how do you programmatically navigate pages via SSR?

something like
export async function getServerSideProps({ params, req }: any) {
  const router = useRouter();

  if (user_id) {
    const result = await queryUserItems(params.id, user_id);
  }
  else
    router.push("login");

  const props = {};

  return {
    props,
  };
}