diff --git a/.github/workflows/build-wasm_of_ocaml.yml b/.github/workflows/build-wasm_of_ocaml.yml
index bffb5bf824..28bebcb26a 100644
--- a/.github/workflows/build-wasm_of_ocaml.yml
+++ b/.github/workflows/build-wasm_of_ocaml.yml
@@ -75,7 +75,7 @@ jobs:
       - name: Set-up Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: latest
+          node-version: 'v24.0.0-v8-canary202412116884e26428'
 
       - name: Set-up OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v3
diff --git a/benchmarks/Makefile b/benchmarks/Makefile
index 3d12c8b96a..7e0ef4214e 100644
--- a/benchmarks/Makefile
+++ b/benchmarks/Makefile
@@ -21,8 +21,8 @@ bench:
 	$(MAKE) -C benchmark-fiat-crypto bench
 	$(MAKE) -C benchmark-ocamlc bench
 	$(MAKE) -C benchmark-partial-render-table bench
-	$(MAKE) -C benchmark-camlboy bench
-	$(MAKE) -C benchmark-others bench
+	#$(MAKE) -C benchmark-camlboy bench
+	$(MAKE) -C benchmark-others bench # To try later!
 
 microbenchmarks:
 	@date -u +"%FT%TZ - Microbenchmarks: starting"
diff --git a/benchmarks/benchmark-fiat-crypto/Makefile b/benchmarks/benchmark-fiat-crypto/Makefile
index cfff448c45..02dc6b9111 100644
--- a/benchmarks/benchmark-fiat-crypto/Makefile
+++ b/benchmarks/benchmark-fiat-crypto/Makefile
@@ -2,7 +2,7 @@
 
 export NAME=Fiat-Crypto
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME): starting"
@@ -11,7 +11,7 @@ bench:
 	@date -u +"%FT%TZ - $(NAME): done"
 
 perform: bedrock2_fiat_crypto.byte
-	/usr/bin/time -f "%E %R" $(COMPILER) --debug times --source-map $(EXTRA_ARGS) $< -o out.js 2>&1 | \
+	env time -f "%E %R" $(COMPILER) --debug times --source-map $(EXTRA_ARGS) $< -o out.js 2>&1 | \
 	ocaml -I +str str.cma ../utils/compilation_metrics.ml $(COMPILER) $(NAME) out.js | \
 	sh ../utils/aggregate.sh $(KIND)
 
diff --git a/benchmarks/benchmark-ocamlc/Makefile b/benchmarks/benchmark-ocamlc/Makefile
index be6f814711..f6b8915c35 100644
--- a/benchmarks/benchmark-ocamlc/Makefile
+++ b/benchmarks/benchmark-ocamlc/Makefile
@@ -2,7 +2,7 @@
 
 export NAME=Ocamlc
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME): starting"
@@ -14,9 +14,9 @@ bench:
 ARGS=ml/*.ml ml/*.ml ml/*.ml ml/*.ml ml/*.ml ml/*.ml ml/*.ml ml/*.ml
 
 perform:
-	/usr/bin/time -f "%E %R" $(COMPILER) --debug times --opt 2 --pretty `which ocamlc.byte` -o $(SCRIPT) 2>&1 | \
+	env time -f "%E %R" $(COMPILER) --debug times --opt 2 --pretty `which ocamlc.byte` -o $(SCRIPT) 2>&1 | \
 	ocaml -I +str str.cma ../utils/compilation_metrics.ml $(COMPILER) $(NAME) $(SCRIPT) | \
 	sh ../utils/aggregate.sh $(KIND)
-	/usr/bin/time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) -c $(ARGS) 2>&1 | \
+	env time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) -c $(ARGS) 2>&1 | \
 	sh ../utils/format_metrics.sh exec | \
 	sh ../utils/aggregate.sh $(KIND)
diff --git a/benchmarks/benchmark-others/bigarrays/Makefile b/benchmarks/benchmark-others/bigarrays/Makefile
index dd5b69e3c4..2b538242ff 100644
--- a/benchmarks/benchmark-others/bigarrays/Makefile
+++ b/benchmarks/benchmark-others/bigarrays/Makefile
@@ -3,17 +3,17 @@
 export NAME=Others
 export SUBNAME=bigarrays
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): starting"
 	ocamlc bench.ml -o bench
-	$(MAKE) perform COMPILER=js_of_ocaml SCRIPT=bench.js KIND=js
-	$(MAKE) perform COMPILER=wasm_of_ocaml SCRIPT=bench.wasm.js KIND=wasm
+	#$(MAKE) perform COMPILER=js_of_ocaml FLAGS= SCRIPT=bench.js KIND=js
+	$(MAKE) perform COMPILER=wasm_of_ocaml FLAGS="--enable use-js-string" SCRIPT=bench.wasm.js KIND=wasm
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): done"
 
 perform:
-	$(COMPILER) --opt 2 --pretty bench -o $(SCRIPT)
-	/usr/bin/time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 | \
+	$(COMPILER) $(FLAGS) --opt 2 --pretty bench -o $(SCRIPT)
+	env time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 | \
 	sh ../../utils/format_metrics.sh exec | \
 	sh ../../utils/aggregate.sh $(KIND)
diff --git a/benchmarks/benchmark-others/bin_prot/Makefile b/benchmarks/benchmark-others/bin_prot/Makefile
index 51effc0579..4e4b7cf1df 100644
--- a/benchmarks/benchmark-others/bin_prot/Makefile
+++ b/benchmarks/benchmark-others/bin_prot/Makefile
@@ -3,7 +3,7 @@
 export NAME=Others
 export SUBNAME=bin_prot
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): starting"
@@ -14,7 +14,7 @@ bench:
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): done"
 
 perform:
-	/usr/bin/time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 1> /dev/null | \
+	env time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 1> /dev/null | \
 	tee /dev/stderr | \
 	sh ../../utils/format_metrics.sh exec | \
 	sh ../../utils/aggregate.sh $(KIND)
diff --git a/benchmarks/benchmark-others/bin_prot/dune b/benchmarks/benchmark-others/bin_prot/dune
index 3f60080847..9e7ea8a5f0 100644
--- a/benchmarks/benchmark-others/bin_prot/dune
+++ b/benchmarks/benchmark-others/bin_prot/dune
@@ -4,7 +4,7 @@
  (js_of_ocaml
   (flags --opt 2))
  (wasm_of_ocaml
-  (flags --opt 2))
+  (flags --opt 2 --enable use-js-string))
  (preprocess
   (pps ppx_bin_prot))
  (libraries unix))
diff --git a/benchmarks/benchmark-others/lexifi-g2pp/Makefile b/benchmarks/benchmark-others/lexifi-g2pp/Makefile
index c7358d6037..936b003d13 100644
--- a/benchmarks/benchmark-others/lexifi-g2pp/Makefile
+++ b/benchmarks/benchmark-others/lexifi-g2pp/Makefile
@@ -3,7 +3,7 @@
 export NAME=Others
 export SUBNAME=lexifi-g2pp
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): starting"
@@ -13,6 +13,6 @@ bench:
 	@date -u +"%FT%TZ - $(NAME)/$(SUBNAME): done"
 
 perform:
-	/usr/bin/time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 1> /dev/null | \
+	env time -f '{"compiler": "$(COMPILER)", "time":"%E"}' node $(SCRIPT) 2>&1 1> /dev/null | \
 	sh ../../utils/format_metrics.sh exec | \
 	sh ../../utils/aggregate.sh $(KIND)
diff --git a/benchmarks/benchmark-partial-render-table/Makefile b/benchmarks/benchmark-partial-render-table/Makefile
index 92889796d5..db90259441 100644
--- a/benchmarks/benchmark-partial-render-table/Makefile
+++ b/benchmarks/benchmark-partial-render-table/Makefile
@@ -2,7 +2,7 @@
 
 export NAME=Partial Render Table
 
-SHELL=/bin/bash -o pipefail
+SHELL=/usr/bin/env bash -o pipefail
 
 bench:
 	@date -u +"%FT%TZ - $(NAME): starting"
@@ -14,7 +14,7 @@ bench:
 	@date -u +"%FT%TZ - $(NAME): done"
 
 perform:
-	/usr/bin/time -f "%E %R" $(COMPILER) --debug times --opt 2 --pretty main.bc-for-jsoo -o out.js 2>&1 | \
+	env time -f "%E %R" $(COMPILER) --debug times --opt 2 --pretty --enable use-js-string main.bc-for-jsoo -o out.js 2>&1 | \
 	tee /dev/stderr | \
 	ocaml -I +str str.cma ../utils/compilation_metrics.ml $(COMPILER) "$(NAME)" out.js | \
 	sh ../utils/aggregate.sh $(KIND)
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index d0f39f6c28..faa9bb98be 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -84,6 +84,7 @@ let preprocessor_variables () =
         | `Disabled | `Jspi -> "jspi"
         | `Cps -> "cps"
         | `Double_translation -> assert false) )
+  ; "use-js-string", Wat_preprocess.Bool (Config.Flag.use_js_string ())
   ]
 
 let with_runtime_files ~runtime_wasm_files f =
@@ -126,6 +127,7 @@ let build_runtime ~runtime_file =
              [ "bindings"
              ; "Math"
              ; "js"
+             ; "str"
              ; "wasm:js-string"
              ; "wasm:text-encoder"
              ; "wasm:text-decoder"
@@ -249,10 +251,10 @@ let generate_prelude ~out_file =
     Driver.optimize_for_wasm ~profile code
   in
   let context = Generate.start () 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
@@ -260,14 +262,14 @@ let generate_prelude ~out_file =
       program
   in
   Generate.wasm_output ch ~opt_source_map_file:None ~context;
-  uinfo.provides
+  uinfo.provides, generated_js
 
 let build_prelude z =
   Fs.with_intermediate_file (Filename.temp_file "prelude" ".wasm")
   @@ fun prelude_file ->
-  let predefined_exceptions = generate_prelude ~out_file:prelude_file in
+  let info = generate_prelude ~out_file:prelude_file in
   Zip.add_file z ~name:"prelude.wasm" ~file:prelude_file;
-  predefined_exceptions
+  info
 
 let build_js_runtime ~primitives ?runtime_arguments () =
   let always_required_js, primitives =
@@ -443,12 +445,17 @@ 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, 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
+           ; fragments
+           }
+         ]
        ();
      Zip.close_out z)
    else
diff --git a/compiler/bin-wasm_of_ocaml/gen/gen.ml b/compiler/bin-wasm_of_ocaml/gen/gen.ml
index 0310943921..0658671d23 100644
--- a/compiler/bin-wasm_of_ocaml/gen/gen.ml
+++ b/compiler/bin-wasm_of_ocaml/gen/gen.ml
@@ -27,7 +27,12 @@ let check_js_file fname =
 
 let default_flags = []
 
