AJAX - The Complete Reference

88 Part I: Core Ideas

{

alert("Before open method: readyState: " + xhr.readyState );

xhr.open("GET",url,true);

xhr.onreadystatechange = function(){alert("In onreadystatechange

function: readyState: " + xhr.readyState);};

xhr.send(null);

}

The alert is useful as it blocks the progress of the request so you can watch the process

closely. However, if you want to see the progress of a request in a more real-time style, try

the example at http://ajaxref.com/ch3/readystate.html, which is displayed here:

Like many of the details of XHRs, readyState values can be a bit quirky depending

on the code and browser. For example, mysteriously, a readyState value of 2 may not be

seen in Opera browsers, at least in version 9 or before. Moving the position of the

onreadystatechange assignment, very different results will be experienced. Most of

readyState Value Meaning Description

0 Uninitialized The XHR has been instantiated, but the open()

method has not been called yet.

1 Open The XHR has been instantiated and the open()

method called, but send() has not been invoked.

2 Sent The send() method has been called, but no headers

or data have been received yet.

3 Receiving Some data has been received. Looking at headers or

content. This phase of loading may cause an error in

some browsers and not in others.

4 Loaded All the data has been received and can be looked at.

Note that the XHR may enter this state in abort and

error conditions.

TABLE 3-7 readyState Values