First... AJAX is a methodology, not a language. The language is JavaScript. The whole process looks like this.

As the request is processed the ready states change. The current state can be found in the readyState property...

• 1 - is when the request object is created.
• 2 - means the request object has been sent
• 3 - the request object is comming back but not all back
• 4 - the response is received.

The response property is null until the ready state is set to 4. You will get a null object and a statusCode of undefined.

• OnReadyStateChange - Executes when the state changes (see states). This is usually a function.
• readyState - the current state (see states)
• response - what the server said.
• responseText - the response in text form (null until readyState is 4)
• responseType - the type of response (null until readyState is 4)
• responseURL - the url the response came from
• responseXML - the response as an XML document (null until readyState is 4)
• status - HTTP status code received from the server e.g. 200 or 404
• statusText - HTTP status code description from the server e.g. OK or NOT FOUND
• timeout - how long to wait for the response before giving up
• upload - an upload object (XMLHttpRequestUpload)
• withCredentials - a boolean that indicates if cross site access control request should be made with cred

These are the methods of the request.

• abort()
• getAllResponseHeaders()
• getResponseHeader(String name)
• open(String [method], String [url[, async[, username[, password]]]])
• overrideMimeType(String mimeType)
• send(String body)
• setRequestHeader(String name, String value)

req.onError = function(){    alert(An error just happend) }

These are the Events of the request.

• abort - abort method was called
• error - an error occurs
• load - request completes successfully
• loadEnd - request completes regardless of status
• loadStart - request starts to load data
• progress - request recieves more data
• timeout - the timeout property has been exceeded

Events can be loaded by using the addEventListener(eventName) method OR by loading a call back in the on[eventName] property.