-let interesting_runtimes = [ [ "effects", `S "jspi" ]; [ "effects", `S "cps" ] ]
+let interesting_runtimes =
+  [ [ "effects", `S "jspi"; "use-js-string", `B false ]
+  ; [ "effects", `S "cps"; "use-js-string", `B false ]
+  ; [ "effects", `S "jspi"; "use-js-string", `B true ]
+  ; [ "effects", `S "cps"; "use-js-string", `B true ]
+  ]
 
 let name_runtime standard l =
   let flags =
diff --git a/compiler/bin-wasm_of_ocaml/link_wasm.ml b/compiler/bin-wasm_of_ocaml/link_wasm.ml
index 07032da073..0cfb7d00cf 100644
--- a/compiler/bin-wasm_of_ocaml/link_wasm.ml
+++ b/compiler/bin-wasm_of_ocaml/link_wasm.ml
@@ -69,7 +69,14 @@ let options =
   in
   let build_t input_modules output_file variables allowed_imports common opt merge =
     let allowed_imports =
-      if List.is_empty allowed_imports then None else Some (List.concat allowed_imports)
+      match allowed_imports with
+      | [] -> Some [ "str" ]
+      | _ :: _ as l ->
+          let l = List.concat l in
+          if List.mem ~eq:String.equal "str" l then
+            Some l
+          else
+            Some ("str" :: l)
     in
     `Ok
       { input_modules
diff --git a/compiler/lib-wasm/gc_target.ml b/compiler/lib-wasm/gc_target.ml
index 686b62d096..d62bcf4176 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
@@ -788,13 +805,48 @@ 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' = wasm_array_get ~ty:Type.string_type e e'
+  let bytes_get e e' = wasm_array_get ~ty:Type.bytes_type e e'
+
+  let bytes_set e e' e'' = wasm_array_set ~ty:Type.bytes_type e e' e''
 
-  let bytes_set e e' e'' = wasm_array_set ~ty:Type.string_type e e' 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' = e' in
+      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)))
 
@@ -913,6 +965,17 @@ module Constant = struct
     | Const_named of string
     | Mutated
 
+  let translate_js_string s =
+    let* x =
+      register_import
+        ~import_module:"str"
+        ~name:s
+        (Global { mut = false; typ = Ref { nullable = false; typ = Extern } })
+    in
+    let* ty = Type.js_type in
+    return
+      (Const_named ("str_" ^ s), W.StructNew (ty, [ AnyConvertExtern (GlobalGet x) ]))
+
   let rec translate_rec c =
     match c with
     | Code.Int i -> return (Const, W.RefI31 (Const (I32 (Targetint.to_int32 i))))
@@ -971,34 +1034,29 @@ module Constant = struct
           | Utf (Utf8 s) -> s
           | Byte s -> byte_string s
         in
-        let* x =
-          register_import
-            ~import_module:"str"
-            ~name:s
-            (Global { mut = false; typ = Ref { nullable = false; typ = Extern } })
-        in
-        let* ty = Type.js_type in
-        return
-          (Const_named ("str_" ^ s), W.StructNew (ty, [ AnyConvertExtern (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 (byte_string 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 (Int64.float_of_bits f)) ]))
diff --git a/compiler/lib-wasm/generate.ml b/compiler/lib-wasm/generate.ml
index d65a71d04d..5c0944c080 100644
--- a/compiler/lib-wasm/generate.ml
+++ b/compiler/lib-wasm/generate.ml
@@ -300,29 +300,30 @@ module Generate (Target : Target_sig.S) = struct
         seq (Memory.array_set x y z) Value.unit);
     register_tern_prim "caml_floatarray_unsafe_set" ~ty:(Int Normalized) (fun x y z ->
         seq (Memory.float_array_set x y z) Value.unit);
-    register_bin_prim "caml_string_unsafe_get" `Pure ~ty:(Int Normalized) Memory.bytes_get;
+    register_bin_prim "caml_string_unsafe_get" `Pure ~ty:(Int Normalized) Memory.string_get;
     register_bin_prim
       "caml_bytes_unsafe_get"
       `Mutable
       ~ty:(Int Normalized)
       Memory.bytes_get;
-    register_tern_prim
-      "caml_string_unsafe_set"
-      ~ty:(Int Normalized)
-      ~tz:(Int Unnormalized)
-      (fun x y z -> seq (Memory.bytes_set x y z) Value.unit);
     register_tern_prim
       "caml_bytes_unsafe_set"
       ~ty:(Int Normalized)
       ~tz:(Int Unnormalized)
       (fun x y z -> seq (Memory.bytes_set x y z) Value.unit);
+    let string_get context x y =
+      seq
+        (let* cond = Arith.uge y (Memory.string_length x) in
+         instr (W.Br_if (label_index context bound_error_pc, cond)))
+        (Memory.string_get x y)
+    in
+    register_bin_prim_ctx "caml_string_get" ~ty:(Int Normalized) string_get;
     let bytes_get context x y =
       seq
         (let* cond = Arith.uge y (Memory.bytes_length x) in
          instr (W.Br_if (label_index context bound_error_pc, cond)))
         (Memory.bytes_get x y)
     in
-    register_bin_prim_ctx "caml_string_get" ~ty:(Int Normalized) bytes_get;
     register_bin_prim_ctx "caml_bytes_get" ~ty:(Int Normalized) bytes_get;
     let bytes_set context x y z =
       seq
@@ -331,17 +332,12 @@ module Generate (Target : Target_sig.S) = struct
          Memory.bytes_set x y z)
         Value.unit
     in
-    register_tern_prim_ctx
-      "caml_string_set"
-      ~ty:(Int Normalized)
-      ~tz:(Int Unnormalized)
-      bytes_set;
     register_tern_prim_ctx
       "caml_bytes_set"
       ~ty:(Int Normalized)
       ~tz:(Int Unnormalized)
       bytes_set;
-    register_un_prim "caml_ml_string_length" `Pure (fun x -> Memory.bytes_length x);
+    register_un_prim "caml_ml_string_length" `Pure (fun x -> Memory.string_length x);
     register_un_prim "caml_ml_bytes_length" `Pure (fun x -> Memory.bytes_length x);
     register_bin_prim
       "%int_add"
@@ -1025,7 +1021,6 @@ 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/link.ml b/compiler/lib-wasm/link.ml
index f2c527323b..6d31387308 100644
--- a/compiler/lib-wasm/link.ml
+++ b/compiler/lib-wasm/link.ml
@@ -393,7 +393,7 @@ 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 ~opt_source_map_file:None ~context;
   if times () then Format.eprintf "    generate start: %a@." Timer.print t1
 
@@ -644,11 +644,11 @@ 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
@@ -745,7 +745,8 @@ 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 428327a3df..45d6ce1fe7 100644
--- a/compiler/lib-wasm/target_sig.ml
+++ b/compiler/lib-wasm/target_sig.ml
@@ -80,6 +80,10 @@ 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/wat_preprocess.ml b/compiler/lib-wasm/wat_preprocess.ml
index f753c4672d..7fbba2d23c 100644
--- a/compiler/lib-wasm/wat_preprocess.ml
+++ b/compiler/lib-wasm/wat_preprocess.ml
@@ -286,6 +286,9 @@ type st =
   ; mutable pos : pos
   ; variables : value StringMap.t
   ; buf : Buffer.t
+  ; mutable head : int
+  ; head_buf : Buffer.t
+  ; mutable id : int (* to generate distinct string id names *)
   }
 
 let value_type v : typ =
@@ -412,6 +415,11 @@ let insert st s =
 let pred_position { loc; byte_loc } =
   { loc = { loc with pos_cnum = loc.pos_cnum - 1 }; byte_loc = byte_loc - 1 }
 
+let generate_id st _ =
+  let id = Printf.sprintf "$js$string$%d$" st.id in
+  st.id <- st.id + 1;
+  id
+
 let rec rewrite_list st l = List.iter ~f:(rewrite st) l
 
