Ajax响应始终在第一次调用后返回2

Question

I'm trying to use Ajax to get an updated num from my DB.
The main function is :

function checkInvitations() {
    if (username ="-1") {
        username = document.getElementById("username").value;
    }
    req.open('GET', 'check_num_of_invitations.php?username='+username);
    req.onreadystatechange = handleNumOfInvitationsResponse;
    req.send(null);
}

The function is called from the HTML file ...
The called function ( handleNumOfInvitationsResponse ) :

function handleNumOfInvitationsResponse() {
if(req.readyState == 4) {
    // update div in HTML
} else {
    // update div in HTML
}
setTimeout(function(){checkInvitations();}, 5000);

}

Notice the recursive call to checkInvitations .

On the first try, the response returns 4 and everything works, from there on it returns with 2.
Thank you

Answer 1

I had a similar issue in Internet Explorer 9 and I solved it by appending a timestamp to query string:

 var url = 'check_num_of_invitations.php?username='+username+'&timestamp='+Date.now();
 req.open('GET', url);