In an ideal case, users will enter valid data in a proper format in your application. But, as you might realize, the real world is not so ideal. Users will enter incorrect data in your application. As a developer, it is your responsibility to validate the user input in your application. If the entered input is not valid, you need to inform the user, explaining what has gone wrong, so that the user can correct the input data and submit the form again.

Validation can be done on the client side, on the server side, or at both ends. If the validation is done before sending the data to the server, it is called client-side validation. For example, if the user does not enter any data in a mandatory field, we can validate (by finding the data that is not entered) the form, at the client side itself.

There is no need to send the form data to the server. JavaScript is the most commonly-used language for client-side validation:

If the validation is done at the server side (sending...