.ts
TypeScript
(application/typescript)
import type { preHandlerHookHandler } from "fastify";

export const guestOrRedirect =
  (redirectToPath: string, statusCode: number = 302): preHandlerHookHandler =>
  async (request, reply) => {
    if (request.session.data.authenticated) {
      reply.redirect(statusCode, redirectToPath);
    }
  };