Ajax without jquery

ajax.js

/**
 * Taken from http://stackoverflow.com/a/18078705 with minor tweaks.
 */
var ajax = {};

/**
 * Attempt to get the XMLHttpRequest object. If unavailable try fallbacks of increasing age.
 */
ajax.x = function() {
  if (typeof XMLHttpRequest !== 'undefined') {
    return new XMLHttpRequest();
  }
  var versions = [
    "MSXML2.XmlHttp.5.0",
    "MSXML2.XmlHttp.4.0",
    "MSXML2.XmlHttp.3.0",
    "MSXML2.XmlHttp.2.0",
    "Microsoft.XmlHttp"
  ];

  var xhr;
  for(var i = 0; i < versions.length; i++) {
    try {
      xhr = new ActiveXObject(versions[i]);
        break;
    } catch (e) {}
  }
  return xhr;
};

/**
 * Send the request.
 *
 * @param url - Send the request to url.
 * @param headers - A list of header tuples. The tulpe is [<header-type>, <header-value>].
 * @param callback - Callback function to handle the response.
 * @param method - The type of request, generally either 'POST' or 'GET'.
 * @param data - Data to send as part of the request.
 * @param async - True if the request should be asynchronous, False if synchronous. Default is true.
 */
ajax.send = function(url, headers, callback, method, data, async) {
  if (typeof async === 'undefined') async = true;  // default to async
  var x = ajax.x();
  x.open(method, url, async);
  x.onreadystatechange = function() {
    if (x.readyState == 4) {
      callback(x.responseText)
    }
  };
  if (method == 'POST') {
    x.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
  }
  for (var i in headers) {
      x.setRequestHeader(header[i][0], header[i][1]);
  }
  x.send(data)
};

/**
 * Send a GET request
 *
 * @param url - Send the get request to url.
 * @param data - Dictionary of key, value parameters to send.
 * @param headers - A list of header tuples. The tulpe is [<header-type>, <header-value>].
 * @param callback - Callback function to handle the response.
 * @param async - True if the request should be asynchronous, False if synchronous. Default is true.
 */
ajax.get = function(url, data, headers, callback, async) {
  var query = [];
  for (var key in data) {
    query.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
  }
  ajax.send(url + '?' + query.join('&'), headers, callback, 'GET', null, async)
};

/**
 * Send a POST request
 *
 * @param url - Send the get request to url.
 * @param data - Dictionary of key, value parameters to send.
 * @param headers - A list of header tuples. The tulpe is [<header-type>, <header-value>].
 * @param callback - Callback function to handle the response.
 * @param async - True if the request should be asynchronous, False if synchronous. Default is true.
 */
ajax.post = function(url, data, headers, callback, async) {
  var query = [];
  for (var key in data) {
    query.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
  }
  ajax.send(url, headers, callback, 'POST', query.join('&'), async)
};