 and rewrite st elt =
@@ -508,35 +516,116 @@ and rewrite st elt =
       then raise (Error (position_of_loc loc_value, "Expecting a string"));
       let s = parse_string loc_value value in
       write st pos;
+      if variable_is_set st "use-js-string"
+      then (
+        Printf.bprintf
+          st.head_buf
+          "(import \"str\" %s (global %s$string externref)) "
+          value
+          name;
+        insert
+          st
+          (Printf.sprintf
+             "(global %s (ref eq) (struct.new $string (any.convert_extern (global.get \
+              %s$string))))"
+             name
+             name))
+      else
+        insert
+          st
+          (Format.asprintf
+             "(global %s (ref eq) (array.new_fixed $bytes %d%a))"
+             name
+             (String.length s)
+             (fun f s ->
+               String.iter
+                 ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c))
+                 s)
+             s);
+      skip st pos'
+  | { desc = List [ { desc = Atom "@string"; _ }; { desc = Atom value; loc = loc_value } ]
+    ; loc = pos, pos'
+    } ->
+      if not (is_string value)
+      then raise (Error (position_of_loc loc_value, "Expecting a string"));
+      let s = parse_string loc_value value in
+      let name = generate_id st s in
+      write st pos;
+      if variable_is_set st "use-js-string"
+      then (
+        Printf.bprintf
+          st.head_buf
+          "(import \"str\" %s (global %s$string externref)) "
+          value
+          name;
+        insert
+          st
+          (Printf.sprintf
+             "(struct.new $string (any.convert_extern (global.get %s$string)))"
+             name))
+      else
+        insert
+          st
+          (Format.asprintf
+             "(array.new_fixed $bytes %d%a)"
+             (String.length s)
+             (fun f s ->
+               String.iter
+                 ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c))
+                 s)
+             s);
+      skip st pos'
+  | { desc =
+        List
+          [ { desc = Atom "@jsstring"; _ }
+          ; { desc = Atom name; _ }
+          ; { desc = Atom value; _ }
+          ]
+    ; loc = pos, pos'
+    } ->
+      write st pos;
+      Printf.bprintf
+        st.head_buf
+        "(import \"str\" %s (global %s$string externref)) "
+        value
+        name;
       insert
         st
-        (Format.asprintf
-           "(global %s (ref eq) (array.new_fixed $bytes %d%a))"
+        (Printf.sprintf
+           "(global %s (ref eq) (struct.new $js (any.convert_extern (global.get \
+            %s$string))))"
            name
-           (String.length s)
-           (fun f s ->
-             String.iter ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c)) s)
-           s);
+           name);
       skip st pos'
-  | { desc = List [ { desc = Atom "@string"; _ }; { desc = Atom value; loc = loc_value } ]
+  | { desc =
+        List [ { desc = Atom "@jsstring"; _ }; { desc = Atom value; loc = loc_value } ]
     ; loc = pos, pos'
     } ->
       if not (is_string value)
       then raise (Error (position_of_loc loc_value, "Expecting a string"));
       let s = parse_string loc_value value in
+      let name = generate_id st s in
       write st pos;
+      Printf.bprintf
+        st.head_buf
+        "(import \"str\" %s (global %s$string externref)) "
+        value
+        name;
       insert
         st
-        (Format.asprintf
-           "(array.new_fixed $bytes %d%a)"
-           (String.length s)
-           (fun f s ->
-             String.iter ~f:(fun c -> Format.fprintf f " (i32.const %d)" (Char.code c)) s)
-           s);
+        (Printf.sprintf
+           "(struct.new $%s (any.convert_extern (global.get %s$string))))"
+           (if variable_is_set st "use-js-string" then "string" else "js")
+           name);
       skip st pos'
-  | { desc = List [ { desc = Atom "@string"; loc = _, pos } ]; loc = _, pos' } ->
+  | { desc = List [ { desc = Atom ("@string" | "@jsstring"); loc = _, pos } ]
+    ; loc = _, pos'
+    } ->
       raise (Error ((pos.loc, pos'.loc), Printf.sprintf "Expecting an id or a string.\n"))
-  | { desc = List ({ desc = Atom "@string"; _ } :: _ :: _ :: { loc; _ } :: _); _ } ->
+  | { desc =
+        List ({ desc = Atom ("@string" | "@jsstring"); _ } :: _ :: _ :: { loc; _ } :: _)
+    ; _
+    } ->
       raise
         (Error (position_of_loc loc, Printf.sprintf "Expecting a closing parenthesis.\n"))
   | { desc = List [ { desc = Atom "@char"; _ }; { desc = Atom value; loc = loc_value } ]
@@ -576,6 +665,9 @@ and rewrite st elt =
       insert st (Printf.sprintf " $%s " (parse_string export_loc export_name));
       skip st pos';
       rewrite_list st l
+  | { 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
   | _ -> ()
 
@@ -585,7 +677,7 @@ let ocaml_version =
   Scanf.sscanf Sys.ocaml_version "%d.%d.%d" (fun major minor patchlevel ->
       Version (major, minor, patchlevel))
 
-let default_settings = [ "name-wasm-functions", Bool true ]
+let default_settings = [ "name-wasm-functions", Bool true; "use-js-string", Bool false ]
 
 let f ~variables ~filename ~contents:text =
   let variables =
@@ -599,10 +691,23 @@ let f ~variables ~filename ~contents:text =
   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) } in
+    let st =
+      { text
+      ; pos
+      ; variables
+      ; buf = Buffer.create (String.length text)
+      ; head_buf = Buffer.create 128
+      ; head = 0
+      ; id = 0
+      }
+    in
     rewrite_list st t;
     write st end_pos;
-    Buffer.contents st.buf
+    let head = Buffer.contents st.head_buf in
+    let contents = Buffer.contents st.buf in
+    String.sub contents ~pos:0 ~len:st.head
+    ^ head
+    ^ String.sub contents ~pos:st.head ~len:(String.length contents - st.head)
   with Error (loc, msg) -> report_error loc msg
 
 type source =
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/dune b/compiler/tests-wasm_of_ocaml/preprocess/dune
index bb37a89e75..547a267ef9 100644
--- a/compiler/tests-wasm_of_ocaml/preprocess/dune
+++ b/compiler/tests-wasm_of_ocaml/preprocess/dune
@@ -17,5 +17,26 @@
  (action
   (diff tests.expected tests.output)))
 
+(rule
+ (with-stdout-to
+  tests-js-string.output
+  (run
+   %{bin:wasm_of_ocaml}
+   pp
+   --enable
+   use-js-string
+   --enable
+   a
+   --disable
+   b
+   --set
+   c=1
+   %{dep:tests.txt})))
+
+(rule
+ (alias runtest)
+ (action
+  (diff tests-js-string.expected tests-js-string.output)))
+
 (cram
  (deps %{bin:wasm_of_ocaml}))
diff --git a/compiler/tests-wasm_of_ocaml/preprocess/tests-js-string.expected b/compiler/tests-wasm_of_ocaml/preprocess/tests-js-string.expected
new file mode 100644
index 0000000000..2fc07cc905
--- /dev/null
+++ b/compiler/tests-wasm_of_ocaml/preprocess/tests-js-string.expected
@@ -0,0 +1,79 @@
+(import "str" "abcd" (global $s$string externref)) (import "str" "abcd" (global $js$string$0$$string externref)) (import "str" "\\\'\28\n" (global $js$string$1$$string externref)) (import "str" "abcd" (global $js$string$2$$string externref)) (import "str" "abcd" (global $js$string$3$$string externref)) (import "str" "0" (global $js$string$4$$string externref)) (import "str" "\n" (global $js$string$5$$string externref)) ;; conditional
+              a is true                     
+                                b is false  
+              a is true  
+                         
+
+;; nested conditionals
+                                                     a is true and b is false  
+                                                                            
+
+;; not
+                                
+                    b is false  
+
+;; and
+                  true  
+                    a is true  
+                               
+                                        
+                            a is true and b is false  
+                                                      
+                                                     
+
+;; or
+                        
+                   a is true  
+                              
+                     a or b is true  
+                           a is true or b is false  
+                                                    
+                                 a or b is false  
+
+;; strings
+                           newline  
+                         quote  
+
+;; string comparisons
+                      c is 1  
+                              
+                                   
+                       c is not 2  
+
+;; version comparisons
+                                                   
+                                (4 1 1) = (4 1 1)  
+                                                   
+                                 (4 1 1) <> (4 1 0)  
+                                                     
+                                 (4 1 1) <> (4 1 2)  
+                                                     
+                                 (4 1 1) <= (4 1 1)  
+                                 (4 1 1) <= (4 1 2)  
+                                 (4 1 1) >= (4 1 0)  
+                                 (4 1 1) >= (4 1 1)  
+                                                     
+                                (4 1 1) > (4 1 0)  
+                                                   
+                                                   
+
+;; version comparisons: lexicographic order
+                                                   
+                                                   
+                                (4 1 1) < (4 1 2)  
+                                                   
+                                (4 1 1) < (4 2 0)  
+                                (4 1 1) < (5 0 1)  
+                                                   
+
+;; strings
+(global $s (ref eq) (struct.new $string (any.convert_extern (global.get $s$string))))
+(struct.new $string (any.convert_extern (global.get $js$string$0$$string)))
+(struct.new $string (any.convert_extern (global.get $js$string$1$$string)))
+                  (struct.new $string (any.convert_extern (global.get $js$string$2$$string)))  
+                         (struct.new $string (any.convert_extern (global.get $js$string$3$$string)))  
+
+;; chars
+(struct.new $string (any.convert_extern (global.get $js$string$4$$string)))
+(struct.new $string (any.convert_extern (global.get $js$string$5$$string)))
+
diff --git a/dune b/dune
index fa9ca7d14d..a6bf4376db 100644
--- a/dune
+++ b/dune
@@ -8,7 +8,10 @@
     (:standard)))
   (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/wasm/array.wat b/runtime/wasm/array.wat
index 235a9aa52b..0d88c0632c 100644
--- a/runtime/wasm/array.wat
+++ b/runtime/wasm/array.wat
@@ -21,6 +21,7 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
 
diff --git a/runtime/wasm/backtrace.wat b/runtime/wasm/backtrace.wat
index 9b63e4e554..b2266f0db2 100644
--- a/runtime/wasm/backtrace.wat
+++ b/runtime/wasm/backtrace.wat
@@ -25,6 +25,7 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (func (export "caml_get_exception_raw_backtrace")
       (param (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/bigarray.wat b/runtime/wasm/bigarray.wat
index 59cc22cd7f..d2ff177365 100644
--- a/runtime/wasm/bigarray.wat
+++ b/runtime/wasm/bigarray.wat
@@ -72,6 +72,12 @@
    (import "bindings" "dv_set_i8"
       (func $dv_set_i8 (param externref i32 i32)))
    (import "bindings" "littleEndian" (global $littleEndian i32))
+   (import "bindings" "ta_blit_from_string"
+      (func $ta_blit_from_string
+         (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"
@@ -121,9 +127,12 @@
       (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 $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
 
@@ -138,7 +147,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -2036,10 +2051,23 @@
          (local.get $view) (local.get $p) (local.get $d) (i32.const 1))
       (ref.i31 (i32.const 0)))
 
-   (export "caml_bytes_of_uint8_array" (func $caml_string_of_uint8_array))
-   (func $caml_string_of_uint8_array (export "caml_string_of_uint8_array")
+(@if use-js-string
+(@then
+   (func (export "caml_string_of_uint8_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_uint8_array" (func $caml_bytes_of_uint8_array))
+))
+
+   (func $caml_bytes_of_uint8_array (export "caml_bytes_of_uint8_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 $bytes))
       (local.set $a
@@ -2051,8 +2079,30 @@
          (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 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
+            (local.get $len)))
+      (call $ta_blit_from_string
+         (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)
diff --git a/runtime/wasm/bigstring.wat b/runtime/wasm/bigstring.wat
index 1cf4428dcb..08f62eb75c 100644
--- a/runtime/wasm/bigstring.wat
+++ b/runtime/wasm/bigstring.wat
@@ -58,10 +58,31 @@
       (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 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 $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)
@@ -103,7 +124,7 @@
          (local.set $h (call $caml_hash_mix_int (local.get $h) (local.get $w))))
       (i32.xor (local.get $h) (local.get $len)))
 
