diff --git a/Makefile b/Makefile
index 2f60cdc..8411bfc 100644
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,7 @@ help: ## Print this help message
 
 .PHONY: create-switch
 create-switch: ## Create opam switch
-	opam switch create . 4.14.1 -y --deps-only
+	opam switch create . 5.1.0~rc3 -y --deps-only
 
 .PHONY: init
 init: create-switch install ## Configure everything to develop this repository in local
@@ -23,8 +23,7 @@ install: ## Install development dependencies
 	yarn
 	opam update
 	opam install -y . --deps-only --with-test
-	# used to build the tests
-	opam pin add melange-jest.dev -y git+https://github.com/melange-community/melange-jest.git#main
+	opam-check-npm-deps
 
 .PHONY: build
 build: ## Build the project
diff --git a/examples/decode.ml b/examples/decode.ml
index 12b5087..307b9af 100644
--- a/examples/decode.ml
+++ b/examples/decode.ml
@@ -1,29 +1,28 @@
 (* Decoding a fixed JSON data structure using Json.Decode *)
-let mapJsonObjectString f decoder (encoder: int -> Js.Json.t) str =
+let mapJsonObjectString f decoder (encoder : int -> Js.Json.t) str =
   let json = Json.parseOrRaise str in
   Json.Decode.(dict decoder json)
-    |> Js.Dict.map ((fun v -> f v) [@bs])
-    |> Json.Encode.dict encoder
-    |> Json.stringify
+  |> Js.Dict.map (fun [@u] v -> f v)
+  |> Json.Encode.dict encoder |> Json.stringify
 
-let sum =
-  Array.fold_left (+) 0
+let sum = Array.fold_left ( + ) 0
 
 (* prints `{ "foo": 6, "bar": 24 }` *)
 let _ =
-  Js.log @@
-    (mapJsonObjectString sum Json.Decode.(array int) Json.Encode.int {|
+  Js.log
+  @@ mapJsonObjectString sum
+       Json.Decode.(array int)
+       Json.Encode.int
+       {|
       {
         "foo": [1, 2, 3],
         "bar": [9, 8, 7]
       }
-    |})
+    |}
 
 (* Error handling *)
 let _ =
   let json = {|{ "y": 42 } |} |> Json.parseOrRaise in
   match Json.Decode.(field "x" int json) with
-  | x ->
-    Js.log x
-  | exception Json.Decode.DecodeError msg ->
-    Js.log ("Error:" ^ msg)
\ No newline at end of file
+  | x -> Js.log x
+  | exception Json.Decode.DecodeError msg -> Js.log ("Error:" ^ msg)
diff --git a/melange-json.opam b/melange-json.opam
index eeaba36..0cb8e2e 100644
--- a/melange-json.opam
+++ b/melange-json.opam
@@ -7,16 +7,17 @@ maintainer: [
 authors: [
   "glennsl"
 ]
-license: "(LGPL-3.0 OR MPL-2.0)"
+license: ["LGPL-3.0-only" "MPL-2.0"]
 homepage: "https://github.com/melange-community/melange-json/"
 doc: "https://github.com/melange-community/melange-json/"
 bug-reports: "https://github.com/melange-community/melange-json/issues"
 depends: [
   "dune" {>= "3.8"}
-  "ocaml" {>= "4.14.0"}
-  "melange" {>= "1.0.0"}
+  "ocaml"
+  "melange" {>= "2.0.0"}
   "melange-jest" {with-test}
   "reason" {with-test}
+  "opam-check-npm-deps" {with-test} # todo: use with-dev-setup once opam 2.2 is out
   "ocaml-lsp-server" {with-test}
   "odoc" {with-doc}
 ]
@@ -35,3 +36,10 @@ build: [
   ]
 ]
 dev-repo: "git+https://github.com/melange-community/melange-json.git"
+pin-depends: [
+  [ "melange.2.0.0"         "git+https://github.com/melange-re/melange.git#e114ad55d185badeb32b3c766c9ab547495eac1b" ]
+  [ "reason.3.10.0"         "git+https://github.com/reasonml/reason.git#972261dab3b651ff8ab9b8b9fcc32940595073dc" ]
+  [ "melange-jest.dev"      "git+https://github.com/melange-community/melange-jest.git#acb6ef50beef3c486805d616b90aa7b56b51172d" ]
+  [ "melange-webapi.dev"    "git+https://github.com/melange-community/melange-webapi.git#96dc1e2a867624d18050fad25cf1c71af7a098e1" ]
+  [ "melange-fetch.dev"     "git+https://github.com/melange-community/melange-fetch.git#796f941b6b85eb7e6182ac6e4f40708bfde7a9a9" ]
+]
diff --git a/src/Json.ml b/src/Json.ml
index f2f8ccd..c1dea7c 100644
--- a/src/Json.ml
+++ b/src/Json.ml
@@ -3,17 +3,14 @@ module Encode = Json_encode
 
