function httpGetAsync(url, callback) {
  var xmlHttp = new XMLHttpRequest();
  xmlHttp.onreadystatechange = function() { 
    if (xmlHttp.readyState == 4 && xmlHttp.status == 200)
      callback(xmlHttp.responseText);
  }
  xmlHttp.open("GET", url, true); // true for asynchronous 
  xmlHttp.send(null);
}