Return a normal JSONAPI response for user endpoint.

app/controllers/api/v3/private/user_controller.rb

@@ -1,11 +1,11 @@
 module API
   module V3
     module Private
-      class Api::V3::Private::UserController < ::ApplicationController
+      class Api::V3::Private::UserController < JSONAPI::ResourceController
         include JwtAuthorizationConcern
 
-        def index
-          render json: { username: @current_user.id }
+        def context
+          {current_user: @current_user, params: params}
         end
       end
     end

app/resources/api/v3/private/user_resource.rb

@@ -7,6 +7,8 @@ class Api::V3::Private::UserResource < JSONAPI::Resource
           key_type :string
 
           paginator :none
+
+          exclude_links :default
         end
       end
     end

config/routes.rb

@@ -6,7 +6,7 @@
         # Don't generate links or relationship routes for decks.
         jsonapi_resources :decks do
         end
-        resources :user, only: [:index]
+        jsonapi_resources :user, only: [:index, :show]
       end
       namespace :public, defaults: { format: :json } do
         jsonapi_resources :card_cycles, only: [:index, :show]