A reusable form validator for ASP pages

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What it's all about

To make sure that only valid data is entered, some checks should occur before it is stored somewhere. Valid means here reasonable in the current context (e.g. no birthdays in the future) as well as suitable for the data store, like limitations in length or data type.

This validation can take place on the client side via JavaScript or on the server within the receiving Active Server Page. Both have well known advantages and disadvantages. This article contains a method for the JavaScript variant.

Writing the same code again and again

The mission objectives

• Write once use everywhere
• No dynamic code generation (ASP scripts that generate client side JavaScript.....)
• Separate JavaScript code from HTML (move it into a separate file)
• Browser independent

The solution

<script language="JavaScript" src="/formvalidator.js"></script>

<form name="RegisterUserForm" action="register.save.asp" method="post" 
      OnSubmit="return CheckForm(document.RegisterUserForm);">

Logon Name: <INPUT TYPE="TEXT" NAME="logon_name" VALUE="">

<INPUT TYPE="HIDDEN" NAME="CHK_logon_name" VALUE="STR|5|255|True|Logon name">

</form>

So what are we doing here? The first two lines are more or less stratight forward. Include the JavaScript file that contains the form checker code and include an OnSubmit event handler. The interesting part is the last hidden input field. It contains instructions for the form checker. In this case they are:

• The field to be checked has the name logon_name
• It's a string value
• Minimum of 5 characters
• Maximum of 255
• It's a mandatory field
• The friendly name (for the error message) is Logon name

For strings it's not too far beyond what HTML can do for you right out of the box. Basically only the enforcement of mandatory fields and the minimum string length are enhancements. It gets more interesting when it comes to numeric values and dates.

According to the features for strings the form checker checks these values for numeric values (integers and floats):

• Minimum and maximum values (not character length)
• Validity of the string. Only numeric characters are allowed.

And for dates:

• Validity of the string. Format of the string
• Minimum and maximum date
• Validity of the date (e.g February 30th is not allowed)

So what the CheckForm function actually does, is to go through all form elements of the form passed as parameter. It the looks for the CHK_ prefix and interprets the values as instructions to check the content of the corresponding field.

Some further thoughts

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)