 exception ParseError of string
 
-let parse s =
-  try Some (Js.Json.parseExn s) with
-  | _ -> None
+let parse s = try Some (Js.Json.parseExn s) with _ -> None
 
 let parseOrRaise s =
-  try Js.Json.parseExn s with
-  | Js.Exn.Error e ->
+  try Js.Json.parseExn s
+  with Js.Exn.Error e ->
     let message =
-      match Js.Exn.message e with
-      | Some m  -> m
-      | None    -> "Unknown error"
-    in raise @@ ParseError message
+      match Js.Exn.message e with Some m -> m | None -> "Unknown error"
+    in
+    raise @@ ParseError message
 
-external stringify : Js.Json.t -> string = "JSON.stringify" [@@bs.val]
\ No newline at end of file
+external stringify : Js.Json.t -> string = "JSON.stringify"
diff --git a/src/Json_decode.ml b/src/Json_decode.ml
index 5168254..d6718c3 100644
--- a/src/Json_decode.ml
+++ b/src/Json_decode.ml
@@ -1,6 +1,5 @@
-external _unsafeCreateUninitializedArray : int -> 'a array = "Array" [@@bs.new]
-
-external _stringify : Js.Json.t -> string = "JSON.stringify" [@@bs.val]
+external _unsafeCreateUninitializedArray : int -> 'a array = "Array" [@@mel.new]
+external _stringify : Js.Json.t -> string = "JSON.stringify"
 
 let _isInteger value =
   Js.Float.isFinite value && Js.Math.floor_float value == value
@@ -11,207 +10,175 @@ exception DecodeError of string
 
 let id json = json
 
-let bool json = 
-  if Js.typeof json = "boolean" then
-    (Obj.magic (json : Js.Json.t) : bool)
-  else
-    raise @@ DecodeError ("Expected boolean, got " ^ _stringify json)
+let bool json =
+  if Js.typeof json = "boolean" then (Obj.magic (json : Js.Json.t) : bool)
+  else raise @@ DecodeError ("Expected boolean, got " ^ _stringify json)
 
-let float json = 
-  if Js.typeof json = "number" then
-    (Obj.magic (json : Js.Json.t) : float)
-  else
-    raise @@ DecodeError ("Expected number, got " ^ _stringify json)
+let float json =
+  if Js.typeof json = "number" then (Obj.magic (json : Js.Json.t) : float)
+  else raise @@ DecodeError ("Expected number, got " ^ _stringify json)
 
-let int json = 
+let int json =
   let f = float json in
-  if _isInteger f then
-    (Obj.magic (f : float) : int)
-  else
-    raise @@ DecodeError ("Expected integer, got " ^ _stringify json)
+  if _isInteger f then (Obj.magic (f : float) : int)
+  else raise @@ DecodeError ("Expected integer, got " ^ _stringify json)
 