-   (@string $buffer "buffer")
+   (@jsstring $buffer "buffer")
 
    (func (export "bigstring_to_array_buffer")
       (param $bs (ref eq)) (result (ref eq))
@@ -164,10 +185,16 @@
       (local $i i32) (local $pos1 i32) (local $pos2 i32) (local $len i32)
       (local $c1 i32) (local $c2 i32)
       (local $v1 (ref extern))
+(@if use-js-string
+(@then
+      (local $s2 externref)
+)
+(@else
       (local $s2 (ref $bytes))
+))
       (local.set $v1 (call $caml_ba_get_view (local.get $s1)))
       (local.set $pos1 (i31.get_s (ref.cast (ref i31) (local.get $vpos1))))
-      (local.set $s2 (ref.cast (ref $bytes) (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
@@ -177,7 +204,7 @@
                   (call $dv_get_ui8 (local.get $v1)
                      (i32.add (local.get $pos1) (local.get $i))))
                (local.set $c2
-                  (array.get_u $bytes (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)))
@@ -229,8 +256,32 @@
                (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))
diff --git a/runtime/wasm/blake2.wat b/runtime/wasm/blake2.wat
index 25ad007838..0a0fd32e4a 100644
--- a/runtime/wasm/blake2.wat
+++ b/runtime/wasm/blake2.wat
@@ -7,6 +7,8 @@
       (func $caml_string_of_jsbytes (param (ref eq)) (result (ref eq))))
    (import "jslib" "caml_jsbytes_of_string"
       (func $caml_jsbytes_of_string (param (ref eq)) (result (ref eq))))
+   (import "jsstring" "jsbytes_of_bytes"
+      (func $jsbytes_of_bytes (param (ref $bytes)) (result anyref)))
 
    (import "js" "blake2_js_for_wasm_create"
       (func $blake2_js_for_wasm_create (param (ref eq) anyref) (result anyref)))
@@ -30,7 +32,7 @@
             (local.get $hashlen)
             (call $unwrap (call $caml_jsbytes_of_string (local.get $key))))))
 
-   (func $jsbytes_of_substring
+   (func $jsbytes_of_subbytes
       (param $vbuf (ref eq)) (param $vofs (ref eq)) (param $vlen (ref eq))
       (result anyref i32 i32)
       (local $buf (ref $bytes)) (local $buf' (ref $bytes))
@@ -49,17 +51,27 @@
             (local.set $buf (local.get $buf'))
             (local.set $ofs (i32.const 0))))
       (tuple.make 3
-         (call $unwrap (call $caml_jsbytes_of_string (local.get $buf)))
+         (call $jsbytes_of_bytes (local.get $buf))
          (local.get $ofs)
          (local.get $len)))
 
    (func (export "caml_blake2_update")
       (param $ctx (ref eq)) (param $buf (ref eq)) (param $ofs (ref eq))
       (param $len (ref eq)) (result (ref eq))
+(@if (and use-js-string (< ocaml_version (5 3 0)))
+(@then
       (call $blake2_js_for_wasm_update
          (call $unwrap (local.get $ctx))
-         (call $jsbytes_of_substring
+         (call $unwrap (local.get $buf))
+         (i31.get_u (ref.cast (ref i31) (local.get $ofs)))
+         (i31.get_u (ref.cast (ref i31) (local.get $len))))
+)
+(@else
+      (call $blake2_js_for_wasm_update
+         (call $unwrap (local.get $ctx))
+         (call $jsbytes_of_subbytes
             (local.get $buf) (local.get $ofs) (local.get $len)))
+))
       (ref.i31 (i32.const 0)))
 
    (func (export "caml_blake2_final")
@@ -70,7 +82,29 @@
                (call $unwrap (local.get $ctx))
                (local.get $hashlen)))))
 
-   (func (export "caml_blake2_string") (export "caml_blake2_bytes")
+(@if use-js-string
+(@then
+   (func (export "caml_blake2_string")
+      (param $hashlen (ref eq)) (param $key (ref eq)) (param $buf (ref eq))
+      (param $ofs (ref eq)) (param $len (ref eq)) (result (ref eq))
+      (local $ctx anyref)
+      (local.set $ctx
+         (call $blake2_js_for_wasm_create
+            (local.get $hashlen)
+            (call $unwrap (local.get $key))))
+      (call $blake2_js_for_wasm_update
+         (local.get $ctx)
+         (call $unwrap (local.get $buf))
+         (i31.get_u (ref.cast (ref i31) (local.get $ofs)))
+         (i31.get_u (ref.cast (ref i31) (local.get $len))))
+      (return_call $wrap
+         (call $blake2_js_for_wasm_final
+            (local.get $ctx) (local.get $hashlen))))
+)
+(@else
+   (export "caml_blake2_string" (func $caml_blake2_bytes))
+))
+   (func $caml_blake2_bytes (export "caml_blake2_bytes")
       (param $hashlen (ref eq)) (param $key (ref eq)) (param $buf (ref eq))
       (param $ofs (ref eq)) (param $len (ref eq)) (result (ref eq))
       (local $ctx anyref)
@@ -80,7 +114,7 @@
             (call $unwrap (call $caml_jsbytes_of_string (local.get $key)))))
       (call $blake2_js_for_wasm_update
          (local.get $ctx)
-         (call $jsbytes_of_substring
+         (call $jsbytes_of_subbytes
             (local.get $buf) (local.get $ofs) (local.get $len)))
       (return_call $caml_string_of_jsbytes
          (call $wrap
diff --git a/runtime/wasm/compare.wat b/runtime/wasm/compare.wat
index b6a48a62b7..b922068daf 100644
--- a/runtime/wasm/compare.wat
+++ b/runtime/wasm/compare.wat
@@ -39,6 +39,7 @@
 
    (type $block (array (mut (ref eq))))
    (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 +63,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
diff --git a/runtime/wasm/custom.wat b/runtime/wasm/custom.wat
index 526d36ca62..6f1f2c0769 100644
--- a/runtime/wasm/custom.wat
+++ b/runtime/wasm/custom.wat
@@ -22,11 +22,12 @@
    (import "int64" "int64_ops" (global $int64_ops (ref $custom_operations)))
    (import "bigarray" "bigarray_ops"
       (global $bigarray_ops (ref $custom_operations)))
-   (import "string" "caml_bytes_equal"
-      (func $caml_bytes_equal
+   (import "string" "caml_string_equal"
+      (func $caml_string_equal
          (param (ref eq)) (param (ref eq)) (result (ref eq))))
 
    (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
@@ -38,7 +39,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -109,14 +116,14 @@
             (local.get $ops) (global.get $custom_operations))))
 
    (func (export "caml_find_custom_operations")
-      (param $id (ref $bytes)) (result (ref null $custom_operations))
+      (param $id (ref eq)) (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_bytes_equal (local.get $id)
+                       (call $caml_string_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 a9305e7a41..56c2c6f035 100644
--- a/runtime/wasm/dune
+++ b/runtime/wasm/dune
@@ -17,6 +17,7 @@
    --binaryen=-g
    --binaryen-opt=-O3
    --set=effects=jspi
+   --disable=use-js-string
    --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
    %{target}
    %{read-lines:args})))
@@ -32,6 +33,39 @@
    --binaryen=-g
    --binaryen-opt=-O3
    --set=effects=cps
+   --disable=use-js-string
+   --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
+   %{target}
+   %{read-lines:args})))
+
+(rule
+ (target runtime-jspi-use-js-string.wasm)
+ (deps
+  args
+  (glob_files *.wat))
+ (action
+  (run
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --set=effects=jspi
+   --enable=use-js-string
+   --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
+   %{target}
+   %{read-lines:args})))
+
+(rule
+ (target runtime-cps-use-js-string.wasm)
+ (deps
+  args
+  (glob_files *.wat))
+ (action
+  (run
+   ../../compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.exe
+   --binaryen=-g
+   --binaryen-opt=-O3
+   --set=effects=cps
+   --enable=use-js-string
    --allowed-imports=bindings,Math,js,wasm:js-string,wasm:text-encoder,wasm:text-decoder
    %{target}
    %{read-lines:args})))
diff --git a/runtime/wasm/effect.wat b/runtime/wasm/effect.wat
index 05bc0ad9c2..4ca192929c 100644
--- a/runtime/wasm/effect.wat
+++ b/runtime/wasm/effect.wat
@@ -44,6 +44,7 @@
 
    (type $block (array (mut (ref eq))))
    (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
diff --git a/runtime/wasm/fail.wat b/runtime/wasm/fail.wat
index 04a6092a0e..84932e11aa 100644
--- a/runtime/wasm/fail.wat
+++ b/runtime/wasm/fail.wat
@@ -22,6 +22,7 @@
 
    (type $block (array (mut (ref eq))))
    (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))
 
diff --git a/runtime/wasm/float.wat b/runtime/wasm/float.wat
index b0bf76e609..1e682a9cad 100644
--- a/runtime/wasm/float.wat
+++ b/runtime/wasm/float.wat
@@ -31,9 +31,32 @@
       (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)))
+(@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))
@@ -189,24 +212,32 @@
                (then
                   (array.set $bytes (local.get $s) (i32.const 0)
                      (local.get $style))))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
    (@string $format_error "format_float: bad format")
 
    (func $parse_format
-      (param $s (ref $bytes)) (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 $bytes (local.get $s) (i32.const 0))
+               (i32.ne (call $string_get (local.get $s) (i32.const 0))
                        (@char "%")))
-            (local.set $c (array.get_u $bytes (local.get $s) (i32.const 1)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 1)))
             (if (i32.eq (local.get $c) (@char "+"))
                (then
                   (local.set $sign_style (i32.const 1))
@@ -217,13 +248,13 @@
                   (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 $bytes (local.get $s) (local.get $i))
-                       (@char ".")))
+               (i32.ne (call $string_get (local.get $s) (local.get $i))
+                       (@char "."))) ;; '.'
             (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 $bytes (local.get $s) (local.get $i)))
+                  (call $string_get (local.get $s) (local.get $i)))
                (if (i32.and (i32.ge_u (local.get $c) (@char "0"))
                             (i32.le_u (local.get $c) (@char "9")))
                   (then
@@ -259,8 +290,7 @@
       (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 $bytes) (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)))
@@ -328,23 +358,31 @@
                         (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)))
 
