use hidden iframe to check whether the url alive or dead

helpers.js

  function checkUrl(url, callback)
  {
      var iframe = document.createElement('IFRAME');

      iframe.src = url;
      iframe.style.visibility = 'hidden';

      document.body.appendChild(iframe);

      iframe.onload = function()
      {
        if(callback)
        try
        {
          callback(typeof iframe.contentWindow.document!=="null");  
        }catch(e)
        {
          callback(false);
        }
        
        document.body.removeChild(iframe);
      }

  }