ขอถามเรื่องการใช้ function clear ใน ajax shopping cart หน่อยครับ

<script language="JavaScript">
	   var HttPRequest = false;

	   function doCallAjax(ProductID,Qty) {
		  HttPRequest = false;
		  if (window.XMLHttpRequest) { // Mozilla, Safari,...
			 HttPRequest = new XMLHttpRequest();
			 if (HttPRequest.overrideMimeType) {
				HttPRequest.overrideMimeType('text/html');
			 }
		  } else if (window.ActiveXObject) { // IE
			 try {
				HttPRequest = new ActiveXObject("Msxml2.XMLHTTP");
			 } catch (e) {
				try {
				   HttPRequest = new ActiveXObject("Microsoft.XMLHTTP");
				} catch (e) {}
			 }
		  } 
		  
		  if (!HttPRequest) {
			 alert('Cannot create XMLHTTP instance');
			 return false;
		  }
	
		  var url = 'AjaxPHPShoppingCart2.php';
		  var pmeters = "tProductID=" + ProductID+
						"&tQty=" + Qty;
			HttPRequest.open('POST',url,true);

			HttPRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
			HttPRequest.setRequestHeader("Content-length", pmeters.length);
			HttPRequest.setRequestHeader("Connection", "close");
			HttPRequest.send(pmeters);
			
			
			HttPRequest.onreadystatechange = function()
			{

				 if(HttPRequest.readyState == 3)  // Loading Request
				  {
				   document.getElementById("mySpan").innerHTML = "Now is Loading...";
				  }

				 if(HttPRequest.readyState == 4) // Return Request
				  {			  
					  document.getElementById('mySpan').innerHTML = HttPRequest.responseText;
				  }				
			}

	   }
	   
	   function CheckOut()
	   {
	   window.location = 'AjaxPHPShoppingCart3.php';
	   }
	</script>

<?php
	/*** By Weerachai Nukitram ***/
	/***  http://www.ThaiCreate.Com ***/

	session_start();
	$strProductID = $_POST["tProductID"];
	$strQty = $_POST["tQty"];	

	$objConnect = mysql_connect("localhost","root","root") or die("Error Connect to Database");
	$objDB = mysql_select_db("mydatabase");

	if($strProductID != "" and $strQty  != "")
	{
		$strSQL = "INSERT INTO cart ";
		$strSQL .="(SID,ProductID,Qty) ";
		$strSQL .="VALUES ";
		$strSQL .="('".session_id()."','".$strProductID."','".$strQty."') ";
		$objQuery = mysql_query($strSQL);
	}
?>
<center>
<h1>Your Cart</h1>
<table width="408" border="1" cellspacing="0" cellpadding="0">
  <tr>
    <td width="51"><div align="center">ID</div></td>
    <td width="154" height="26"><div align="center">Product</div></td>
    <td width="69"><div align="center">Price</div></td>
    <td width="57"><div align="center">Qty</div></td>
    <td width="65"><div align="center">Total</div></td>
  </tr>
<?
$intSumTotal = 0;
$intRows = 0;
$strSQL = "SELECT * FROM cart  WHERE SID = '".session_id()."' ";
$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
while($objResult = mysql_fetch_array($objQuery))
{
	$intRows ++;
	//*** Product ***//
	$strSQL = "SELECT * FROM product  WHERE ProductID = '".$objResult["ProductID"]."' ";
	$objQueryPro = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
	$objResultPro = mysql_fetch_array($objQueryPro);
	$intTotal = $objResult["Qty"] * $objResultPro["Price"];
	$intSumTotal = $intSumTotal + $intTotal;
?>
  <tr>
    <td><div align="center"><?=$intRows;?></div></td>
    <td><?=$objResultPro["ProductName"];?></td>
    <td><div align="right"><?=number_format($objResultPro["Price"],2);?></div></td>
    <td><div align="center"><?=$objResult["Qty"];?></div></td>
    <td><div align="right"><?=number_format($intTotal,2);?></div></td>
  </tr>
<?
}
?>    
  <tr>
    <td colspan="4"><div align="right">Total Amount </div></td>
    <td>
	<div align="right"><?=number_format($intSumTotal,2);?></div></td>
  </tr>
</table>
<?
if($intSumTotal > 0)
{
?>
<br><input name="btnCheckOut" type="submit" id="btnCheckOut" value="Check Out" onClick="JavaScript:CheckOut();">
<?
}
?>
</center>

$("form").live('submit',function(){
 var productid=$("input[name=productid]");
 var qty=$("input[name=qty]");
 if(productid.val()==""){
   productid.focus();
   return false;
 }
 if(qty.val()==""){
   qty.focus();
   return false;
 }
 $.ajax({
   url:"AjaxPHPShoppingCart2.php",
   type:"POST",
   data:"tProductID=" + productid.val()+"&tQty=" + qty.val();
   success:function(res){
    $("#mySpan").html(res);
    productid.val("");  // clear ค่า
    qty.val(""); // clear ค่า
  },
  error:function(err){
   alert("error : "+err);
  }
 });
 return false; // ไม่ให้มัน refresh
});