Is it necessary to have a 404 page in a single-page application (SPA) built with React?

Yes, it's highly recommended to have a 404 page in an SPA built with React. Even in an SPA, users can still try to access routes that don't exist, whether by typing them directly into the address bar, following broken links, or encountering errors in your application's routing logic. Without a 404 page, the user might see a blank page or an unhelpful error message, leading to a poor user experience. A well-designed 404 page provides guidance and helps the user navigate back to a functional part of the application.