@chimurai I found same issue as @Floriferous. IMO his example is bad. To the point.
In your example:

  proxyRes: responseInterceptor(async (responseBuffer, proxyRes, req, res) => {
    const response = responseBuffer.toString('utf8'); // convert buffer to string
    return response.replace('Hello', 'Goodbye'); // manipulate response and return the result
  }),
},

event handler uses response interceptor which is an async but types of ProxyResCallback and ResponseInterceptor mismatch.
proxyRes?: httpProxy.ProxyResCallback<TReq, TRes>;
ProxyResCallback is void NOT Promise

Originally posted by @tobiw777 in #1139