Track Configured Values

app.go

@@ -114,6 +114,8 @@ type App struct {
 	customMethod bool
 	// Mount fields
 	mountFields *mountFields
+	// Indicates if the value was explicitly configured
+	configured Config
 }
 
 // Config is a struct holding the server settings.
@@ -513,6 +515,9 @@ func New(config ...Config) *App {
 		app.config = config[0]
 	}
 
+	// Initialize configured before defaults are set
+	app.configured = app.config
+
 	if app.config.ETag {
 		if !IsChild() {
 			fmt.Println("[Warning] Config.ETag is deprecated since v2.0.6, please use 'middleware/etag'.")
@@ -999,7 +1004,10 @@ func (app *App) ErrorHandler(ctx *Ctx, err error) error {
 		if prefix != "" && strings.HasPrefix(ctx.path, prefix) {
 			parts := len(strings.Split(prefix, "/"))
 			if mountedPrefixParts <= parts {
-				mountedErrHandler = subApp.config.ErrorHandler
+				if subApp.configured.ErrorHandler != nil {
+					mountedErrHandler = subApp.config.ErrorHandler
+				}
+
 				mountedPrefixParts = parts
 			}
 		}

mount_test.go

@@ -139,6 +139,24 @@ func Test_App_Group_Mount(t *testing.T) {
 	utils.AssertEqual(t, uint32(2), app.handlersCount)
 }
 
+func Test_App_UseParentErrorHandler(t *testing.T) {
+	app := New(Config{
+		ErrorHandler: func(ctx *Ctx, err error) error {
+			return ctx.Status(500).SendString("hi, i'm a custom error")
+		},
+	})
+
+	fiber := New()
+	fiber.Get("/", func(c *Ctx) error {
+		return errors.New("something happened")
+	})
+
+	app.Mount("/api", fiber)
+
+	resp, err := app.Test(httptest.NewRequest(MethodGet, "/api", nil))
+	testErrorResponse(t, err, resp, "hi, i'm a custom error")
+}
+
 func Test_App_UseMountedErrorHandler(t *testing.T) {
 	app := New()