Introducing Key Concepts for Form Validation with the DOM
If you're looking for ways to validate the data submitted on your web forms, you'll be interested in this article. It's the first in a series that explains how to use the DOM to supplement your server-side validation techniques.
Introducing Key Concepts for Form Validation with the DOM - Completing the validation script (Page 5 of 5 )
Since the pair of JavaScript functions that I showed you in the previous section were responsible for displaying and removing error nodes from the corresponding web document, in consonance with the values entered on the online form, the only piece of the validation script that remains undefined is the function that checks whether all the required form fields has been filled in or not.
To satisfy this requirement, below I coded a simple function, called "validateForm()," which not only verifies whether any required filed has been left blank, but it also calls the respective "showError()" function when applicable.
Here is the signature of this new function. Take a look at its source code:
// validate form
function validateForm(formObj){
valid=true;
var fname=formObj.elements[0];
if(!fname){return};
if(!fname.value){showError(fname,'*Enter your First Name')};
var lname=formObj.elements[1];
if(!lname){return};
if(!lname.value){showError(lname,'*Enter your Last Name')};
var email=formObj.elements[2];
if(!email){return};
if(!email.value){showError(email,'*Enter your email
address')};
var age=formObj.elements[3];
if(!age){return};
if(!age.value){showError(age,'*Enter your age (1-99)')};
var postadd=formObj.elements[4];
if(!postadd){return};
if(!postadd.value){showError(postadd,'*Enter your postal
address')};
return valid;
}
In this case, I'm not going to bore you with irrelevant details of how the above function works, since I'm sure that you've worked with similar scripts hundreds of times. However, I'd like to highlight at least briefly how the referenced function calls "showError()" when a particular form field has been left empty.
Of course, the validation process can be easily changed to perform a strict verification on each form field; consider this possibility seriously if you're going to use this script with your own web applications.
Right, at this stage you've understood how each of the JavaScript functions defined previously works as an isolated piece, but I'm certain that you want to see them working together. Given that, below I listed the full source code for this DOM-based validation script, in conjunction with the corresponding (X)HTML markup.
Here's the script in question, ready to be copied and pasted:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html;
charset=iso-8859-1" />
<title>DOM-based form validator</title>
<script language="javascript">
// validate form
function validateForm(formObj){
valid=true;
var fname=formObj.elements[0];
if(!fname){return};
if(!fname.value){showError(fname,'*Enter your First
Name')};
var lname=formObj.elements[1];
if(!lname){return};
if(!lname.value){showError(lname,'*Enter your Last
Name')};
var email=formObj.elements[2];
if(!email){return};
if(!email.value){showError(email,'*Enter your email
address')};
var age=formObj.elements[3];
if(!age){return};
if(!age.value){showError(age,'*Enter your age (1-99)')};
var postadd=formObj.elements[4];
if(!postadd){return};
if(!postadd.value){showError(postadd,'*Enter your postal
address')};
return valid;
}
// display error messages
function showError(obj,message){
if(!obj.errorNode){
obj.onchange=hideError;
var span=document.createElement('span');
span.appendChild(document.createTextNode
(message));
obj.parentNode.appendChild(span);
obj.errorNode=span;
}
valid=false;
return
}
// hide error messages
function hideError(){
this.parentNode.removeChild(this.errorNode);
this.errorNode=null;
this.onchange=null;
}
// execute 'ValidateForm()' function when page is loaded
window.onload=function(){
// check if browser is W3CDOM compatible
if(document.getElementById&&document.
getElementsByTagName&&document.createElement){
var theform=document.
getElementsByTagName('form')[0];
if(theform){theform.onsubmit=function()
{return validateForm(this)}};
}
}
</script>
</head>
<body>
<div id="formcontainer">
<form action="processform.php" method="post">
First Name <input type="text" name="fname" /><br />
Last Name <input type="text" name="lname" /><br />
Email <input type="text" name="email" /><br />
Age <input type="text" name="age" /><br />
Postal Address <input type="text" name="paddress" /><br/>
Comments<br/>
<textarea rows="10" cols="20"></textarea><br />
<input type="submit" name="send" value="Send Data" />
</form>
</div>
</body>
</html>
Final thoughts
In this first article of the series, I hopefully demonstrated how to create a simple form validation script that uses the DOM for displaying and removing error messages from the corresponding web document. But if you're already tested the script, certainly you'll have noticed that it really lacks a good look and feeling.
That's why in the next part, I'll be adding some styles to the original form checking application so it can look a bit more polished and professional. Meet you in the next article!
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JavaScript 
A Complete Example of Form Validation with the DOM
