How to validate types in API Requests and Responses?

// routes/api/users/index.ts
export const handler: Handlers<User | null> = {
  async POST(req, _ctx) {
    const user = (await req.json()) as User;
    const userKey = ["user", user.id];
    const ok = await kv.atomic().set(userKey, user).commit();
    if (!ok) throw new Error("Something went wrong.");
    return new Response(JSON.stringify(user));
  },
};