It is important to validate the form submitted by the user because it can have inappropriate values. So, validation is must to authenticate user.
Form validation normally used to occur at the server, after the client had entered all the necessary data and then pressed the Submit button. If the data entered by a client was incorrect or was simply missing, the server would have to send all the data back to the client and request that the form be resubmitted with correct information. This was really a lengthy process which used to put a lot of burden on the server.
Basic Validation − First of all, the form must be checked to make sure all the mandatory fields are filled in. It would require just a loop through each field in the form and check for data.
Data Format Validation − Secondly, the data that is entered must be checked for correct form and value. Your code must include appropriate logic to test correctness of data.
First let us see how to do a basic form validation. In the above form, we are calling validate() to validate data when onsubmit event is occurring. The following code shows the implementation of this validate() function.
Now we will see how we can validate our entered form data before submitting it to the web server.
The following example shows how to validate an entered email address. An email address must contain at least a ‘@’ sign and a dot (.). Also, the ‘@’ must not be the first character of the email address, and the last dot must at least be one character after the ‘@’ sign.