✨ Add JSONResponder type and tests

- Introduced JSONResponder type in handler.go, combining JSON body parsing with ResponseHelper handling.
- Implemented ServeHTTP for JSONResponder to automatically decode JSON requests and return structured responses.
- Added comprehensive unit tests for JSONResponder in handler_test.go, covering successful responses, invalid JSON, empty bodies, and error propagation.
- Ensured all new tests adhere to existing project conventions and pass linting checks.

handler.go

@@ -62,3 +62,27 @@ func (fn JSONBodyHandler[V]) ServeHTTP(w http.ResponseWriter, r *http.Request) e
 
 	return fn(w, r, body)
 }
+
+// JSONResponder can be used as a [Handler] which automatically parses the json body into a value, which is passed to the function.
+// It also returns a [ResponseHelper], instead of passing an [http.ResponseWriter].
+type JSONResponder[V any] func(r *http.Request, body V) (ResponseHelper, error)
+
+var _ Handler = JSONResponder[map[string]any](nil)
+
+func (fn JSONResponder[V]) ServeHTTP(w http.ResponseWriter, r *http.Request) error {
+	reqBody := r.Body
+	//nolint:errcheck // This is usually fine
+	defer reqBody.Close()
+
+	dec := json.NewDecoder(reqBody)
+	var body V
+	if err := dec.Decode(&body); err != nil {
+		return err
+	}
+
+	var rh ResponseHandler = func(req *http.Request) (ResponseHelper, error) {
+		return fn(req, body)
+	}
+
+	return rh.ServeHTTP(w, r)
+}

handler_test.go

@@ -1,6 +1,7 @@
 package ezhandler_test
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -144,3 +145,80 @@ func runJSONBodyHandlerTest(tt *testing.T, tc jsonBodyTest) {
 
 	})
 }
+
+type jsonBodyResponderTest struct {
+	name                string
+	requestBody         string
+	handler             ezhandler.JSONResponder[testBody]
+	expectedStatus      int
+	expectedBody        testBody
+	expectedErr         error
+	expectedErrContains string
+}
+
+var jsonBodyResponderTests = []jsonBodyResponderTest{
+	{
+		name:        "successful decoding and response",
+		requestBody: `{"name":"John Doe","age":30}`,
+		handler: ezhandler.JSONResponder[testBody](func(r *http.Request, body testBody) (ezhandler.ResponseHelper, error) {
+			return ezhandler.JSONResponse(body), nil
+		}),
+		expectedStatus: http.StatusOK,
+		expectedBody:   testBody{Name: "John Doe", Age: 30},
+		expectedErr:    nil,
+	},
+	{
+		name:        "invalid JSON body",
+		requestBody: `{"name":"John Doe","age":}`,
+		handler: ezhandler.JSONResponder[testBody](func(r *http.Request, body testBody) (ezhandler.ResponseHelper, error) {
+			return nil, nil // Should not be called
+		}),
+		expectedErrContains: "invalid character",
+	},
+	{
+		name:        "empty body for struct type",
+		requestBody: "",
+		handler: ezhandler.JSONResponder[testBody](func(r *http.Request, body testBody) (ezhandler.ResponseHelper, error) {
+			return nil, nil // Should not be called
+		}),
+		expectedErr: io.EOF,
+	},
+	{
+		name:        "handler function returns error",
+		requestBody: `{"name":"Jane Doe","age":25}`,
+		handler: ezhandler.JSONResponder[testBody](func(r *http.Request, body testBody) (ezhandler.ResponseHelper, error) {
+			return nil, errTest
+		}),
+		expectedErr: errTest,
+	},
+}
+
+func TestJSONResponder_ServeHTTP(t *testing.T) {
+	for _, tc := range jsonBodyResponderTests {
+		runJSONResponderTest(t, tc)
+	}
+}
+
+func runJSONResponderTest(tt *testing.T, tc jsonBodyResponderTest) {
+	tt.Run(tc.name, func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(tc.requestBody))
+		rec := httptest.NewRecorder()
+
+		err := tc.handler.ServeHTTP(rec, req)
+
+		switch {
+		case tc.expectedErr != nil:
+			assert.Equal(t, tc.expectedErr, err)
+		case tc.expectedErrContains != "":
+			assert.ErrorContains(t, err, tc.expectedErrContains)
+		default:
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expectedStatus, rec.Code)
+			// Marshal expectedBody to JSON string for comparison
+			expectedBodyBytes, marshalErr := json.Marshal(tc.expectedBody)
+			assert.NoError(t, marshalErr)
+			assert.JSONEq(t, string(expectedBodyBytes), rec.Body.String())
+			assert.Equal(t, "application/json", rec.Header().Get("Content-Type"))
+		}
+	})
+}