Form fields are designed to collect information from visitors of a certain page. But despite the best efforts and explanations, getting the correct information from an average user can be difficult. Impatient or careless visitors may miss important fields, only partially fill one or several fields, or simply press the wrong keys. After filling the form this way they click "Send" button. The server program receives a set of data and if data validation is not implemented on the server side, the server program has no idea what to do with it.

A professional web page requires data validation, i.e. a method for detecting errors in the input data, or even better - a way to avoid mistakes in the first place. Over the years, web developers have been doing this using JavaScript procedures of their own design or through professional JavaScript libraries. To be honest, these approaches gave excellent results.