按需加载UI

fuction showHelp()
{
   var request = new Sys.Net.WebRequest();
   request.set_httpVerb("GET");
   request.set_url('help.aspx');
   request.add_completed(fuction(executor)
   { 
      if (executor.get_responseAvailable())
      {
          var helpDiv = $get('HelpDiv');
          var helpLink = $get('HelpLink');
          var helpLinkBounds = Sys.UI.DomElement.getBounds(helpLink);
          helpDiv.style.top = (helpLinkBounds.y+helpLinkBounds.height)+"px";
          var content = executor.get_responseData();
          helpDiv.innerHtml = content;
          helpDiv.style.display = "block";
      }
   });
   var executor = new Sys.Net.XMLHttpExecutor();
   request.set_executor(executor);
   executor.executeRequest();
}

摘自 asp.net 3.5 构建web 2.0 门户站点，P245

需引用asp.net ajax 的库