import { Client, Dispatcher } from "undici";

const insertHeaderInterceptor = dispatch => {
  return function InterceptedDispatch(opts, handler){
    opts.headers.push('Authorization', 'Bearer [Some token]')
    return dispatch(opts, handler)
  }
}

const client = new Client('https://localhost:3000', {
  interceptors: { Client: [insertHeaderInterceptor] } // error here
})

import { Client, Dispatcher } from "undici";

// the change from above is explicitly defining the type for the interceptor to match
// the intent
const insertHeaderInterceptor: Dispatcher.DispatchInterceptor = (
  dispatch: Dispatcher["dispatch"]
) => {
  return function InterceptedDispatch(
    opts: Dispatcher.DispatchOptions,
    handler: Dispatcher.DispatchHandlers
  ) {
    return dispatch(opts, handler);
  };
};

const client = new Client("https://localhost:3000", {
  interceptors: { Client: [insertHeaderInterceptor] },
});