function xdr(url, method, data, callback, errback) {
    var req;
    if (!callback)
      callback = function(){};
    if (!errback)
      errback = function(){};

    if(XMLHttpRequest && ('withCredentials' in (req = new XMLHttpRequest()))) {
      req.open(method, url, true);
      req.withCredentials = "true";
      req.onerror = errback;
      req.onreadystatechange = function() {
        if (req.readyState === 4) {
          if (req.status >= 200 && req.status < 400) {
            callback(req.responseText);
          } else {
            errback(new Error('Response returned with non-OK status'), req);
          }
        }
      };
      req.setRequestHeader("Accept", "application/json");
      if (data)
        req.setRequestHeader("Content-type", "application/json");
      req.send(data);
    } else if(XDomainRequest) {
      var reqUrl = url;
      if (url.indexOf('http:') === 0) {
        reqUrl = url.substr(5);
      }
      else if (url.indexOf('https:') === 0) {
        reqUrl = url.substr(6);
      }
      req = new XDomainRequest();
      req.open(method, reqUrl);
      req.onerror = function(err) { errback(err, req); };
      req.onload = function() {
        callback(req.responseText);
      };
      req.ontimeout = function() {};
      req.onprogress = function() {};
      req.timeout = 0;
      req.send(data);
    } else {
      errback(new Error('CORS not supported'));
    }
  };
  function jsonGET(url, callback, errback){
    xdr(url, 'GET', null, function(data){
      callback(JSON.parse(data));
    }, errback);
  }
  function jsonPOST(url, data, callback, errback){
    xdr(url, 'POST', data, function(data){
      callback(JSON.parse(data));
    }, errback);
  }

function startGallery(items, index) {

    var pswpElement = document.querySelectorAll('.pswp')[0];

    var options = {
        index: index || 0
    };

    // Initializes and opens PhotoSwipe
    var gallery = new PhotoSwipe( pswpElement, PhotoSwipeUI_Default, items, options);
    gallery.init();

    return gallery;
}