readyState vs status==200

Question

xmlhttp.onreadystatechange = function() {     if (xmlhttp.readyState == 4 && xmlhttp.status == 200)     {         document.getElementById("myDiv").innerHTML = xmlhttp.responseText;     } } 

Above code is from:http://www.w3schools.com/ajax/ajax_xmlhttprequest_onreadystatechange.asp.

Question:

According to this tutorial:

readyState: 4: request finished and response is ready   status: 200: "OK"   When readyState is 4 and status is 200, the response is ready: 

since when xmlhttp.readyState == 4, response is ready, why do we still need xmlhttp.status == 200? what is the difference between xmlhttp.readyState == 4 and xmlhttp.status == 200?

Answer 1

The status of the response, xhr.status, is (generally) used to determine whether the request was successful or not. xhr.readyState is simply used to determine the state of the request, such as "has not yet been sent" (0), "complete and response received" (4), etc.

The server is responsible for providing the status, while the user agent provides the readyState.

Answer 2

status indicates if server response is ok.
In general words, when you got an status

500 - 599: the server had an error

400 - 499: this is a client error (Ex: 404 page not found)

300 - 399: then exists a redirect

200 - 299: then it is correct and

100 - 199: means information message

Then the status==200 is getting you a message where the server says: 'Hey man I do the work!'