-let string json = 
-  if Js.typeof json = "string" then
-    (Obj.magic (json : Js.Json.t) : string)
-  else
-    raise @@ DecodeError ("Expected string, got " ^ _stringify json)
+let string json =
+  if Js.typeof json = "string" then (Obj.magic (json : Js.Json.t) : string)
+  else raise @@ DecodeError ("Expected string, got " ^ _stringify json)
 
 let char json =
   let s = string json in
-  if String.length s = 1 then
-    String.get s 0
+  if String.length s = 1 then String.get s 0
   else
-    raise @@ DecodeError ("Expected single-character string, got " ^ _stringify json)
+    raise
+    @@ DecodeError ("Expected single-character string, got " ^ _stringify json)
 
-let date json =
-  json |> string
-       |> Js.Date.fromString
+let date json = json |> string |> Js.Date.fromString
 
 let nullable decode json =
-  if (Obj.magic json : 'a Js.null) == Js.null then
-    Js.null
-  else
-    Js.Null.return (decode json)
+  if (Obj.magic json : 'a Js.null) == Js.null then Js.null
+  else Js.Null.return (decode json)
 
 (* TODO: remove this? *)
-let nullAs value json = 
-  if (Obj.magic json : 'a Js.null) == Js.null then
-    value
-  else 
-    raise @@ DecodeError ("Expected null, got " ^ _stringify json)
-
-let array decode json = 
-  if Js.Array.isArray json then begin
+let nullAs value json =
+  if (Obj.magic json : 'a Js.null) == Js.null then value
+  else raise @@ DecodeError ("Expected null, got " ^ _stringify json)
+
+let array decode json =
+  if Js.Array.isArray json then (
     let source = (Obj.magic (json : Js.Json.t) : Js.Json.t array) in
     let length = Js.Array.length source in
     let target = _unsafeCreateUninitializedArray length in
     for i = 0 to length - 1 do
-      let value = 
-        try
-          decode (Array.unsafe_get source i)
-        with
-          DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin array at index " ^ string_of_int i)
-        in
-      Array.unsafe_set target i value;
+      let value =
+        try decode (Array.unsafe_get source i)
+        with DecodeError msg ->
+          raise @@ DecodeError (msg ^ "\n\tin array at index " ^ string_of_int i)
+      in
+      Array.unsafe_set target i value
     done;
-    target
-  end
-  else
-    raise @@ DecodeError ("Expected array, got " ^ _stringify json)
+    target)
+  else raise @@ DecodeError ("Expected array, got " ^ _stringify json)
 
-let list decode json =
-  json |> array decode |> Array.to_list
+let list decode json = json |> array decode |> Array.to_list
 
 let pair decodeA decodeB json =
-  if Js.Array.isArray json then begin
+  if Js.Array.isArray json then
     let source = (Obj.magic (json : Js.Json.t) : Js.Json.t array) in
     let length = Js.Array.length source in
     if length = 2 then
       try
-        decodeA (Array.unsafe_get source 0),
-        decodeB (Array.unsafe_get source 1)
-      with
-        DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin pair/tuple2")
+        ( decodeA (Array.unsafe_get source 0),
+          decodeB (Array.unsafe_get source 1) )
+      with DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin pair/tuple2")
     else
-      raise @@ DecodeError ({j|Expected array of length 2, got array of length $length|j})
-  end
-  else
-    raise @@ DecodeError ("Expected array, got " ^ _stringify json)
+      raise
+      @@ DecodeError
+           {j|Expected array of length 2, got array of length $length|j}
+  else raise @@ DecodeError ("Expected array, got " ^ _stringify json)
 
 let tuple2 = pair
 
 let tuple3 decodeA decodeB decodeC json =
-  if Js.Array.isArray json then begin
+  if Js.Array.isArray json then
     let source = (Obj.magic (json : Js.Json.t) : Js.Json.t array) in
     let length = Js.Array.length source in
     if length = 3 then
       try
-        decodeA (Array.unsafe_get source 0),
-        decodeB (Array.unsafe_get source 1),
-        decodeC (Array.unsafe_get source 2)
-      with
-        DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin tuple3")
+        ( decodeA (Array.unsafe_get source 0),
+          decodeB (Array.unsafe_get source 1),
+          decodeC (Array.unsafe_get source 2) )
+      with DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin tuple3")
     else
-      raise @@ DecodeError ({j|Expected array of length 3, got array of length $length|j})
-  end
-  else
-    raise @@ DecodeError ("Expected array, got " ^ _stringify json)
+      raise
+      @@ DecodeError
+           {j|Expected array of length 3, got array of length $length|j}
+  else raise @@ DecodeError ("Expected array, got " ^ _stringify json)
 
