What is the value of ajax.responseText when there is no response being sent?

Question

I wrote a javascript function that checks if a username is available in a database. If the username is NOT available the ajax send a text response back which changes some css and adds an error message. When the username is available the ajax doesn't send a response, which is fine but I just need to know what is being returned from ajax.responseText since there is no value. I've tried '' and null.

function _(x) {
return document.getElementById(x);
}

function ajaxObj(meth, url) {
var x = new XMLHttpRequest(); 
x.open( meth, url, true );  
x.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); 
return x;
}
function ajaxReturn(x) {
if(x.readyState == 4 && x.status == 200) {
    return true;
    }
}
function verifyEmail(){

var email = _("email").value;
var status = _("estatus");     
var emailRegEx = /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i;


if (document.signupform.email.value.search(emailRegEx) == -1) {
    _("emaildiv").style.border = "1px solid #d33e3e";
    _("emaildiv").style.backgroundColor = "#fadede";
    status.innerHTML = "<br />Please enter a valid email address";
 } else {
    _("emaildiv").style.border = "";
    _("emaildiv").style.backgroundColor = "";
    status.innerHTML = "";

    if (email != "") {
        status.innerHTML = "checking. . . "; 
        var ajax = ajaxObj("POST", "fan_signup_local.php");    
        ajax.onreadystatechange = function() {
            if (ajaxReturn(ajax) == true) {
                status.innerHTML = ajax.responseText;
                console.log(status.innerHTML);
                if (status.innerHTML !== '') {
                    _("emaildiv").style.border = "1px solid #d33e3e";
                    _("emaildiv").style.backgroundColor = "#fadede";
                    console.log(ajax.responseText);
                } else {
                    _("emaildiv").style.border = "";
                    _("emaildiv").style.backgroundColor = "";
                }
            }
        }
        ajax.send("email="+email);
    }
 }
}

Answer 1

There's always a response, unless the request times out. If the server script exits without printing anything, the response will be an empty string.