Building a request, step by step

First step: create an instance

This is just a classical instance of class, but two options must be tried, for browser compatibility.

if (window.XMLHttpRequest)     // Object of the current windows
{ 
    xhr = new XMLHttpRequest();     // Firefox, Safari, ...
} 
else 
 if (window.ActiveXObject)   // ActiveX version
 {
    xhr = new ActiveXObject("Microsoft.XMLHTTP");  // Internet Explorer 
 } 

Or exceptions may be used instead:

try {
   xhr = new ActiveXObject("Microsoft.XMLHTTP");    // Trying Internet Explorer 
}
catch(e)    // Failed 
{
  xhr = new XMLHttpRequest();    // Other browsers.
}

Second step: wait for the response

The response and further processing are included in a function and the return of the function will be assigned to the onreadystatechange attribute of the object previously created.

xhr.onreadystatechange = function() { // instructions to process the response };                   

if (xhr.readyState == 4)
{
   // Received, OK
} else 
{
  // Wait...
}

Third step: make the request itself

Two methods of XMLHttpRequest are used:
- open: command GET or POST, URL of the document, true for asynchronous.
- send: with POST only, the data to send to the server.

The request below read a document on the server.

xhr.open('GET', 'http://www.xul.fr/somefile.xml', true);                  
xhr.send(null);