AJAX GET request function

http-get-async.js

// Send a 'GET' request to the specified url and run the callback function when it completes.
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);
  xmlHttp.send(null);
}