function ajax(url, callback, onError) {
  var req = getAjaxRequest();

  req.onreadystatechange = processRequest;
        
  function getAjaxRequest() {
    if( window.XMLHttpRequest ) {
      return new XMLHttpRequest();
    }
    else if (window.ActiveXObject ) {
      return new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
    
  function processRequest () {
    if( req.readyState == 4 ) {
      if( req.status == 200 ) {
        if( callback ) {
          callback(req);
        }
      }
      else if( onError ) {
        onError(req);
      }
    }
  }

  this.doGet = function() {
    req.open("GET", url, true);
    req.send(null);
  }
    
  this.doPost = function(body) {
    req.open("POST", url, true);
    req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    req.send(body);
  }
}