function shorten(ref, callback) {
  var url = "/api/" + escape(ref);
  var req = new XMLHttpRequest(); 
  req.onreadystatechange = function shortenIt() {
    if ( this.readyState == 4 && this.status == 200 ) {
      callback(req.responseText);
    };
  };
  req.open( "GET", url );
  req.send();
};