<!--------- Ajax JS ------------>
var http = createRequestObject();
function createRequestObject()
{
	var ro;
	var browser = navigator.appName;
	if(browser == "Microsoft Internet Explorer")
	{
		ro = new ActiveXObject("Microsoft.XMLHTTP");
	}
	else
	{
		ro = new XMLHttpRequest();
	}
	return ro;
}
function GetXmlHttpObject()
{
	var xmlHttp=null;
	try
 	{
 		// Firefox, Opera 8.0+, Safari
 		xmlHttp=new XMLHttpRequest();
 	}
	catch (e)
 	{
		//Internet Explorer
		try { xmlHttp=new ActiveXObject("Msxml2.XMLHTTP"); }
		catch (e) { xmlHttp=new ActiveXObject("Microsoft.XMLHTTP"); }
 	}
 	return xmlHttp;
}
<!--------- Ajax JS ------------>

function getnetprice()
{
	var discount = document.getElementById('couponcode').value;
	xmlHttp=GetXmlHttpObject()
	if (xmlHttp==null)
	{
		 alert ("Browser does not support HTTP Request")
		 return
	}
	
	if(discount == '')
	{
		document.getElementById('msgdiv').innerHTML = "Enter discount code.";
	}
	else
	{
		var url="netpriceajax.php";
		url=url+"?code="+discount;
		xmlHttp.onreadystatechange=netprice;
		xmlHttp.open("GET",url,true);
		xmlHttp.send(null); //
	}
}
function netprice()
{  
	if(xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
	{ 
		  var ar = xmlHttp.responseText.split('~');
		  if(ar[1] == 'true')
		  {
		  	document.getElementById("netprice").value = ar[0];
			document.getElementById("netpriceval").innerHTML = ar[0];
			document.getElementById("getdiscountblock").style.display ="none";
		  }
		  else
		  {
		  	document.getElementById('msgdiv').innerHTML = "Invalid code entered.";
		  	document.getElementById("getdiscountblock").style.display ="block";
		  }
	}
}

function cartshow()
{
	if(xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
	{
		var res = xmlHttp.responseText;
		if(res == 'ok')
			window.location.href = "cart.php";
		else
			alert(xmlHttp.responseText);
	}
}
function addtocart(productid)
{
	xmlHttp=GetXmlHttpObject()
	if (xmlHttp==null)
	{
		 alert ("Browser does not support HTTP Request")
		 return
	}
	else
	{
		var url="addtocart.php";
		url=url+"?productid="+productid;
		//alert(url);
		xmlHttp.onreadystatechange=cartshow;
		xmlHttp.open("GET",url,true);
		xmlHttp.send(null); 
	}
}