If there is a mandatory input field name, the user may type a number or leave the field blank, type just one letter, etc.In such cases you will get the alert message which guides you to make the changes according to the form requirements.If you entered phone number 9 digits you will get alert message to enter the 10 digit number.If you left the Email box empty then you will get the alert that Email is mandatory or if the email format is incorrect then you will email format is incorrect. All these validations can be easily done using JavaScript.