Question

I am requesting a simple php file that echos a single line; using AJAX, but I am getting a HTTP 501 response from my localhost server on linux. My question is, What could be causing this kind of error?

JavaScript:

var request = new XMLHttpRequest();

request.onreadystatechange = function () {
    //console.log(request.responseText);
    if (request.readyState == 4 && request.status == 200) {
        console.log(0);
    } else if (request.status == 501) {
        console.log(request.responseText);
    }
}
request.open('test.php', 'GET', true);
request.send();

test.php:

<?php echo 'this is a test'; ?>

Answer 1

As far as know, you can not detect server the status before readyState. If it’s 4 (so DONE) then can check the server status.

So you need to check first readyState before checking status.

if (req.readyState === 4) {
    // 200 is not enough to detect successful requests
    // cos there are many success status codes
    if (req.status === 200) {
        // every thing ok
    } else {
        // something is wrong with server?
    }
}

Have a look HTTP status code here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

Maybe this could be useful for you: https://github.com/qeremy/mii/blob/master/mii.ajax.js#L248