From f336335ecec1f04339b18abee5effa2d835a298f Mon Sep 17 00:00:00 2001
From: Matt Brophy <matt@brophy.org>
Date: Wed, 26 Mar 2025 13:34:21 -0600
Subject: [PATCH] Fix middleware return types

---
 .changeset/gentle-gifts-kneel.md                | 5 +++++
 packages/react-router/lib/router/utils.ts       | 2 +-
 packages/react-router/lib/types/route-module.ts | 4 ++--
 3 files changed, 8 insertions(+), 3 deletions(-)
 create mode 100644 .changeset/gentle-gifts-kneel.md

diff --git a/.changeset/gentle-gifts-kneel.md b/.changeset/gentle-gifts-kneel.md
new file mode 100644
index 0000000000..64d8ebc9c9
--- /dev/null
+++ b/.changeset/gentle-gifts-kneel.md
@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix types on `unstable_MiddlewareFunction` to avoid type errors when a middleware doesn't return a value
diff --git a/packages/react-router/lib/router/utils.ts b/packages/react-router/lib/router/utils.ts
index b6523eb2fa..977185daf7 100644
--- a/packages/react-router/lib/router/utils.ts
+++ b/packages/react-router/lib/router/utils.ts
@@ -220,7 +220,7 @@ export interface unstable_MiddlewareNextFunction<Result = unknown> {
 export type unstable_MiddlewareFunction<Result = unknown> = (
   args: DataFunctionArgs<unstable_RouterContextProvider>,
   next: unstable_MiddlewareNextFunction<Result>
-) => MaybePromise<Result | undefined>;
+) => MaybePromise<Result | void>;
 
 /**
  * Arguments passed to loader functions
diff --git a/packages/react-router/lib/types/route-module.ts b/packages/react-router/lib/types/route-module.ts
index 3a813b6b44..465b895007 100644
--- a/packages/react-router/lib/types/route-module.ts
+++ b/packages/react-router/lib/types/route-module.ts
@@ -190,12 +190,12 @@ type ServerDataFunctionArgs<T extends RouteInfo> = {
 export type CreateServerMiddlewareFunction<T extends RouteInfo> = (
   args: ServerDataFunctionArgs<T>,
   next: unstable_MiddlewareNextFunction<Response>
-) => MaybePromise<Response | undefined>;
+) => MaybePromise<Response | void>;
 
 export type CreateClientMiddlewareFunction<T extends RouteInfo> = (
   args: ClientDataFunctionArgs<T>,
   next: unstable_MiddlewareNextFunction<undefined>
-) => MaybePromise<undefined>;
+) => MaybePromise<void>;
 
 export type CreateServerLoaderArgs<T extends RouteInfo> =
   ServerDataFunctionArgs<T>;