Use the built-in JSON library

config/config.exs

@@ -5,7 +5,7 @@ config :logger, :console,
   format: "\n$time $metadata[$level] $message\n"
 
 config :phoenix,
-  json_library: Jason,
+  json_library: JSON,
   stacktrace_depth: 20,
   trim_on_html_eex_engine: false
 

guides/controllers.md

@@ -94,7 +94,7 @@ end
 
 Now [`/hello/Frank`] in your browser should display `From messenger Frank` as plain text without any HTML.
 
-A step beyond this is rendering pure JSON with the [`json/2`] function. We need to pass it something that the [Jason library](`Jason`) can decode into JSON, such as a map. (Jason is one of Phoenix's dependencies.)
+A step beyond this is rendering pure JSON with the [`json/2`] function. We need to pass it something that the [JSON library](https://hexdocs.pm/elixir/JSON.html) can decode into JSON, such as a map.
 
 ```elixir
 def show(conn, %{"messenger" => messenger}) do

guides/json_and_apis.md

@@ -171,7 +171,7 @@ defmodule HelloWeb.UrlJSON do
 end
 ```
 
-This view is very simple. The `index` function receives all URLs, and converts them into a list of maps. Those maps are placed inside the data key at the root, exactly as we saw when interfacing with our application from `cURL`. In other words, our JSON view converts our complex data into simple Elixir data-structures. Once our view layer returns, Phoenix uses the `Jason` library to encode JSON and send the response to the client.
+This view is very simple. The `index` function receives all URLs, and converts them into a list of maps. Those maps are placed inside the data key at the root, exactly as we saw when interfacing with our application from `cURL`. In other words, our JSON view converts our complex data into simple Elixir data-structures. Once our view layer returns, Phoenix uses the built-in [JSON library](https://hexdocs.pm/elixir/1.18/JSON.html) to encode JSON and send the response to the client.
 
 If you explore the remaining controller, you will learn the `show` action is similar to the `index` one. For `create`, `update`, and `delete` actions, Phoenix uses one other important feature, called "Action fallback".
 

guides/plug.md

@@ -153,7 +153,7 @@ The default endpoint plugs do quite a lot of work. Here they are in order:
 
 - `Plug.Telemetry` - adds instrumentation points so Phoenix can log the request path, status code and request time by default.
 
-- `Plug.Parsers` - parses the request body when a known parser is available. By default, this plug can handle URL-encoded, multipart and JSON content (with `Jason`). The request body is left untouched if the request content-type cannot be parsed.
+- `Plug.Parsers` - parses the request body when a known parser is available. By default, this plug can handle URL-encoded, multipart and JSON content. The request body is left untouched if the request content-type cannot be parsed.
 
 - `Plug.MethodOverride` - converts the request method to PUT, PATCH or DELETE for POST requests with a valid `_method` parameter.
 

installer/templates/phx_single/config/config.exs

@@ -59,8 +59,8 @@ config :logger, :default_formatter,
   format: "$time $metadata[$level] $message\n",
   metadata: [:request_id]
 
-# Use Jason for JSON parsing in Phoenix
-config :phoenix, :json_library, Jason
+# Use built-in `JSON` for JSON parsing in Phoenix
+config :phoenix, :json_library, JSON
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.

installer/templates/phx_umbrella/config/extra_config.exs

@@ -5,8 +5,8 @@ config :logger, :default_formatter,
   format: "$time $metadata[$level] $message\n",
   metadata: [:request_id]
 
-# Use Jason for JSON parsing in Phoenix
-config :phoenix, :json_library, Jason
+# Use built-in `JSON` for JSON parsing in Phoenix
+config :phoenix, :json_library, JSON
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.

installer/test/phx_new_test.exs

@@ -54,7 +54,7 @@ defmodule Mix.Tasks.Phx.NewTest do
       assert_file("phx_blog/config/config.exs", fn file ->
         assert file =~ "ecto_repos: [PhxBlog.Repo]"
         assert file =~ "generators: [timestamp_type: :utc_datetime]"
-        assert file =~ "config :phoenix, :json_library, Jason"
+        assert file =~ "config :phoenix, :json_library, JSON"
         assert file =~ ~s[cd: Path.expand("../assets", __DIR__),]
         refute file =~ "namespace: PhxBlog"
         refute file =~ "config :phx_blog, :generators"

installer/test/phx_new_umbrella_test.exs

@@ -63,7 +63,7 @@ defmodule Mix.Tasks.Phx.New.UmbrellaTest do
         assert file =~ ~r/config :esbuild/
         assert file =~ "cd: Path.expand(\"../apps/phx_umb_web/assets\", __DIR__)"
         assert file =~ ~S[import_config "#{config_env()}.exs"]
-        assert file =~ "config :phoenix, :json_library, Jason"
+        assert file =~ "config :phoenix, :json_library, JSON"
         assert file =~ "ecto_repos: [PhxUmb.Repo]"
         assert file =~ ":phx_umb_web, PhxUmbWeb.Endpoint"
         assert file =~ "generators: [context_app: :phx_umb]\n"

