jQuery AJAX setup test

HTML

<button type="button">Click Me</button>

CSS

body.loading {
  background-color: #123456;
}

JavaScript

var
    body= $('body'),
    btn = $('button'),
    dfd
;

jQuery.ajaxSetup({
  beforeSend: function() {
     body.addClass('loading');
  },
  complete: function(){
     body.removeClass('loading');
  },
  success: function() {}
});

btn.click(function() {
    dfd = $.ajax({
        url: 'http://jsfiddle.net/'
    });
});