    (@string $float_of_string "float_of_string")
 
-   (func $caml_float_of_hex (param $s (ref $bytes)) (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 $bytes (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) (@char "."))
                      (then
@@ -357,7 +395,7 @@
                      (then
                         (br_if $error (i32.eq (local.get $i) (local.get $len)))
                         (local.set $c
-                           (array.get_u $bytes (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) (@char "-"))
                            (then
@@ -365,7 +403,7 @@
                               (br_if $error
                                  (i32.eq (local.get $i) (local.get $len)))
                               (local.set $c
-                                 (array.get_u $bytes
+                                 (call $string_get
                                     (local.get $s) (local.get $i)))
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))))
@@ -374,7 +412,7 @@
                               (br_if $error
                                  (i32.eq (local.get $i) (local.get $len)))
                               (local.set $c
-                                 (array.get_u $bytes
+                                 (call $string_get
                                     (local.get $s) (local.get $i)))
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))))
@@ -394,7 +432,7 @@
                               (if (i32.ne (local.get $i) (local.get $len))
                                  (then
                                     (local.set $c
-                                       (array.get_u $bytes
+                                       (call $string_get
                                           (local.get $s) (local.get $i)))
                                     (local.set $i
                                        (i32.add (local.get $i) (i32.const 1)))
@@ -474,24 +512,39 @@
       (call $caml_failwith (global.get $float_of_string))
       (f64.const 0))
 
-   (func $on_whitespace (param $s (ref $bytes)) (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 $bytes (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) (@char " "))
          (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 $bytes)) (local $len i32) (local $i i32) (local $j i32)
+(@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 $bytes) (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 (@char "_")
-                      (array.get_u $bytes (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)))
@@ -507,7 +560,7 @@
                (if (i32.lt_u (local.get $i) (local.get $len))
                   (then
                      (local.set $c
-                        (array.get_u $bytes (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) (@char "_"))
                         (then
@@ -517,7 +570,15 @@
                               (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))
@@ -531,7 +592,7 @@
          (br_if $error
             (call $on_whitespace
                (local.get $s) (i32.sub (local.get $len) (i32.const 1))))
-         (local.set $c (array.get_u $bytes (local.get $s) (i32.const 0)))
+         (local.set $c (call $string_get (local.get $s) (i32.const 0)))
          (if (i32.eq (local.get $c) (@char "-"))
             (then
                (local.set $negative (i32.const 1))
@@ -541,11 +602,11 @@
                (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 $bytes (local.get $s) (local.get $i))
+               (if (i32.eq (call $string_get (local.get $s) (local.get $i))
                            (@char "0"))
                   (then
                      (if (i32.eq (i32.and
-                                    (array.get_u $bytes (local.get $s)
+                                    (call $string_get (local.get $s)
                                        (i32.add (local.get $i) (i32.const 1)))
                                     (i32.const 0xdf))
                                  (@char "X"))
@@ -558,17 +619,17 @@
                            (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 $bytes (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))
                            (@char "N")) (then
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $bytes (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))
                               (@char "A")) (then
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $bytes (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))
                                  (@char "N"))
                         (then
@@ -578,12 +639,12 @@
                            (@char "I")) (then
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $bytes (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))
                               (@char "N")) (then
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $bytes (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))
                                  (@char "F"))
                         (then
@@ -595,34 +656,34 @@
                                     (local.get $negative))))))))))))
          (if (i32.eq (i32.add (local.get $i) (i32.const 8)) (local.get $len))
             (then
-               (local.set $c (array.get_u $bytes (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))
                            (@char "I")) (then
                   (local.set $i (i32.add (local.get $i) (i32.const 1)))
                   (local.set $c
-                     (array.get_u $bytes (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))
                               (@char "N")) (then
                      (local.set $i (i32.add (local.get $i) (i32.const 1)))
                      (local.set $c
-                        (array.get_u $bytes (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))
                                  (@char "F")) (then
                         (local.set $i (i32.add (local.get $i) (i32.const 1)))
                         (local.set $c
-                           (array.get_u $bytes (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))
                                     (@char "I")) (then
                            (local.set $i (i32.add (local.get $i) (i32.const 1)))
                            (local.set $c
-                              (array.get_u $bytes
+                              (call $string_get
                                  (local.get $s) (local.get $i)))
                            (if (i32.eq (i32.and (local.get $c) (i32.const 0xdf))
                                        (@char "N")) (then
                               (local.set $i
                                  (i32.add (local.get $i) (i32.const 1)))
                               (local.set $c
-                                 (array.get_u $bytes
+                                 (call $string_get
                                      (local.get $s) (local.get $i)))
                               (if (i32.eq
                                      (i32.and (local.get $c) (i32.const 0xdf))
@@ -630,7 +691,7 @@
                                  (local.set $i
                                     (i32.add (local.get $i) (i32.const 1)))
                                  (local.set $c
-                                    (array.get_u $bytes
+                                    (call $string_get
                                        (local.get $s) (local.get $i)))
                                  (if (i32.eq
                                         (i32.and (local.get $c) (i32.const 0xdf))
@@ -638,7 +699,7 @@
                                     (local.set $i
                                        (i32.add (local.get $i) (i32.const 1)))
                                     (local.set $c
-                                       (array.get_u $bytes
+                                       (call $string_get
                                            (local.get $s) (local.get $i)))
                                     (if (i32.eq
                                            (i32.and (local.get $c)
@@ -652,7 +713,15 @@
                                                 (local.get $negative))))
                                        ))))))))))))))))))
          (local.set $f
-            (call $parse_float (call $jsstring_of_bytes (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 (global.get $float_of_string))
diff --git a/runtime/wasm/fs.wat b/runtime/wasm/fs.wat
index 898dabc6a6..ede02efacd 100644
--- a/runtime/wasm/fs.wat
+++ b/runtime/wasm/fs.wat
@@ -50,6 +50,7 @@
       (func $caml_string_concat (param (ref eq) (ref eq)) (result (ref eq))))
 
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (func (export "caml_sys_getcwd")
       (param (ref eq)) (result (ref eq))
diff --git a/runtime/wasm/hash.wat b/runtime/wasm/hash.wat
index a7a78c9e49..7528549c0c 100644
--- a/runtime/wasm/hash.wat
+++ b/runtime/wasm/hash.wat
@@ -25,6 +25,7 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
    (type $js (struct (field anyref)))
 
@@ -39,7 +40,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -120,7 +127,7 @@
          (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")
+   (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)))
@@ -168,6 +175,19 @@
          (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))
+))
+
    (global $HASH_QUEUE_SIZE i32 (i32.const 256))
    (global $MAX_FORWARD_DEREFERENCE i32 (i32.const 1000))
 
@@ -216,10 +236,10 @@
                               (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 $bytes)
+                        (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)))
@@ -323,6 +343,8 @@
       (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)
@@ -330,7 +352,20 @@
          (i32.and
             (call $caml_hash_mix_final
                (call $caml_hash_mix_string
+                  (i31.get_s (ref.cast (ref i31) (local.get 0)))
+                  (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 bb3126fb53..ba52e1a71c 100644
--- a/runtime/wasm/int32.wat
+++ b/runtime/wasm/int32.wat
@@ -33,6 +33,7 @@
       (func $caml_deserialize_int_4 (param (ref eq)) (result i32)))
 
    (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 +45,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
diff --git a/runtime/wasm/int64.wat b/runtime/wasm/int64.wat
index 6b2a4fb964..06bd3303a6 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 $bytes)) (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 $bytes)) (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 $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 +82,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -137,17 +180,24 @@
    ;; package "integers".
    (func $caml_i64_of_digits (export "caml_i64_of_digits")
       (param $base i32) (param $signedness i32) (param $sign i32)
-      (param $s (ref $bytes)) (param $i i32) (param $errmsg (ref eq))
+(@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 $bytes (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)))
@@ -155,7 +205,7 @@
          (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 $bytes (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) (@char "_")))
                (local.set $d (call $parse_digit (local.get $c)))
                (if (i32.ge_u (local.get $d) (local.get $base))
@@ -186,10 +236,16 @@
       (local.get $res))
 
    (func (export "caml_int64_of_string") (param $v (ref eq)) (result (ref eq))
+(@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 $bytes) (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)))
@@ -227,13 +283,20 @@
          (br_if $write (i64.ne (local.get $d) (i64.const 0))))
       (if (local.get $negative)
          (then (array.set $bytes (local.get $s) (i32.const 0) (@char "-"))))
-      (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)
+(@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)
@@ -242,14 +305,14 @@
       (local $i i32)
       (local $n i64)
       (local $chars (ref $chars))
-      (local.set $s (ref.cast (ref $bytes) (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 $bytes (local.get $s) (i32.const 1))
+            (if (i32.eq (call $string_get (local.get $fmt) (i32.const 1))
                         (@char "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)))
@@ -315,6 +378,6 @@
                         (array.set $bytes (local.get $s) (i32.const 1)
                            (select (@char "X") (@char "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 a524ae9a77..eded6ca049 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))))
+
+(@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 $bytes)) (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 $bytes (local.get $s) (i32.const 0)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 0)))
             (if (i32.eq (local.get $c) (@char "-"))
                (then
                   (local.set $sign (i32.const -1))
@@ -47,11 +77,11 @@
                (else (if (i32.eq (local.get $c) (@char "+"))
                   (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 $bytes (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 $bytes (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) (@char "X"))
                            (i32.eq (local.get $c) (@char "x")))
@@ -96,13 +126,19 @@
    (func $parse_int (export "parse_int")
       (param $v (ref eq)) (param $nbits i32) (param $errmsg (ref eq))
       (result i32)
+(@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 $bytes) (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 $bytes (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 $bytes (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) (@char "_")))
                (local.set $d (call $parse_digit (local.get $c)))
                (if (i32.ge_u (local.get $d) (local.get $base))
@@ -213,24 +249,31 @@
          (br_if $write (local.get $d)))
       (if (local.get $negative)
          (then (array.set $bytes (local.get $s) (i32.const 0) (@char "-"))))
-      (local.get $s))
+      (return_call $caml_string_of_bytes (local.get $s)))
 
    (@string $format_error "format_int: bad format")
 
