Where should validation of a required field be designed: on the front end or on the back end?

Validation of required fields should be designed both on the frontend and on the backend, but for different purposes.

On the frontend, validation is needed to improve the user experience: it allows quickly informing the user about incorrect data without unnecessary server requests. For example, checking that a field is not empty can be implemented on the form using JavaScript.

On the backend, validation is mandatory to ensure data security and integrity, as the frontend can be bypassed or altered. The server must check required fields before saving or processing data.

Thus, the frontend is for user convenience, and the backend is for system reliability and security.