ajax xhr对象状态

I have a weird problem, that the xmlhttpDelUser.status is never being received 200 in a simple html file on my local machine, but the same code works in my google chrome app. Request is made to a remote server i.e. twitter.

Any guess?

Thanks for help.

xmlhttpDelUser=new XMLHttpRequest();
xmlhttpDelUser.onreadystatechange=function()
{   
    if (xmlhttpDelUser.readyState==4 && xmlhttpDelUser.status==200)
    {   
        xmlTreeDelUser=xmlhttpDelUser.responseText;
        alert(xmlTreeDelUser);
        document.getElementById("res").innerHTML="hell";    
    }   
}