jQuery: Handle fallback for failed AJAX Request

You will need to either use the lower level $.ajax call, or the ajaxError function. Here it is with the $.ajax method:

function update() {
  $.ajax({
    type: 'GET',
    dataType: 'json',
    url: url,
    timeout: 5000,
    success: function(data, textStatus ){
       alert('request successful');
    },
    fail: function(xhr, textStatus, errorThrown){
       alert('request failed');
    }
  });
}

EDIT I added a timeout to the $.ajax call and set it to five seconds.