In JavaScript, possible readyState values after sending an XMLHttpRequest are:
0 = open has not yet been called
1 = send has not yet been called but open has been called
2 = send has been called but no response from server
3 = data is in the process of being received from the server
4 = response from server has arrived
Related articles:
JavaScript Creating XMLHttpRequest Object for Ajax
JavaScript XMLHttpRequest send Method: Send a URL in Ajax
JavaScript XMLHttpRequest Send Method: Send a URL in Ajax
JavaScript XMLHttpRequest onreadystatechange for Ajax requests
