct.get $string 0
+               (ref.cast (ref $string) (local.get $s))))))
+)
+(#else
+   (func (export "caml_string_length") (param $s (ref eq)) (result i32)
+      (array.len (ref.cast (ref $bytes) (local.get $s))))
+))
+
 )
diff --git a/runtime/wasm/sync.wat b/runtime/wasm/sync.wat
index cd14209dd1..6adc3f6aee 100644
--- a/runtime/wasm/sync.wat
+++ b/runtime/wasm/sync.wat
@@ -24,7 +24,8 @@
       (func $custom_hash_id (param (ref eq)) (result i32)))
    (import "custom" "custom_next_id" (func $custom_next_id (result i64)))
 
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $compare
       (func (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
    (type $hash
@@ -36,7 +37,7 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
-         (field $id (ref $string))
+         (field $id (ref $bytes))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -53,7 +54,7 @@
 
    (global $mutex_ops (ref $custom_operations)
       (struct.new $custom_operations
-         (array.new_fixed $string 6 ;; "_mutex"
+         (array.new_fixed $bytes 6 ;; "_mutex"
             (i32.const 95) (i32.const 109) (i32.const 117) (i32.const 116)
             (i32.const 101) (i32.const 120))
          (ref.func $custom_compare_id)
@@ -75,16 +76,13 @@
       (struct.new $mutex
          (global.get $mutex_ops) (call $custom_next_id) (i32.const 0)))
 
-   (data $lock_failure "Mutex.lock: mutex already locked. Cannot wait.")
+   (#string $lock_failure "Mutex.lock: mutex already locked. Cannot wait.")
 
    (func (export "caml_ml_mutex_lock") (param (ref eq)) (result (ref eq))
       (local $t (ref $mutex))
       (local.set $t (ref.cast (ref $mutex) (local.get 0)))
       (if (struct.get $mutex $state (local.get $t))
-         (then
-            (call $caml_failwith
-               (array.new_data $string $lock_failure
-                  (i32.const 0) (i32.const 46)))))
+         (then (call $caml_failwith (global.get $lock_failure))))
       (struct.set $mutex $state (local.get $t) (i32.const 1))
       (ref.i31 (i32.const 0)))
 
@@ -106,13 +104,11 @@
    (func (export "caml_ml_condition_new") (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
-   (data $condition_failure "Condition.wait: cannot wait")
+   (#string $condition_failure "Condition.wait: cannot wait")
 
    (func (export "caml_ml_condition_wait")
       (param (ref eq)) (param (ref eq)) (result (ref eq))
-      (call $caml_failwith
-         (array.new_data $string $condition_failure
-            (i32.const 0) (i32.const 27)))
+      (call $caml_failwith (global.get $condition_failure))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_ml_condition_signal") (param (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/sys.wat b/runtime/wasm/sys.wat
index b397c89a82..a40de69017 100644
--- a/runtime/wasm/sys.wat
+++ b/runtime/wasm/sys.wat
@@ -49,7 +49,8 @@
       (func $jsstring_test (param anyref) (result i32)))
 
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
 
    (tag $ocaml_exit (export "ocaml_exit") (param i32))
@@ -148,14 +149,14 @@
       ;; ZZZ
       (ref.i31 (i32.const 0)))
 
-   (data $Unix "Unix")
+   (#string $Unix "Unix")
 
    (func (export "caml_sys_get_config")
       (param (ref eq)) (result (ref eq))
       ;; ZZZ
       ;; (call $log_js (string.const "caml_sys_get_config"))
       (array.new_fixed $block 4 (ref.i31 (i32.const 0))
-         (array.new_data $string $Unix (i32.const 0) (i32.const 4))
+         (global.get $Unix)
          (ref.i31 (i32.const 32))
          (ref.i31 (i32.const 0))))
 
@@ -163,11 +164,13 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (i32.const 0)))
 
+   (#string $empty_string "")
+
    (func (export "caml_runtime_variant") (param (ref eq)) (result (ref eq))
-      (array.new_fixed $string 0))
+      (global.get $empty_string))
 
    (func (export "caml_runtime_parameters") (param (ref eq)) (result (ref eq))
-      (array.new_fixed $string 0))
+      (global.get $empty_string))
 
    (func (export "caml_install_signal_handler")
       (param (ref eq) (ref eq)) (result (ref eq))
@@ -185,7 +188,7 @@
       (param (ref eq)) (result (ref eq))
       (ref.i31 (global.get $caml_runtime_warnings)))
 
-   (data $toString "toString")
+   (#string $toString "toString")
 
    (func $caml_handle_sys_error (export "caml_handle_sys_error")
       (param $exn externref)
@@ -193,6 +196,6 @@
          (call $caml_string_of_jsstring
             (call $caml_js_meth_call
                (call $wrap (any.convert_extern (local.get $exn)))
-               (array.new_data $string $toString (i32.const 0) (i32.const 8))
+               (global.get $toString)
                (array.new_fixed $block 1 (ref.i31 (i32.const 0)))))))
 )
diff --git a/runtime/wasm/weak.wat b/runtime/wasm/weak.wat
index 5b54df26e0..4deb1b47b3 100644
--- a/runtime/wasm/weak.wat
+++ b/runtime/wasm/weak.wat
@@ -33,7 +33,8 @@
    (import "jslib" "unwrap" (func $unwrap (param (ref eq)) (result anyref)))
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (type $block (array (mut (ref eq))))
-   (type $string (array (mut i8)))
+   (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $js (struct (field anyref)))
 
    ;; A weak array is a an abstract value composed of possibly some
@@ -275,7 +276,7 @@
       (call $caml_ephe_set_data_opt (local.get $vx) (local.get $d))
       (ref.i31 (i32.const 0)))
 
-   (data $Weak_create "Weak.create")
+   (#string $Weak_create "Weak.create")
 
    (export "caml_weak_create" (func $caml_ephe_create))
    (func $caml_ephe_create (export "caml_ephe_create")
@@ -284,10 +285,7 @@
       (local $res (ref $block))
       (local.set $len (i31.get_s (ref.cast (ref i31) (local.get $vlen))))
       (if (i32.lt_s (local.get $len) (i32.const 0))
-         (then
-            (call $caml_invalid_argument
-               (array.new_data $string $Weak_create
-                  (i32.const 0) (i32.const 11)))))
+         (then (call $caml_invalid_argument (global.get $Weak_create))))
       (local.set $res
          (array.new $block (global.get $caml_ephe_none)
             (i32.add (local.get $len) (global.get $caml_ephe_key_offset))))
diff --git a/tools/ci_setup.ml b/tools/ci_setup.ml
index d64dfbc7fc..d33f15f591 100644
--- a/tools/ci_setup.ml
+++ b/tools/ci_setup.ml
@@ -32,6 +32,9 @@ let dune_workspace =
  (_
   (env-vars (TESTING_FRAMEWORK inline-test))
   (js_of_ocaml (enabled_if false))
+  (wasm_of_ocaml
+   (flags
+    (:standard --enable use-js-string)))
   (flags :standard -warn-error -8-32-34-49-52-55 -w -67-69)))
 |}