Data validation in an application is one of the major tasks for the developer now-a-days. Data validation is the process of ensuring that the data entered by any users is correct and is useful. It uses routines, often called validation rules, validation constraints or check routines, which check for the correctness, meaningfulness, and security of data, that gets put into a web application. These rules are mainly implemented in UI, business logic, and databases.
If we check in any web application, we will mainly find validations in the UI. As the user interface is the primary source, where the user can enter invalid data in the application, we mainly focus on UI validation.