Using a different decorator name with TypeScript

[kerolloz]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.28.1

Plugin version

8.0.1

Node.js version

using Bun

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

24.04

Description

How can I make TypeSciprt understand the new decorator for my custom decorator name?

For example if I have the following:

export default fp(async (fastify) => {
  fastify.register(fastifyJwt, {
    secret: env('JWT_SECRET'),
    decoratorName: "jwtUser"
  });

  fastify.decorate(
    'authenticate',
    async (request: FastifyRequest, reply: FastifyReply) => {
      try {
        await request.jwtVerify();
      } catch (err) {
        reply.send(err);
      }
    },
  );
});

How can I make TypeScript understand that I have a new property jwtUser?

I have tried the following but it doesn't seem to work:

// fastify-jwt.d.ts
import '@fastify/jwt';
import type { ObjectId } from 'mongodb';

type UserPayload = { id: ObjectId };
type DecodedUser = UserPayload;

declare module '@fastify/jwt' {
  interface FastifyJWT {
    // payload type is used for signing and verifying
    payload: UserPayload;
    // user type is return type of `request.user` object
    jwtUser: DecodedUser;
  }
  interface FastifyRequest {
    jwtUser: DecodedUser;
  }
}

declare module 'fastify' {
  interface FastifyInstance {
    authenticate: FastifyMiddleware;
  }
}

Link to code that reproduces the bug

No response

Expected Behavior

No response

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