ajax with xhr cannot run success callback function

I am writing a html page which uses ajax with xhr to upload a form data（including text and files）to the nodejs server.
But after nodejs server receives the data and send back a string "1" to the frontend.The js code cannot run its success callback function.

function show_status(status) {
     alert(status);
     if (status == '1') {
         $('#status').text('Add game succeed!！');
     } else {
         $('#status').text('Add game failed!！');
     }
 }
 $(document).ready(function() {
     $('#addformonline').submit(function(e) {
         $('#status').text('');
         $.ajax({
             type: 'POST',
             dataType: 'text',
             xhr:function() {
                 myXhr = $.ajaxSettings.xhr();
                 return myXhr;
             },
             data: new FormData($('#addformonline')),
             success: show_status,
             error: function(){alert('error');}
          });
          return false;
       }); 
  });