title: What is the underscore for in the query string of a jQuery jsonp ajax request?
body: <p>When I look at the query string from a jsonp request (client code below), there are 2 objects, a "callback" string that you need to use in the response (so the client codes directs to the success handler) and one with a key of <code>_</code>... what is this underscore for? I can't find any reference to this in any documentation, it appears to be a number of some kind. </p>
  
  <p>I though that it might be used to direct to the error handler (either on its on, in combination with the callback, or replacing the number after the underscore in the callback string), but it doesn't appear to be.</p>
  
  <pre><code>url = 'http://localhost:11767/Handlers/MyHandler.ashx';
  </code></pre>
  
  <p>...</p>
  
  <pre><code>$.ajax({
      url: url,
      dataType: "jsonp",
      error: function (jqXHR, textStatus, errorThrown) {
          //...
      },
      success : function(d) {
          //...
      }
  });
  </code></pre>
  
  <p>or</p>
  
  <pre><code>    $.getJSON(url + "?callback=?", function(d) {
      }).success(function(d) {
          //...
      }).error(function(jqXHR, textStatus, errorThrown) {
          //...
      }).complete(function(d) {
          //...
      });
  </code></pre>
  
  <p>Side note in case this helps anyone reading this: as this is a jsonp request, error will only be hit if the exception occurs client side, e.g. there is a timeout or a problem with the formatting of response (i.e. not using the callback), to overcome this, I always log and swallow the exceptions in the handlers, but give a standard response object (which all response are made up of) that has a state property of exception and a message property.</p>