Sample javascript implementing a cross platform xmlhttprequest function:

• Method 1 – Public domain XMLHttpRequest Object¹
• Method 2 – XHConn, CC-SA XMLHttpRequest Object² Returning Content Through XMLHttpRequest³

Method 1

In this method, you can instantiate an XMLHttpRequest object that can then call functions from the server. The code is in the public domain.

function getNewHTTPObject()
{
        var xmlhttp;

        /** Special IE only code ... */
        /*@cc_on
          @if (@_jscript_version >= 5)
              try
              {
                  xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
              }
              catch (e)
              {
                  try
                  {
                      xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                  }
                  catch (E)
                  {
                      xmlhttp = false;
                  }
             }
          @else
             xmlhttp = false;
        @end @*/

        /** Every other browser on the planet */
        if (!xmlhttp && typeof XMLHttpRequest != 'undefined')
        {
            try
            {
                xmlhttp = new XMLHttpRequest();
            }
            catch (e)
            {
                xmlhttp = false;
            }
        }

        return xmlhttp;
}

Now you can call XMLHTTPRequest

var xmlHttp = getHTTPObject();

function getDynamicData()
{
  var url = "http://url/that/returns/dynamic/content";

  xmlHttp.open('GET', url, true);
  xmlHttp.onreadystatechange = callbackFunction;
  xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  xmlHttp.send(null);
}

The callback function set in xmlHttp.onreadystatechange will be called when data is back from the server

var syndLinkRequest = getNewHTTPObject();

function callbackFunction()
{
  if (syndLinkRequest.readyState != 4)
    return;

  var result = xmlHttp.responseText;

  /* if you've returned javascript instead of xml or text, 
  you can eval(result) to access the javascript variables returned.
  */
}

Method 2 – XHConn

XHConn is an open source project that attempts to create a very simple and cross platform way to invoke Ajax requests

/** XHConn - Simple XMLHTTP Interface - bfults@gmail.com - 2005-04-08        **
 ** Code licensed under Creative Commons Attribution-ShareAlike License      **
 ** http://creativecommons.org/licenses/by-sa/2.0/                           **/
function XHConn()
{
  var xmlhttp, bComplete = false;
  try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); }
  catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); }
  catch (e) { try { xmlhttp = new XMLHttpRequest(); }
  catch (e) { xmlhttp = false; }}}
  if (!xmlhttp) return null;
  this.connect = function(sURL, sMethod, sVars, fnDone)
  {
    if (!xmlhttp) return false;
    bComplete = false;
    sMethod = sMethod.toUpperCase();
    try {
      if (sMethod == "GET")
      {
        xmlhttp.open(sMethod, sURL+"?"+sVars, true);
        sVars = "";
      }
      else
      {
        xmlhttp.open(sMethod, sURL, true);
        xmlhttp.setRequestHeader("Method", "POST "+sURL+" HTTP/1.1");
        xmlhttp.setRequestHeader("Content-Type",
          "application/x-www-form-urlencoded");
      }
      xmlhttp.onreadystatechange = function(){
        if (xmlhttp.readyState == 4 && !bComplete)
        {
          bComplete = true;
          fnDone(xmlhttp);
        }};
      xmlhttp.send(sVars);
    }
    catch(z) { return false; }
    return true;
  };
  return this;
}

Using XHConn:

//initialize XHConn (if XHConn isn't created successfully, 
//the client doesnt' support Ajax)
var ajaxConn = new XHConn();

//post to mypage.php with args foo and baz
ajaxConn.connect("mypage.php", "POST", 
  "foo=bar&baz=qux",fnWhenDone);

//when the server responds, javascript 
//will trigger this callback function
fnWhenDone(XML)
{
  alert(XML.responseText);
}

Returning content through the XMLHTTPRequest method:

There are various ways you can return content through XMLHTTPRequest:

• Plain text (XML.responseText)
• Javascript variables (eval(XML.responseText))
• JSON – Javascript objects (eval(XML.responseText))
• XML (XML.responseText = XML)

Of these, plain text is the simplest, as all it requires is servicing a request with a simple plain text dump of data. The downside to this method is that it’s not always a clean way to return more than one variable.

Javascript variables are a nice way to return multiple variables through an XMLHTTPRequest call. Instead of dumping plain text, the server decorates the different returned variables with javascript variable declarations. On the client end, the XMLHTTPRequest callback function evals the server response and now has the variables set by the server available to it as native javascript variables.

One key part of this method however is remembering to escape the content in the javascript variable assignment. link

The third way of passing back content through XMLHTTPRequest is XML, which despite the name of the function, is often the slowest and most difficult way due to difficulties parsing xml. However for highly structured content or when E4X becomes more standard, this might be the way to go.