 let tuple4 decodeA decodeB decodeC decodeD json =
-  if Js.Array.isArray json then begin
+  if Js.Array.isArray json then
     let source = (Obj.magic (json : Js.Json.t) : Js.Json.t array) in
     let length = Js.Array.length source in
     if length = 4 then
       try
-        decodeA (Array.unsafe_get source 0),
-        decodeB (Array.unsafe_get source 1),
-        decodeC (Array.unsafe_get source 2),
-        decodeD (Array.unsafe_get source 3)
-      with
-        DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin tuple4")
+        ( decodeA (Array.unsafe_get source 0),
+          decodeB (Array.unsafe_get source 1),
+          decodeC (Array.unsafe_get source 2),
+          decodeD (Array.unsafe_get source 3) )
+      with DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin tuple4")
     else
-      raise @@ DecodeError ({j|Expected array of length 4, got array of length $length|j})
-  end
-  else
-    raise @@ DecodeError ("Expected array, got " ^ _stringify json)
-
-let dict decode json = 
-  if Js.typeof json = "object" && 
-      not (Js.Array.isArray json) && 
-      not ((Obj.magic json : 'a Js.null) == Js.null)
-  then begin
+      raise
+      @@ DecodeError
+           {j|Expected array of length 4, got array of length $length|j}
+  else raise @@ DecodeError ("Expected array, got " ^ _stringify json)
+
+let dict decode json =
+  if
+    Js.typeof json = "object"
+    && (not (Js.Array.isArray json))
+    && not ((Obj.magic json : 'a Js.null) == Js.null)
+  then (
     let source = (Obj.magic (json : Js.Json.t) : Js.Json.t Js.Dict.t) in
     let keys = Js.Dict.keys source in
     let l = Js.Array.length keys in
     let target = Js.Dict.empty () in
     for i = 0 to l - 1 do
-        let key = (Array.unsafe_get keys i) in
-        let value =
-          try
-            decode (Js.Dict.unsafeGet source key)
-          with
-            DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin dict")
-          in
-        Js.Dict.set target key value;
+      let key = Array.unsafe_get keys i in
+      let value =
+        try decode (Js.Dict.unsafeGet source key)
+        with DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tin dict")
+      in
+      Js.Dict.set target key value
     done;
-    target
-  end
-  else
-    raise @@ DecodeError ("Expected object, got " ^ _stringify json)
+    target)
+  else raise @@ DecodeError ("Expected object, got " ^ _stringify json)
 
 let field key decode json =
-  if 
-    Js.typeof json = "object" && 
-    not (Js.Array.isArray json) && 
-    not ((Obj.magic json : 'a Js.null) == Js.null)
-  then begin
-    let dict =
-      (Obj.magic (json : Js.Json.t) : Js.Json.t Js.Dict.t) in
+  if
+    Js.typeof json = "object"
+    && (not (Js.Array.isArray json))
+    && not ((Obj.magic json : 'a Js.null) == Js.null)
+  then
+    let dict = (Obj.magic (json : Js.Json.t) : Js.Json.t Js.Dict.t) in
     match Js.Dict.get dict key with
-    | Some value -> begin
-      try
-        decode value
-      with
-        DecodeError msg -> raise @@ DecodeError (msg ^ "\n\tat field '" ^ key ^ "'")
-      end
-    | None ->
-      raise @@ DecodeError ({j|Expected field '$(key)'|j})
-  end
-  else
-    raise @@ DecodeError ("Expected object, got " ^ _stringify json)
+    | Some value -> (
+        try decode value
+        with DecodeError msg ->
+          raise @@ DecodeError (msg ^ "\n\tat field '" ^ key ^ "'"))
+    | None -> raise @@ DecodeError {j|Expected field '$(key)'|j}
+  else raise @@ DecodeError ("Expected object, got " ^ _stringify json)
 
