IE8 is breaking my AJAX... FF is fine

Feeling very proud of myself after creating a form with an AJAX submit, I test it in IE8 and get "Message: 'quantity' is undefined". I've read that it could be something to do with the fact that earlier versions of IE used ActiveX for AJAX requests, but I'm very new to JS a开发者_如何学Gond have no real understanding of the problem, let alone the ability to implement a fix.

Here's my code:

var time_variable;

function getXMLObject()  //XML OBJECT
{
  var xmlHttp = false;
   try {
     xmlHttp = new ActiveXObject("Msxml2.XMLHTTP")  // For Old Microsoft Browsers
   }
   catch (e) {
     try {
       xmlHttp = new ActiveXObject("Microsoft.XMLHTTP")  // For Microsoft IE 6.0+
     }
     catch (e2) {
       xmlHttp = false   // No Browser accepts the XMLHTTP Object then false
     }
   }
   if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
     xmlHttp = new XMLHttpRequest();        //For Mozilla, Opera Browsers
   }
   return xmlHttp;  // Mandatory Statement returning the ajax object created
}

var xmlhttp = new getXMLObject();   //xmlhttp holds the ajax object

function ajaxFunction() {
  var getdate = new Date();  //Used to prevent caching during ajax call
  if(xmlhttp) { 
    var txtname = document.getElementById("txtname");
    xmlhttp.open("POST","slots.php",true); //calling     testing.php using POST method
    xmlhttp.onreadystatechange  = handleServerResponse;
    xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xmlhttp.send("quantity=" + quantity.value + "&price=" + price.value + "&slot=" +         slot.value + "&store=" + store.value); //Posting txtname to PHP File


  }
}

function handleServerResponse() {
   if (xmlhttp.readyState == 4) {
     if(xmlhttp.status == 200) {
       document.getElementById("message").innerHTML=xmlhttp.responseText; //Update the     HTML Form element 
     }
     else {
        alert("Error during AJAX call. Please try again");
     }
   }
}

From your last comment on your question, I suspect you are not defining 'quantity' anywhere and assuming that it will reference the form field. Try this:

if(xmlhttp) { 
    var txtname = document.getElementById("txtname");
    var quantity = document.getElementById("quantity");
    var price = document.getElementById("price");
    var store = document.getElementById("store");
    xmlhttp.open("POST","slots.php",true); //calling     testing.php using POST method
    xmlhttp.onreadystatechange  = handleServerResponse;
    xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    xmlhttp.send("quantity=" + quantity.value + "&price=" + price.value + "&slot=" +               slot.value + "&store=" + store.value); //Posting txtname to PHP File
}

If quantity is a form field you need to get it using getElementById before using it just like you did with txtname:

var quantity = document.getElementById("quantity");

You cant use it directly from the form.