title: What is "!!" in Javascript or JQuery
body: <p>I have the following code, can anyone please tell me what "!!" is here. Thank you.</p>
  
  <pre><code>$('#calendar').fullCalendar({
      events: function(start, end, timezone, callback) {
          jQuery.ajax({
              url: 'schedule.php/load',
              type: 'POST',
              dataType: 'json',
              data: {
                  start: start.format(),
                  end: end.format()
              },
              success: function(doc) {
                  var events = [];
                  if(!!doc.result){
                      $.map( doc.result, function( r ) {
                          events.push({
                              id: r.id,
                              title: r.title,
                              start: r.date_start,
                              end: r.date_end
                          });
                      });
                  }
                  callback(events);
              }
          });
      }
  });
  </code></pre>
  
  <p>I did some Google search, unfortunately I could not find such operator. If you know any web site as reference, please let me know.</p>
  
  <p>Thanks.</p>