Why I Am Getting Request Object Status 0 On Localhost

xhr=new XMLHttpRequest();

var urlDetail="abc.html";
xhr.open("GET",urlDetail,true);

xhr.onreadystatechange=function()
 {
   if(xhr.readyState==4)
    {
      if(xhr.status==200)
        {
           .....
        }
    }
 }
xhr.send(null);