AJAX with memoization

asyncRequest.js

//
// Author: Dustin Diaz
// http://dustindiaz.com/faster-ajax/
//
var asyncRequest = function() {
  function handleReadyState(o, callback, fallback) {
    if (!o || o.readyState != 4) return;
    if (o.status == 200) {
      if (callback) {
        callback(o);
      }
    } else if (fallback) {
      fallback(o);
    } 
  }
  var getXHR = function() {
    var http;
    try {
      http = new XMLHttpRequest;
      getXHR = function() {
        return new XMLHttpRequest;
      };
    }
    catch(e) {
      var msxml = [
        'MSXML2.XMLHTTP.3.0', 
        'MSXML2.XMLHTTP', 
        'Microsoft.XMLHTTP'
      ];
      for (var i=0, len = msxml.length; i < len; ++i) {
        try {
          http = new ActiveXObject(msxml[i]);
          getXHR = function() {
            return new ActiveXObject(msxml[i]);
          };
          break;
        }
        catch(e) {}
      }
    }
    return http;
  };
  return function(method, uri, postData, callback, fallback) {
    var http = getXHR();
    http.open(method, uri, true);
    http.onreadystatechange = function() {
      handleReadyState(http, callback, fallback);
    };
    http.send(postData || null);
    return http;
  };
}();