Ajax : Why success displays 0?

Question

I use ajax with jquery and when I tried to display the result in popup. Then alert always displays 0

success : function(results) { 
alert(results); }

EDIT :

 var dataToSend = {
    action:"ACTION_NAME",
    id : "12"
  };
  url_action = 'http://www.______.com/wp-admin/admin-ajax.php';
  $.ajax({
          url: url_action,
          type:'POST',
          data: dataToSend,
          success:function(results)
          {
             alert(results);
          }
   });

Answer 1

Try to add die(); or exit(); at the very last line of the function.