Validate on client (fast feedback) and server (source of truth). Common validation: required fields, format (email, phone), length, allowed values, cross-field logic. Never trust client validation — always re-validate on the server.
Validate on client (fast feedback) and server (source of truth). Common validation: required fields, format (email, phone), length, allowed values, cross-field logic. Never trust client validation — always re-validate on the server.