 let rec at key_path decoder =
-    match key_path with 
-      | [key] -> field key decoder
-      | first::rest -> field first (at rest decoder) 
-      | [] -> raise @@ Invalid_argument ("Expected key_path to contain at least one element")
+  match key_path with
+  | [ key ] -> field key decoder
+  | first :: rest -> field first (at rest decoder)
+  | [] ->
+      raise
+      @@ Invalid_argument "Expected key_path to contain at least one element"
 
-let optional decode json =
-  try Some (decode json) with
-  | DecodeError _ -> None
+let optional decode json = try Some (decode json) with DecodeError _ -> None
 
 let oneOf decoders json =
   let rec inner decoders errors =
     match decoders with
     | [] ->
-        let formattedErrors = "\n- " ^ Js.Array.joinWith "\n- " (Array.of_list (List.rev errors)) in
-        raise @@ DecodeError
-              ({j|All decoders given to oneOf failed. Here are all the errors: $formattedErrors\nAnd the JSON being decoded: |j} ^ _stringify json)
-    | decode::rest ->
-        try decode json with
-        | DecodeError e ->
-             inner rest (e :: errors) in
+        let formattedErrors =
+          "\n- " ^ Js.Array.joinWith "\n- " (Array.of_list (List.rev errors))
+        in
+        raise
+        @@ DecodeError
+             ({j|All decoders given to oneOf failed. Here are all the errors: $formattedErrors\nAnd the JSON being decoded: |j}
+            ^ _stringify json)
+    | decode :: rest -> (
+        try decode json with DecodeError e -> inner rest (e :: errors))
+  in
   inner decoders []
 
-let either a b =
-  oneOf [a;b]
+let either a b = oneOf [ a; b ]
 
 let withDefault default decode json =
-  try decode json with
-  | DecodeError _ -> default
-
-let map f decode json =
-  f (decode json)
+  try decode json with DecodeError _ -> default
 
-let andThen b a json=
-  b (a json) json
+let map f decode json = f (decode json)
+let andThen b a json = b (a json) json
diff --git a/src/Json_encode.ml b/src/Json_encode.ml
index 0d19b64..a91c3ff 100644
--- a/src/Json_encode.ml
+++ b/src/Json_encode.ml
@@ -1,61 +1,49 @@
 type 'a encoder = 'a -> Js.Json.t
 
-external null : Js.Json.t = "null" [@@bs.val]
+external null : Js.Json.t = "null"
 external string : string -> Js.Json.t = "%identity"
 external float : float -> Js.Json.t = "%identity"
 external int : int -> Js.Json.t = "%identity"
 external bool : bool -> Js.Json.t = "%identity"
 
-let char c =
-  c |> String.make 1
-    |> string
-
-let date d =
-  d |> Js.Date.toJSONUnsafe
-    |> string
-
-let nullable encode = function
-  | None -> null
-  | Some v -> encode v
-
-let withDefault d encode = function
-  | None -> d
-  | Some v -> encode v
+let char c = c |> String.make 1 |> string
+let date d = d |> Js.Date.toJSONUnsafe |> string
+let nullable encode = function None -> null | Some v -> encode v
+let withDefault d encode = function None -> d | Some v -> encode v
 
 external jsonDict : Js.Json.t Js.Dict.t -> Js.Json.t = "%identity"
+
 let dict encode d =
   let pairs = Js.Dict.entries d in
-  let encodedPairs = Array.map (fun (k, v) -> (k, encode(v))) pairs in
+  let encodedPairs = Array.map (fun (k, v) -> (k, encode v)) pairs in
   jsonDict (Js.Dict.fromArray encodedPairs)
 