    (func $parse_int_format (export "parse_int_format")
-      (param $s (ref $bytes)) (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 $bytes (local.get $s) (i32.const 0))
+               (i32.ne (call $string_get (local.get $s) (i32.const 0))
                        (@char "%")))
-            (local.set $c (array.get_u $bytes (local.get $s) (i32.const 1)))
+            (local.set $c (call $string_get (local.get $s) (i32.const 1)))
             (if (i32.eq (local.get $c) (@char "+"))
                (then
                   (local.set $sign_style (i32.const 1))
@@ -244,7 +287,7 @@
                   (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 $bytes (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) (@char "L"))
                                 (i32.eq (local.get $c) (@char "l")))
                         (i32.eq (local.get $c) (@char "n")))
@@ -252,7 +295,7 @@
                   (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 $bytes (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) (@char "d"))
@@ -286,6 +329,13 @@
 
    (func $format_int (export "format_int")
       (param (ref eq)) (param $d i32) (param $small i32) (result (ref eq))
+(@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)
@@ -294,13 +344,13 @@
       (local $i i32)
       (local $n i32)
       (local $chars (ref $chars))
-      (local.set $s (ref.cast (ref $bytes) (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 $bytes (local.get $s) (i32.const 1))
+            (if (i32.eq (call $string_get (local.get $fmt) (i32.const 1))
                         (@char "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)))
@@ -372,5 +422,5 @@
                         (array.set $bytes (local.get $s) (i32.const 1)
                            (select (@char "X") (@char "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 b72f1600b1..a242afa1ca 100644
--- a/runtime/wasm/io.wat
+++ b/runtime/wasm/io.wat
@@ -72,6 +72,10 @@
       (func $dv_get_ui8 (param externref i32) (result i32)))
    (import "bindings" "dv_set_i8"
       (func $dv_set_i8 (param externref i32 i32)))
+   (import "bindings" "ta_blit_from_string"
+      (func $ta_blit_from_string
+         (param anyref) (param i32) (param (ref extern)) (param i32)
+         (param i32)))
    (import "custom" "custom_compare_id"
       (func $custom_compare_id
         (param (ref eq)) (param (ref eq)) (param i32) (result i32)))
@@ -88,6 +92,8 @@
       (func $caml_handle_sys_error (param externref)))
    (import "bigarray" "caml_ba_get_data"
       (func $caml_ba_get_data (param (ref eq)) (result (ref extern))))
+   (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"
@@ -101,6 +107,7 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $offset_array (array (mut i64)))
 
    (type $compare
@@ -114,7 +121,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -850,6 +863,7 @@
       (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_set (local.get $buf)
          (call $ta_subarray (local.get $d)
             (local.get $pos) (i32.add (local.get $pos) (local.get $len)))
@@ -860,6 +874,26 @@
          (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))
+      (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_string
+         (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 (export "caml_really_putblock")
       (param $ch (ref eq)) (param $s (ref $bytes))
       (param $pos i32) (param $len i32)
@@ -889,8 +923,34 @@
                (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)
diff --git a/runtime/wasm/jslib.wat b/runtime/wasm/jslib.wat
index 23542f08e5..a7647a4e90 100644
--- a/runtime/wasm/jslib.wat
+++ b/runtime/wasm/jslib.wat
@@ -90,8 +90,18 @@
       (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 eq)) (result (ref eq))))
+   (import "jsstring" "string_of_jsstring"
+      (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 eq) i32 i32) (result (ref eq))))
    (import "int32" "caml_copy_int32"
       (func $caml_copy_int32 (param i32) (result (ref eq))))
    (import "int32" "Int32_val"
@@ -105,6 +115,7 @@
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $js (struct (field anyref)))
 
    (func $wrap (export "wrap") (param anyref) (result (ref eq))
@@ -261,7 +272,7 @@
                      (array.get $block (local.get $a) (local.get $i))))
                (call $set (local.get $o)
                   (call $unwrap
-                     (call $caml_jsstring_of_bytes
+                     (call $caml_jsstring_of_string
                         (array.get $block (local.get $p) (i32.const 1))))
                   (call $unwrap
                         (array.get $block (local.get $p) (i32.const 2))))
@@ -450,13 +461,33 @@
                                  (local.get $acc)))))))))
       (return_call $unwrap (local.get $acc)))
 
-   (export "caml_js_from_string" (func $caml_jsstring_of_bytes))
-   (func $caml_jsstring_of_bytes (export "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 $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))
@@ -466,110 +497,50 @@
                (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)))))))
+))
+
+(@if use-js-string
+(@then
+   (func (export "caml_jsbytes_of_string")
+      (param (ref eq)) (result (ref eq))
+      (local.get 0))
+)
+(@else
+   (export "caml_jsbytes_of_string" (func $caml_jsbytes_of_bytes))
+))
 
-   (func $caml_jsbytes_of_bytes (export "caml_jsbytes_of_string")
+   (func $caml_jsbytes_of_bytes
       (param (ref eq)) (result (ref eq))
       (local $s (ref $bytes))
-      (local $s' (ref $bytes))
-      (local $l i32) (local $i i32) (local $n i32) (local $c i32)
       (local.set $s (ref.cast (ref $bytes) (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 $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
-               (struct.new $js
-                  (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 (struct.new $js (call $jsstring_of_bytes (local.get $s')))))
-
+      (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 $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 $bytes)) (local $s'' (ref $bytes))
-      (local.set $s'
-         (call $bytes_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 $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''))
+      (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))
@@ -645,7 +616,7 @@
                (call $meth_call
                   (local.get $exn)
                   (call $unwrap
-                     (call $caml_jsstring_of_bytes (global.get $toString)))
+                     (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")
@@ -684,5 +655,13 @@
 
    (func (export "caml_jsoo_flags_use_js_string")
       (param (ref eq)) (result (ref eq))
-      (ref.i31 (i32.const 0)))
+      (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 5f3c4c14e0..48a2e3f2a3 100644
--- a/runtime/wasm/jslib_js_of_ocaml.wat
+++ b/runtime/wasm/jslib_js_of_ocaml.wat
@@ -33,6 +33,7 @@
 
    (type $block (array (mut (ref eq))))
    (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
@@ -42,13 +43,13 @@
       (return_call $wrap
          (call $caml_js_html_entities_js (call $unwrap (local.get 0)))))
 
-   (@string $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)))
          (global.get $console)))
 
-   (@string $XMLHttpRequest "XMLHttpRequest")
+   (@jsstring $XMLHttpRequest "XMLHttpRequest")
 
    (func (export "caml_xmlhttprequest_create") (param (ref eq)) (result (ref eq))
       (return_call $caml_js_new
diff --git a/runtime/wasm/jsstring.wat b/runtime/wasm/jsstring.wat
index 4180f7649e..13fd87ff11 100644
--- a/runtime/wasm/jsstring.wat
+++ b/runtime/wasm/jsstring.wat
@@ -20,11 +20,16 @@
       (func $compare_strings (param externref externref) (result i32)))
    (import "wasm:js-string" "test"
       (func $is_string (param externref) (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 $bytes)) (param i32) (param i32)
@@ -43,8 +48,13 @@
       (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 $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $wstring (array (mut i16)))
 
    (global $text_converters_available (mut i32) (i32.const 0))
@@ -96,10 +106,12 @@
    (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))
 
    (func $jsstring_of_subbytes (export "jsstring_of_subbytes")
-      (export "jsstring_of_substring") ;; compatibility with zarith stubs
       (param $s (ref $bytes)) (param $pos i32) (param $len i32)
       (result anyref)
       (local $i i32) (local $c i32)
@@ -133,16 +145,160 @@
       (return_call $jsstring_of_subbytes_fallback
          (local.get $s) (local.get $pos) (local.get $len)))
 
-   (func (export "jsstring_of_bytes") (param $s (ref $bytes)) (result anyref)
+   (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 "bytes_of_jsstring") (param $s anyref) (result (ref $bytes))
+   (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
 
@@ -210,7 +366,7 @@
       (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 $bytes))
+   (func $bytes_of_jsstring_fallback (param $s anyref) (result (ref $bytes))
       (local $ofs i32) (local $len i32)
       (local $s' (ref $bytes)) (local $s'' (ref $bytes))
       (local $item (ref $stack))
@@ -257,4 +413,106 @@
       (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 5016d8a379..060b75d5bf 100644
--- a/runtime/wasm/lexing.wat
+++ b/runtime/wasm/lexing.wat
@@ -20,15 +20,38 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
-   (func $get (param $a (ref eq)) (param $i i32) (result i32)
-      (local $s (ref $bytes))
-      (local.set $s (ref.cast (ref $bytes) (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 $bytes (local.get $s) (local.get $i))
+         (i32.or (call $string_get (local.get $s) (local.get $i))
             (i32.shl
-               (array.get_u $bytes (local.get $s)
+               (call $string_get (local.get $s)
                   (i32.add (local.get $i) (i32.const 1)))
                (i32.const 8)))))
 
@@ -65,12 +88,23 @@
       (local $buffer (ref $bytes))
       (local $vpos (ref eq)) (local $action (ref eq))
       (local $pos i32) (local $base i32) (local $backtrk i32)
+(@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
@@ -91,22 +125,22 @@
          (else
             (local.set $state (i32.sub (i32.const -1) (local.get $state)))))
       (local.set $lex_base
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base))))
       (local.set $lex_backtrk
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk))))
       (local.set $lex_check
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check))))
       (local.set $lex_check_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check_code))))
       (local.set $lex_trans
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans))))
       (local.set $lex_default
-         (ref.cast (ref $bytes)
+         (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)))
@@ -182,7 +216,14 @@
          (br $loop)))
 
    (func $run_mem
-      (param $s (ref $bytes)) (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))
@@ -190,11 +231,11 @@
          (ref.cast (ref $block)
             (array.get $block (local.get $lexbuf) (global.get $lex_mem))))
       (loop $loop
-         (local.set $dst (array.get_u $bytes (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 $bytes (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)
@@ -208,7 +249,14 @@
          (br $loop)))
 
    (func $run_tag
-      (param $s (ref $bytes)) (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))))
 
