Here are a few form validation scripts I've found usefull for web store. 
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1.  Pull-down menu validation

This script checks all <select> elements, and alerts the customer if he/she
has not chosen a menu item before clicking the submit button.  This script assumes
that the default state of the menu has no value as in this example:

<select name="option|1|0001">
<option value="" selected>SELECT COLOR
<option value="Black|0.50">Black
<option value="Charcoal|0.00">Charcoal
<option value="Ivory|0.00">Ivory


Here's the script:


<script language="JavaScript">

<!--

function validateOptions(){

	var  form = document.forms[0]
    for(var i=0; i<form.elements.length; i++)

        if (form.elements[i].type.indexOf("select")!=-1)
        {
        
            if(form.elements[i].selectedIndex == 0 )
               {
                alert("You forgot to select one or more options.");
       
            return false;
    }
  }
}


//-->

</Script>


Then just add the appropriate event handler to your submit button:


<input type="submit" name="add_to_cart_button" value="ADD TO CART" onClick="return validateOptions()">



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2.  Search Form Validation

This script alerts the user if he/she tries to submit the search form without
entering any key words.  You can use this script for any form with text entry fields.


<script language="JavaScript">

<!--

function validateSearch(){

  var  form = document.forms[0]
  if (form.keywords.value == "") {
  alert("Please specify a search criteria");
  form.keywords.focus();
  return false;
  }else{
  return true;
  }	
}

//-->

</Script>


Then just add the following event handler to your submit button:


<input type="submit" name="search_request_button" value="FIND IT!"  onClick="return validateSearch()"



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3.  FInal Order Form Required Fields validation.


This script alerts the user if he/she did not enter any text into a required field.   The nice thing
about this particular script is that it will alert for any/all input fields left blank.
Make sure and change the definition variables to match your shopping cart.


<script language="JavaScript">
<!--  

function validateOrderform(){
	
	var  form = document.forms[0]
	var errMsg  =  "";
	var bflag  = false;
	

	if (form.elements['01-name'].value == "" ){
		errMsg += "You forgot to fill in your first name. \n";
	}
	

	if (form.elements['12-phone'].value == "" ){
		errMsg += "You forgot to fill in your phone number. \n";
	}
	
	if (form.elements['14-e-mail'].value == "" ){
		errMsg += "You forgot to fill in your email address. \n";
	}

    if (form.elements['02-b_street_address'].value == "" ){
		errMsg += "You forgot to fill in your billing address. \n";
	}

    if (form.elements['03-b_city'].value == "" ){
		errMsg += "You forgot to fill in your city. \n";
	}

	 if (form.elements['04-b_state'].value == "" ){
		errMsg += "You forgot to fill in your state. \n";
	}
	
	 if (form.elements['05-b_zip'].value == "" ){
		errMsg += "You forgot to fill in your zip code. \n";
	}

	if (errMsg == ""){
		bflag = true;
	}
	else {
		alert(errMsg);
	}

	return bflag;
}



Here's the button:


<input type="submit" NAME="confirm_order_button" value="SUBMIT ORDER" onClick="return validateOrderform()">



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4.  Auto Copy Billing information to Mailing fields


This usefull script allows the customer to copy all of order form billing information 
into the mailing fields.


function CopyToShipping()
 {
 
var form = document.forms[0]

form.elements['07-m_street_address'].value = form.elements['02-b_street_address'].value ;
form.elements['08-m_city'].value = form.elements['03-b_city'].value ;
form.elements['09-m_state'].selectedIndex = form.elements['04-b_state'].selectedIndex ;
form.elements['10-m_zip'].value = form.elements['05-b_zip'].value;

 }
 
 
//-->
</script>


Here's the button


<input type="button" value="CLICK TO COPY INFO" OnClick="CopyToShipping()">