-let object_ props: Js.Json.t =
-  props |> Js.Dict.fromList
-        |> jsonDict
+let object_ props : Js.Json.t = props |> Js.Dict.fromList |> jsonDict
 
 external jsonArray : Js.Json.t array -> Js.Json.t = "%identity"
-let array encode l =
-  l |> Array.map encode
-    |> jsonArray
+
+let array encode l = l |> Array.map encode |> jsonArray
+
 let list encode = function
   | [] -> jsonArray [||]
-  | hd::tl as l ->
+  | hd :: tl as l ->
       let a = Array.make (List.length l) (encode hd) in
       let rec fill i = function
         | [] -> a
-        | hd::tl -> (
-          Array.unsafe_set a i (encode hd);
-          fill (i + 1) tl
-        )
+        | hd :: tl ->
+            Array.unsafe_set a i (encode hd);
+            fill (i + 1) tl
       in
-        jsonArray (fill 1 tl)
+      jsonArray (fill 1 tl)
 
-let pair encodeA encodeB (a, b) =
-  jsonArray [|encodeA a; encodeB b|]
+let pair encodeA encodeB (a, b) = jsonArray [| encodeA a; encodeB b |]
 let tuple2 = pair
+
 let tuple3 encodeA encodeB encodeC (a, b, c) =
-  jsonArray [|encodeA a; encodeB b; encodeC c|]
+  jsonArray [| encodeA a; encodeB b; encodeC c |]
+
 let tuple4 encodeA encodeB encodeC encodeD (a, b, c, d) =
-  jsonArray [|encodeA a; encodeB b; encodeC c; encodeD d|]
+  jsonArray [| encodeA a; encodeB b; encodeC c; encodeD d |]
 
 external stringArray : string array -> Js.Json.t = "%identity"
 external numberArray : float array -> Js.Json.t = "%identity"
diff --git a/src/Json_encode.mli b/src/Json_encode.mli
index de8b7be..a4ec3fb 100644
--- a/src/Json_encode.mli
+++ b/src/Json_encode.mli
@@ -3,7 +3,8 @@
 type 'a encoder = 'a -> Js.Json.t
 (** The type of a encoder combinator *)
 
-external null : Js.Json.t = "null" [@@bs.val]
+external null : Js.Json.t = "null"
+
 (** [null] is the singleton null JSON value *)
 
 external string : string -> Js.Json.t = "%identity"
@@ -30,16 +31,22 @@ val nullable : 'a encoder -> 'a option -> Js.Json.t
 val withDefault : Js.Json.t -> 'a encoder -> 'a option -> Js.Json.t
 (** [withDefault default encoder option] returns the encoded value if present, oterwise [default] *)
 
-val pair : 'a encoder -> 'b encoder -> ('a * 'b) -> Js.Json.t
+val pair : 'a encoder -> 'b encoder -> 'a * 'b -> Js.Json.t
 (** [pair encoder encoder tuple] creates a JSON array from a tuple of size 2 *)
 
-val tuple2 : 'a encoder -> 'b encoder -> ('a * 'b) -> Js.Json.t
+val tuple2 : 'a encoder -> 'b encoder -> 'a * 'b -> Js.Json.t
 (** [tuple2 encoder encoder tuple] creates a JSON array from a tuple of size 2. Alias of [pair] *)
 
-val tuple3 : 'a encoder -> 'b encoder -> 'c encoder -> ('a * 'b * 'c) -> Js.Json.t
+val tuple3 : 'a encoder -> 'b encoder -> 'c encoder -> 'a * 'b * 'c -> Js.Json.t
 (** [tuple3 enc enc enc tuple] creates a JSON array from a tuple of size 3 *)
 
-val tuple4 : 'a encoder -> 'b encoder -> 'c encoder -> 'd encoder -> ('a * 'b * 'c * 'd) -> Js.Json.t
+val tuple4 :
+  'a encoder ->
+  'b encoder ->
+  'c encoder ->
+  'd encoder ->
+  'a * 'b * 'c * 'd ->
+  Js.Json.t
 (** [tuple4 enc enc enc enc tuple] creates a JSON array from a tuple of size 4 *)
 
 external jsonDict : Js.Json.t Js.Dict.t -> Js.Json.t = "%identity"