@@ -224,6 +272,21 @@
       (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)
+(@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))
@@ -235,6 +298,7 @@
       (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
@@ -255,37 +319,37 @@
          (else
             (local.set $state (i32.sub (i32.const -1) (local.get $state)))))
       (local.set $lex_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_code))))
       (local.set $lex_base
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base))))
       (local.set $lex_base_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_base_code))))
       (local.set $lex_backtrk
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk))))
       (local.set $lex_backtrk_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_backtrk_code))))
       (local.set $lex_check
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check))))
       (local.set $lex_check_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_check_code))))
       (local.set $lex_trans
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans))))
       (local.set $lex_trans_code
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_trans_code))))
       (local.set $lex_default
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tbl) (global.get $lex_default))))
       (local.set $lex_default_code
-         (ref.cast (ref $bytes)
+         (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)))
diff --git a/runtime/wasm/marshal.wat b/runtime/wasm/marshal.wat
index 4aa53e5936..0fe44401f8 100644
--- a/runtime/wasm/marshal.wat
+++ b/runtime/wasm/marshal.wat
@@ -48,11 +48,34 @@
       (func $caml_init_custom_operations))
    (import "custom" "caml_find_custom_operations"
       (func $caml_find_custom_operations
-         (param (ref $bytes)) (result (ref null $custom_operations))))
+         (param (ref eq)) (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))))
 
    (@string $input_val_from_string "input_value_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 $bytes))
@@ -132,6 +155,7 @@
 
    (type $block (array (mut (ref eq))))
    (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)))
@@ -150,7 +174,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
@@ -427,8 +457,8 @@
             (br_on_null $unknown
                (call
                   $caml_find_custom_operations
-                  (call $readstr
-                     (local.get $s)))))
+                  (call $caml_string_of_bytes
+                     (call $readstr (local.get $s))))))
          (block $no_length
             (if (i32.eq (local.get $code) (global.get $CODE_CUSTOM_FIXED))
                (then
@@ -664,7 +694,7 @@
           ;; read_string
           (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
@@ -995,6 +1025,19 @@
          (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 $bytes)) (local $d i64) (local $i i32)
@@ -1121,7 +1164,7 @@
                (i32.or (local.get $tag)
                   (i32.shl (local.get $sz) (i32.const 10)))))))
 
-   (func $extern_string (param $s (ref $extern_state)) (param $v (ref $bytes))
+   (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))
@@ -1137,6 +1180,22 @@
             (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)))
@@ -1163,6 +1222,10 @@
       (local $fixed_length (ref $fixed_length))
       (local $pos i32) (local $buf (ref $bytes))
       (local $r (tuple i32 i32))
+(@if use-js-string
+(@then
+      (local $id anyref)
+))
       (local.set $ops (struct.get $custom 0 (local.get $v)))
       (block $abstract
          (local.set $serialize
@@ -1174,8 +1237,18 @@
                   (struct.get $custom_operations $fixed_length
                      (local.get $ops))))
             (call $write (local.get $s) (global.get $CODE_CUSTOM_FIXED))
+(@if use-js-string
+(@then
+            (local.set $id
+               (struct.get $string 0
+                  (struct.get $custom_operations $id (local.get $ops))))
+            (call $writestring (local.get $s) (local.get $id)
+               (call $jsstring_length (local.get $id)))
+)
+(@else
             (call $writeblock (local.get $s)
                (struct.get $custom_operations $id (local.get $ops)))
+))
             (call $write (local.get $s) (i32.const 0))
             (local.set $r
                (call_ref $serialize
@@ -1196,8 +1269,18 @@
             (return (local.get $r)))
          ;; variable length
          (call $write (local.get $s) (global.get $CODE_CUSTOM_LEN))
+(@if use-js-string
+(@then
+            (local.set $id
+               (struct.get $string 0
+                  (struct.get $custom_operations $id (local.get $ops))))
+            (call $writestring (local.get $s) (local.get $id)
+               (call $jsstring_length (local.get $id)))
+)
+(@else
          (call $writeblock (local.get $s)
             (struct.get $custom_operations $id (local.get $ops)))
+))
          (call $write (local.get $s) (i32.const 0))
          (local.set $pos
             (call $reserve_extern_output (local.get $s) (i32.const 12)))
@@ -1227,6 +1310,7 @@
       (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))
@@ -1286,7 +1370,7 @@
                (local.set $str
                   (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)
@@ -1328,9 +1412,25 @@
                (then (call $caml_invalid_argument (global.get $func_value))))
             (if (call $caml_is_continuation (local.get $v))
                (then (call $caml_invalid_argument (global.get $cont_value))))
-            (if (ref.test (ref $js) (local.get $v))
-               (then (call $caml_invalid_argument (global.get $js_value))))
-            (call $caml_invalid_argument (global.get $abstract_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
             (local.set $item (br_on_null $done (local.get $sp)))
@@ -1398,8 +1498,18 @@
          (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")
-      (export "caml_output_value_to_bytes")
+      (param $v (ref eq)) (param $flags (ref eq)) (result (ref eq))
+      (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)
diff --git a/runtime/wasm/md5.wat b/runtime/wasm/md5.wat
index 671de14964..6969ecd80c 100644
--- a/runtime/wasm/md5.wat
+++ b/runtime/wasm/md5.wat
@@ -21,6 +21,10 @@
          (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 $bytes (array (mut i8)))
    (type $int_array (array (mut i32)))
@@ -32,7 +36,19 @@
          (field (ref $int_array)) ;; 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))
@@ -484,7 +500,7 @@
                (local.get $input) (local.get $input_pos)
                (local.get $input_len)))))
 
-   (func $MD5Final (param $ctx (ref $context)) (result (ref $bytes))
+   (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 $bytes)) (local $res (ref $bytes))
@@ -547,5 +563,5 @@
                (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 5e06a4a5ed..535a0f9fa6 100644
--- a/runtime/wasm/obj.wat
+++ b/runtime/wasm/obj.wat
@@ -29,8 +29,13 @@
       (func $caml_cps_trampoline (param (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 $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))))
@@ -239,6 +244,8 @@
                (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")
@@ -282,6 +289,13 @@
          (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")
diff --git a/runtime/wasm/parsing.wat b/runtime/wasm/parsing.wat
index 686f411145..47670d534a 100644
--- a/runtime/wasm/parsing.wat
+++ b/runtime/wasm/parsing.wat
@@ -29,19 +29,64 @@
    (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 $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
-   (func $get (param $a (ref eq)) (param $i i32) (result i32)
-      (local $s (ref $bytes))
-      (local.set $s (ref.cast (ref $bytes) (local.get $a)))
+(@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
+(@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 $bytes (local.get $s) (local.get $i))
+         (i32.or (call $string_get (local.get $s) (local.get $i))
             (i32.shl
-               (array.get_u $bytes (local.get $s)
+               (call $string_get (local.get $s)
                   (i32.add (local.get $i) (i32.const 1)))
                (i32.const 8)))))
 
@@ -100,11 +145,19 @@
    (global $tbl_names_const i32 (i32.const 15))
    (global $tbl_names_block i32 (i32.const 16))
 
-   (func $strlen (param $s (ref $bytes)) (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 $bytes (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)))
@@ -115,11 +168,17 @@
 
    (func $token_name
       (param $vnames (ref eq)) (param $number i32) (result (ref eq))
-      (local $names (ref $bytes)) (local $i i32) (local $len i32)
-      (local $name (ref $bytes))
-      (local.set $names (ref.cast (ref $bytes) (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 $bytes (local.get $names) (local.get $i)))
+         (if (i32.eqz (call $string_get (local.get $names) (local.get $i)))
             (then (return (global.get $unknown_token))))
          (if (i32.ne (local.get $number) (i32.const 0))
             (then
@@ -130,19 +189,14 @@
                (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 $bytes (i32.const 0) (local.get $len)))
-      (array.copy $bytes $bytes
-         (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 $bytes))
-      (local.set $s (ref.cast (ref $bytes) (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 (array.len (local.get $s))))))
+            (ref.i31 (call $caml_string_length (local.get $s))))))
 
    (func $output_nl
       (drop
@@ -151,6 +205,8 @@
             (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 (@string "%d") (ref.i31 (local.get 0)))))
@@ -218,6 +274,19 @@
       (local $errflag i32)
       (local $tables (ref $block)) (local $env (ref $block)) (local $cmd i32)
       (local $arg (ref $block))
+(@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))
@@ -227,33 +296,34 @@
       (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 $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_defred))))
       (local.set $tbl_sindex
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_sindex))))
       (local.set $tbl_check
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_check))))
       (local.set $tbl_rindex
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_rindex))))
       (local.set $tbl_table
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_table))))
       (local.set $tbl_len
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_len))))
       (local.set $tbl_lhs
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_lhs))))
       (local.set $tbl_gindex
-         (ref.cast (ref $bytes)
+         (call $string_val
             (array.get $block (local.get $tables) (global.get $tbl_gindex))))
       (local.set $tbl_dgoto
-         (ref.cast (ref $bytes)
+         (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))))
@@ -454,8 +524,7 @@
                                          (br $next)))))))
                        (if (global.get $caml_parser_trace)
                           (then
-                             (call $output
-                                (global.get $discarding_state))
+                             (call $output (global.get $discarding_state))
                              (call $output_int (local.get $state1))
                              (call $output_nl)))
                        (if (i32.le_s (local.get $sp)
diff --git a/runtime/wasm/printexc.wat b/runtime/wasm/printexc.wat
index c39df76412..839aea26f8 100644
--- a/runtime/wasm/printexc.wat
+++ b/runtime/wasm/printexc.wat
@@ -21,8 +21,19 @@
    (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 (struct (field anyref)))
    (type $bytes (array (mut i8)))
 
    (type $buffer
@@ -41,26 +52,41 @@
             (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 $bytes))
-      (local $s (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 $bytes) (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 $bytes $bytes
-         (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))
@@ -119,7 +145,7 @@
                               (call $caml_format_int
                                   (@string "%d")
                                   (ref.cast (ref i31) (local.get $v)))))
-                    (else (if (ref.test (ref $bytes) (local.get $v))
+                    (else (if (call $is_string (local.get $v))
                        (then
                           (call $add_char (local.get $buf) (@char "\""))
                           (call $add_string (local.get $buf) (local.get $v))
@@ -140,7 +166,7 @@
                (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 742122d572..cc92ac7199 100644
--- a/runtime/wasm/runtime.js
+++ b/runtime/wasm/runtime.js
@@ -141,7 +141,27 @@
     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;
   }
 