integration_test/config/config.exs

@@ -1,6 +1,6 @@
 import Config
 
-config :phoenix, :json_library, Jason
+config :phoenix, :json_library, JSON
 
 config :swoosh, api_client: false
 

lib/phoenix.ex

@@ -6,6 +6,8 @@ defmodule Phoenix do
   """
   use Application
 
+  @default_json_library if Code.ensure_loaded?(JSON), do: JSON, else: Jason
+
   @doc false
   def start(_type, _args) do
     # Warm up caches
@@ -45,7 +47,7 @@ defmodule Phoenix do
 
   """
   def json_library do
-    Application.get_env(:phoenix, :json_library, Jason)
+    Application.get_env(:phoenix, :json_library, @default_json_library)
   end
 
   @doc """

test/phoenix/socket/v1_json_serializer_test.exs

@@ -20,11 +20,11 @@
     IO.iodata_to_binary(encoded)
   end
 
   test "encode!/1 encodes `Phoenix.Socket.Message` as JSON" do
     msg = %Message{topic: "t", event: "e", payload: "m"}
     encoded = encode!(@serializer, msg)
 
-    assert Jason.decode!(encoded) == %{
+    assert JSON.decode!(encoded) == %{
              "event" => "e",
              "payload" => "m",
              "ref" => nil,
@@ -36,7 +36,7 @@
     msg = %Reply{topic: "t", ref: "null"}
     encoded = encode!(@serializer, msg)
 
-    assert Jason.decode!(encoded) == %{
+    assert JSON.decode!(encoded) == %{
              "event" => "phx_reply",
              "payload" => %{"response" => nil, "status" => nil},
              "ref" => "null",
@@ -57,11 +57,11 @@
     )
   end
 
   test "fastlane!/1 encodes a broadcast into a message as JSON" do
     msg = %Broadcast{topic: "t", event: "e", payload: "m"}
     encoded = fastlane!(@serializer, msg)
 
-    assert Jason.decode!(encoded) == %{
+    assert JSON.decode!(encoded) == %{
              "event" => "e",
              "payload" => "m",
              "ref" => nil,

test/phoenix/socket/v2_json_serializer_test.exs

@@ -88,7 +88,7 @@
     end
   end
 
   test "encode!/1 encodes `Phoenix.Socket.Message` as JSON" do
     msg = %Message{topic: "t", event: "e", payload: %{m: 1}}
     assert encode!(@serializer, msg) == @v2_msg_json
   end
@@ -98,11 +98,11 @@
     assert_raise ArgumentError, fn -> encode!(@serializer, msg) end
   end
 
   test "encode!/1 encodes `Phoenix.Socket.Reply` as JSON" do
     msg = %Reply{topic: "t", payload: %{m: 1}}
     encoded = encode!(@serializer, msg)
 
-    assert Jason.decode!(encoded) == [
+    assert JSON.decode!(encoded) == [
              nil,
              nil,
              "t",
@@ -111,12 +111,12 @@
            ]
   end
 
   test "decode!/2 decodes `Phoenix.Socket.Message` from JSON" do
     assert %Message{topic: "t", event: "e", payload: %{"m" => 1}} ==
              decode!(@serializer, @v2_msg_json, opcode: :text)
   end
 
   test "fastlane!/1 encodes a broadcast into a message as JSON" do
     msg = %Broadcast{topic: "t", event: "e", payload: %{m: 1}}
     assert fastlane!(@serializer, msg) == @v2_fastlane_json
   end

test/phoenix/test/conn_test.exs

@@ -328,13 +328,13 @@ defmodule Phoenix.Test.ConnTest do
       build_conn(:get, "/") |> resp(200, "ok") |> json_response(200)
     end
 
-    assert_raise Jason.DecodeError,
-                 "unexpected byte at position 0: 0x6F (\"o\")", fn ->
+    assert_raise JSON.DecodeError,
+                 "invalid byte 111 at position (byte offset) 0", fn ->
       build_conn(:get, "/") |> put_resp_content_type("application/json")
                       |> resp(200, "ok") |> json_response(200)
     end
 
-    assert_raise Jason.DecodeError, ~r/unexpected end of input at position 0/, fn ->
+    assert_raise JSON.DecodeError, ~r/unexpected end of JSON binary at position \(byte offset\) 0/, fn ->
       build_conn(:get, "/")
       |> put_resp_content_type("application/json")
       |> resp(200, "")