fix(type): fix RequestHandler return type (#980)

Author: chimurai

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## [next](https://github.com/chimurai/http-proxy-middleware/releases)
+
+- fix(type): fix RequestHandler return type
+
 ## [v3.0.0](https://github.com/chimurai/http-proxy-middleware/releases/tag/v3.0.0)
 
 This release contains some breaking changes.

src/types.ts

@@ -16,7 +16,7 @@ export interface RequestHandler<
   TRes = http.ServerResponse,
   TNext = NextFunction,
 > {
-  (req: TReq, res: TRes, next?: TNext): void | Promise<void>;
+  (req: TReq, res: TRes, next?: TNext): Promise<void>;
   upgrade: (req: http.IncomingMessage, socket: net.Socket, head: Buffer) => void;
 }