Checks

• I understand project setup issues should be asked on StackOverflow or in GitHub Discussions.
• I updated to latest http-proxy-middleware.

Describe the bug (be clear and concise)

I'm seeing a type mismatch when writing the following code:

createProxyMiddleware({
  on: {
    proxyReq: responseInterceptor( ... )
  }
})

The error is detected by typescript-eslint:

Promise-returning function provided to property where a void return was expected.eslint[@typescript-eslint/no-misused-promises](https://typescript-eslint.io/rules/no-misused-promises)

The types are defined as follows:

// on.proxyRes
type ProxyResCallback<TIncomingMessage = http.IncomingMessage, TServerResponse = http.ServerResponse> = (
        proxyRes: TIncomingMessage,
        req: TIncomingMessage,
        res: TServerResponse,
    ) => void;

// responseInterceptor
export declare function responseInterceptor<TReq extends http.IncomingMessage = http.IncomingMessage, TRes extends http.ServerResponse = http.ServerResponse>(interceptor: Interceptor<TReq, TRes>): (proxyRes: TReq, req: TReq, res: TRes) => Promise<void>;

I'm not sure if on.proxyRes should support promises, or if the returnValue of responseInterceptor should just be void.

Step-by-step reproduction instructions

-

Expected behavior (be clear and concise)

There shouldn't be a type mismatch.

How is http-proxy-middleware used in your project?

-

What http-proxy-middleware configuration are you using?

-

What OS/version and node/version are you seeing the problem?

TypeScript 5.8.3
Node 22

Additional context (optional)

No response