Disallow undefined as a return value from LoaderFunction/ActionFunction

.changeset/disallow-return-undefined-type.md

@@ -0,0 +1,5 @@
+---
+"@remix-run/router": patch
+---
+
+Enhance `LoaderFunction`/`ActionFunction` return type to prevent `undefined` from being a valid return value

packages/router/utils.ts

@@ -123,18 +123,25 @@ export interface LoaderFunctionArgs extends DataFunctionArgs {}
  */
 export interface ActionFunctionArgs extends DataFunctionArgs {}
 
+/**
+ * Loaders and actions can return anything except `undefined` (`null` is a
+ * valid return value if there is no data to return).  Responses are preferred
+ * and will ease any future migration to Remix
+ */
+type DataFunctionValue = Response | NonNullable<unknown> | null;
+
 /**
  * Route loader function signature
  */
 export interface LoaderFunction {
-  (args: LoaderFunctionArgs): Promise<Response> | Response | Promise<any> | any;
+  (args: LoaderFunctionArgs): Promise<DataFunctionValue> | DataFunctionValue;
 }
 
 /**
  * Route action function signature
  */
 export interface ActionFunction {
-  (args: ActionFunctionArgs): Promise<Response> | Response | Promise<any> | any;
+  (args: ActionFunctionArgs): Promise<DataFunctionValue> | DataFunctionValue;
 }
 
 /**