@@ -280,6 +300,17 @@
     dv_set_i16: call.bind(DV.setInt16),
     dv_set_i8: call.bind(DV.setInt8),
     littleEndian: new Uint8Array(new Uint32Array([1]).buffer)[0],
+    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) {
@@ -553,10 +584,15 @@
   };
   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),
     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(
     {
diff --git a/runtime/wasm/stdlib.wat b/runtime/wasm/stdlib.wat
index 62ff000f26..2afbc215d1 100644
--- a/runtime/wasm/stdlib.wat
+++ b/runtime/wasm/stdlib.wat
@@ -35,6 +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))
@@ -45,10 +47,11 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
 
    (type $assoc
       (struct
-         (field (ref $bytes))
+         (field (ref eq))
          (field (mut (ref eq)))
          (field (mut (ref null $assoc)))))
 
@@ -115,9 +118,7 @@
          (return (ref.i31 (i32.const 0))))
       (array.set $assoc_array
          (global.get $named_value_table) (local.get $h)
-         (struct.new $assoc
-            (ref.cast (ref $bytes) (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
diff --git a/runtime/wasm/str.wat b/runtime/wasm/str.wat
index e1cbe8ab8c..44cd7bb87b 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))))
+
+(@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 $bytes)) (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 $bytes (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 $bytes)) (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 $bytes)) (local $set (ref $bytes))
+(@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 $bytes))
       (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 $bytes)
-            (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 $bytes
+                                  (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 $bytes
+                                 (call $string_get
                                     (local.get $normtable)
-                                    (array.get_u $bytes
+                                    (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 $bytes)
+                             (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 $bytes (local.get $s')
+                                         (call $string_get (local.get $s')
                                             (local.get $i))
-                                         (array.get_u $bytes (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 $bytes)
+                            (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 $bytes (local.get $s')
+                                        (call $string_get (local.get $s')
                                            (local.get $i))
-                                        (array.get_u $bytes
+                                        (call $string_get
                                            (local.get $normtable)
-                                           (array.get_u $bytes (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 $bytes)
+                                 (call $string_val
                                     (array.get $block (local.get $cpool)
                                        (i32.add (local.get $arg)
                                           (i32.const 1))))
-                                 (array.get_u $bytes (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 (@char "\n")
-                             (array.get_u $bytes (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 (@char "\n")
-                            (array.get_u $bytes (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 $bytes (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 $bytes (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 $bytes (local.get $s)
+                                          (call $string_get (local.get $s)
                                              (i32.sub (local.get $pos)
                                                 (i32.const 1))))
                                        (call $is_word_letter
-                                          (array.get_u $bytes (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 $bytes (local.get $s)
+                                 (call $string_get (local.get $s)
                                     (local.get $i))
-                                 (array.get_u $bytes (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 $bytes)
+                              (call $string_val
                                  (array.get $block (local.get $cpool)
-                                    (i32.add (local.get $arg) (i32.const 1))))
-                              (array.get_u $bytes (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 $bytes)
+                   (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 $bytes (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 $bytes)
+                  (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 $bytes (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 $bytes (local.get $s)
+                              (call $string_get (local.get $s)
                                  (local.get $pos)))))))
                (br $continue))
               ;; GOTO
@@ -513,12 +566,18 @@
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
+(@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 $bytes) (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 (global.get $search_forward))))
       (loop $loop
@@ -538,15 +597,20 @@
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
+(@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 $bytes) (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 (global.get $search_backward))))
+         (then (call $caml_invalid_argument (global.get $search_backward))))
       (loop $loop
          (local.set $res
             (call $re_match
@@ -564,12 +628,18 @@
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
+(@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 $bytes) (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 (global.get $string_match))))
       (local.set $res
@@ -586,15 +656,20 @@
       (param $re (ref eq)) (param $vs (ref eq)) (param $vpos (ref eq))
       (result (ref eq))
       ;; ZZZ startchars
+(@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 $bytes) (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 (global.get $string_partial_match))))
+         (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)))
@@ -607,23 +682,27 @@
    (@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))
+(@if use-js-string
+(@then
+      (local $repl externref)
+)
+(@else
       (local $repl (ref $bytes))
+))
       (local $groups (ref $block))
-      (local $orig (ref $bytes))
       (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 $bytes) (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 $bytes) (local.get $vorig)))
       (loop $loop
          (if (i32.lt_u (local.get $i) (local.get $l))
             (then
                (local.set $c
-                  (array.get_u $bytes (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) (@char "\\"))
                   (then
@@ -632,7 +711,7 @@
                (if (i32.eq (local.get $i) (local.get $l))
                   (then (call $caml_failwith (global.get $illegal_backslash))))
                (local.set $c
-                  (array.get_u $bytes (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) (@char "\\"))
                   (then
@@ -669,7 +748,7 @@
          (if (i32.lt_u (local.get $i) (local.get $l))
             (then
                (local.set $c
-                  (array.get_u $bytes (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) (@char "\\"))
                   (then
@@ -678,7 +757,7 @@
                      (local.set $j (i32.add (local.get $j) (i32.const 1)))
                      (br $loop)))
                (local.set $c
-                  (array.get_u $bytes (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) (@char "\\"))
                   (then
@@ -711,11 +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 $bytes $bytes
-                  (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 66183061b4..413b76b77e 100644
--- a/runtime/wasm/string.wat
+++ b/runtime/wasm/string.wat
@@ -19,11 +19,115 @@
    (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 $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))
+      (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)
@@ -45,14 +149,13 @@
                (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 $bytes)) (local $s2 (ref $bytes))
       (local $l1 i32) (local $l2 i32) (local $len i32) (local $i i32)
@@ -85,39 +188,61 @@
          (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))
+))
 
    (@string $Bytes_create "Bytes.create")
 
@@ -129,8 +254,39 @@
          (then (call $caml_invalid_argument (global.get $Bytes_create))))
       (array.new $bytes (i32.const 0) (local.get $l)))
 
-   (export "caml_blit_bytes" (func $caml_blit_string))
-   (func $caml_blit_string (export "caml_blit_string")
+(@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))
@@ -152,7 +308,111 @@
          (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 $bytes)) (local $p i32)
@@ -169,7 +429,6 @@
                               (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 $bytes)) (local $p i32)
@@ -194,7 +453,6 @@
                         (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 $bytes)) (local $p i32)
@@ -317,6 +575,22 @@
          (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 $bytes)) (local $s2 (ref $bytes))
@@ -336,4 +610,19 @@
          (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 1b498d4a93..21042d6173 100644
--- a/runtime/wasm/sync.wat
+++ b/runtime/wasm/sync.wat
@@ -25,6 +25,7 @@
    (import "custom" "custom_next_id" (func $custom_next_id (result i64)))
 
    (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 +37,13 @@
    (type $dup (func (param (ref eq)) (result (ref eq))))
    (type $custom_operations
       (struct
+(@if use-js-string
+(@then
+         (field $id (ref $string))
+)
+(@else
          (field $id (ref $bytes))
+))
          (field $compare (ref null $compare))
          (field $compare_ext (ref null $compare))
          (field $hash (ref null $hash))
diff --git a/runtime/wasm/sys.wat b/runtime/wasm/sys.wat
index 7a2a582499..05be31a031 100644
--- a/runtime/wasm/sys.wat
+++ b/runtime/wasm/sys.wat
@@ -56,6 +56,7 @@
 
    (type $block (array (mut (ref eq))))
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $float (struct (field f64)))
 
    (tag $ocaml_exit (export "ocaml_exit"))
diff --git a/runtime/wasm/unix.wat b/runtime/wasm/unix.wat
index 0b4a9229ae..561a8c013c 100644
--- a/runtime/wasm/unix.wat
+++ b/runtime/wasm/unix.wat
@@ -118,6 +118,7 @@
       (func $Int64_val (param (ref eq)) (result i64)))
 
    (type $bytes (array (mut i8)))
+   (type $string (struct (field anyref)))
    (type $block (array (mut (ref eq))))
    (type $float (struct (field f64)))
    (type $float_array (array (mut f64)))
diff --git a/runtime/wasm/weak.wat b/runtime/wasm/weak.wat
index 1f704b8071..12fe4f0b91 100644
--- a/runtime/wasm/weak.wat
+++ b/runtime/wasm/weak.wat
@@ -34,6 +34,7 @@
    (import "jslib" "wrap" (func $wrap (param anyref) (result (ref eq))))
    (type $block (array (mut (ref eq))))
    (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
diff --git a/tools/ci_setup.ml b/tools/ci_setup.ml
index cb91554ba4..671f44ceee 100644
--- a/tools/ci_setup.ml
+++ b/tools/ci_setup.ml
@@ -37,6 +37,7 @@ let forked_packages =
     ; "bonsai_web_test"
     ; "core"
     ; "core_kernel"
+    ; "core_unix"
     ; "ocaml_intrinsics_kernel"
     ; "ppx_expect"
     ; "ppx_inline_test"
@@ -54,6 +55,9 @@ 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 -alert -all -warn-error -7-8-27-30-32-34-37-49-52-55 -w -7-27-30-32-34-37-49-52-55-58-67-69)))
 |}
 
@@ -293,12 +297,28 @@ let sync_exec f l =
   let l = List.map f l in
   List.iter (fun f -> f ()) l
 
+let branch nm =
+  if is_forked nm then
+    match nm with
+    | "async_js"
+    | "base"
+    | "core"
+    | "core_kernel"
+    | "core_unix"
+    | "time_now"
+    | "zarith_stubs_js" -> Some "js-strings"
+    | _ -> Some "wasm-v0.18"
+  else
+    None
+
 let pin nm =
+  let branch = Option.value ~default:"wasm-v0.18" (branch nm) in
   exec_async
     (Printf.sprintf
-       "opam pin add -n %s https://github.com/ocaml-wasm/%s.git#wasm-v0.18"
+       "opam pin add -n %s https://github.com/ocaml-wasm/%s.git#%s"
        nm
-       nm)
+       nm
+       branch)
 
 let pin_packages () = sync_exec pin (StringSet.elements do_pin)
 
@@ -354,7 +374,7 @@ let () =
   sync_exec (fun () -> exec_async "opam install uri --deps-only") [ () ];
   sync_exec
     (fun nm ->
-      let branch = if is_forked nm then Some "wasm-v0.18" else None in
+      let branch = branch nm in
       let commit =
         if is_forked nm
         then None