DecoratorHandler is deprecated. How to make Interceptor without it?

[alnikmakh]

Hello there! I've been exploring interceptors in version 7.4 and noticed that DecoratorHandler is deprecated. After digging into how the new approach works, I realized that DecoratorHandler is indeed seems redundant. However, I couldn't find an example or documentation showing the proper way to create an interceptor without DecoratorHandler.

While reviewing the source code of the compose method in Dispatcher, I found an approach that wraps the dispatch method using a Proxy. This seems suitable for wrapping hooks of a handler (an instance of WrapHandler) within the interceptor:

import { type Dispatcher } from 'undici';

const clearHeadersInterceptor: Dispatcher.DispatcherComposeInterceptor =
  (dispatch) => (opts, handler) => {
    const initialOnResponseStart = handler.onResponseStart!.bind(handler);

    const newOnResponseStart: Dispatcher.DispatchHandler['onResponseStart'] = (
      controller,
      statusCode,
      headers,
      statusMessage,
    ) => {
      initialOnResponseStart(controller, statusCode, {}, statusMessage);
    };

    const proxiedHandler = new Proxy(handler, {
      get: (target, key) => {
        const methodOrProp =
          typeof target[key] === 'function' ? target[key].bind(target) : target[key];

        return key === 'onResponseStart' ? newOnResponseStart : methodOrProp;
      },
    });

    return dispatch(opts, proxiedHandler);
  };

Could you please guide me on whether this approach is acceptable, or if there will be another helper utility introduced to replace DecoratorHandler approach?

[metcoder95]

The DecoratorHandler as introduced to abstract the several checks the RequestHandler needs when passing it down to the Dispatcher#dispatch(opts, handler) method.

I'd suggest you to take care of the usage of Proxy as it can hit you on performance pretty easily.

Check this example of an interceptor: https://github.com/nodejs/undici/blob/main/lib/interceptor/dump.js

It uses DecoratorHandler (you can see specifically why), and might give you an idea of how to use it without it and workarounds.

[alnikmakh]

Thanks! Yes! I'm aware that using Proxy might introduce some performance overhead, but it felt cleaner for wrapping a single hook.

Please, correct me if I'm wrong:
DecoratorHandler is deprecated and wasn't intended for conveniently wrapping some of the lifecycle hooks in simple scenarios? Is the recommended pattern to explicitly create a wrapper class implementing all DispatchHandler methods and extending it, similar to extending DecoratorHandler as shown here https://blog.platformatic.dev/http-fundamentals-understanding-undici-and-its-working-mechanism#heading-interceptors ?

[metcoder95]

Just to confirm, DecoratorHandler has not been dropped, but enhanced with further APIs that sync with our new Handler's shape.

So you should be able to use it for your interceptor right away

[alnikmakh]

Thank you for clarifying! The deprecation mark confused me, so I began searching for an alternative approach. https://github.com/nodejs/undici/blob/main/lib/handler/decorator-handler.js#L7

[metcoder95]

Ah got it; I missed that. It should be fine to be used within the v7 release line, if dropped it will be in upcoming v8 or something. Though, the idea is exactly the same, so it must be fine copy/pasting the similar shape for your interceptors