MVC is being used for web development. The M part, model, handles the rules for inputs. This would trigger validation errors. In a typical stack, we code the server-side model in, say, Ruby on Rails, and duplicate the same model in client-side model in JavaScript so it can detect things like minimum password lengths without sending data back to the server. The holy grail is to have both server-side and client-side in JavaScript and also sharing the models as well, with that we reduce amount of things we have to code.