Change middleware return type from void to undefined

.changeset/unlucky-pumas-buy.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+[UNSTABLE] Update `Route.unstable_MiddlewareFunction` to have a return value of `Response | undefined` instead of `Response | void` becaue you should not return anything if you aren't returning the `Response`

packages/react-router/lib/router/router.ts

@@ -22,6 +22,7 @@ import type {
   DataStrategyResult,
   ImmutableRouteKey,
   MapRoutePropertiesFunction,
+  MaybePromise,
   MutationFormMethod,
   RedirectResult,
   RouteData,
@@ -61,8 +62,6 @@ import {
 //#region Types and Constants
 ////////////////////////////////////////////////////////////////////////////////
 
-type MaybePromise<T> = T | Promise<T>;
-
 /**
  * A Router instance manages all navigation and data loading/mutations
  */

packages/react-router/lib/router/utils.ts

@@ -2,6 +2,8 @@ import type { Equal, Expect } from "../types/utils";
 import type { Location, Path, To } from "./history";
 import { invariant, parsePath, warning } from "./history";
 
+export type MaybePromise<T> = T | Promise<T>;
+
 /**
  * Map of routeId -> data returned from a loader/action/error
  */
@@ -206,7 +208,7 @@ interface DataFunctionArgs<Context> {
  * middlewares from the bottom-up
  */
 export interface unstable_MiddlewareNextFunction<Result = unknown> {
-  (): Result | Promise<Result>;
+  (): MaybePromise<Result>;
 }
 
 /**
@@ -218,7 +220,7 @@ export interface unstable_MiddlewareNextFunction<Result = unknown> {
 export type unstable_MiddlewareFunction<Result = unknown> = (
   args: DataFunctionArgs<unstable_RouterContextProvider>,
   next: unstable_MiddlewareNextFunction<Result>
-) => Result | Promise<Result>;
+) => MaybePromise<Result | undefined>;
 
 /**
  * Arguments passed to loader functions
@@ -237,7 +239,7 @@ export interface ActionFunctionArgs<Context = any>
  */
 type DataFunctionValue = unknown;
 
-type DataFunctionReturnValue = Promise<DataFunctionValue> | DataFunctionValue;
+type DataFunctionReturnValue = MaybePromise<DataFunctionValue>;
 
 /**
  * Route loader function signature
@@ -373,7 +375,7 @@ export type AgnosticPatchRoutesOnNavigationFunction<
   M extends AgnosticRouteMatch = AgnosticRouteMatch
 > = (
   opts: AgnosticPatchRoutesOnNavigationFunctionArgs<O, M>
-) => void | Promise<void>;
+) => MaybePromise<void>;
 
 /**
  * Function provided by the framework-aware layers to set any framework-specific

packages/react-router/lib/types/route-module.ts

@@ -190,7 +190,7 @@ type ServerDataFunctionArgs<T extends RouteInfo> = {
 export type CreateServerMiddlewareFunction<T extends RouteInfo> = (
   args: ServerDataFunctionArgs<T>,
   next: unstable_MiddlewareNextFunction<Response>
-) => MaybePromise<Response | void>;
+) => MaybePromise<Response | undefined>;
 
 export type CreateClientMiddlewareFunction<T extends RouteInfo> = (
   args: ClientDataFunctionArgs<T>,