1: Create an xmlhttp object

Firstly we need to create an xmlhttp object to handle data returned by the web-server, the only problem is that Mozilla (the engine behind Firefox for instance) uses XMLHttpRequest and IE uses an ActiveX object that might be called Msxml2.XMLHTTP or Microsoft.XMLHTTP depending on the version of IE.

However, rather than test for the browser it's more robust to test for the functionality, we use a try...catch to first see if the browser is Mozilla, if that fails then check if it's the latest IE, and if it fails try an older IE. Of course if all three fail we're done for!

Here's some pseudocode that shows the outline idea:

create a variable called httpRequest
to hold the request object
if (window understands XMLHttpRequest) { // Mozilla, Safari
   httpRequest = new XMLHttpRequest object;
}
else if (window understands ActiveXObject) { // IE
   try to create {
      httpRequest = new ActiveX Object
                    using Msxml2.XMLHTTP;
   }
   catch any errors and do this instead {
      try to create {
         httpRequest = new ActiveX Object
                       using Microsoft.XMLHTTP;
      }
      catch any errors
   }
}
if (there is no httpRequest object by now) {
   alert('Giving up :( Cannot create an httpRequest');
}

2: Send request to the url

Once we have an httpRequest object -- be it the Mozilla or IE flavour -- we make the request to the server

open the httpRequest giving:
           the method,
           the url,
           and whether asynchronous;
send the httpRequest with any parameters

3: Request completes, do something

When the call completes successfully it will execute whichever function has been stored into the httpRequest object's onreadystatechange attribute. In fact you assign a function to this attribute before you open and send the request, so we'll need to go back a stage and write and add such a function.

set the onreadystatechange of the httpRequest to a function then
open the httpRequest giving:
           the method,
           the url,
           and whether asynchronous;
send the httpRequest with any parameters
define the function to be triggered by the state change
which will do the following:
   if ( the readyState is complete ) {
      if ( the status is ok) {
         do something with the response from the server;
      } else {
         give some error message;
      }
   }
}