My XML HTTP Request function

xhr.js

/*jslint browser: true, windows: true, indent: 2 */
// async: whether the request is asynchronuos or not
// cached: set to false to ignore browser cache
// method: the http method to use
// data: key value pairs, e.g. foo=1&bar=2
// url: the absolute or relative url to send the request to
// success: the success function, will receive the XHR object
// failure: the failure function, will receive the XHR object
// prepare: set to a function to set additional parameters to the xhr object, it should return the xhr object
(function (w) {
  'use strict';
  var HttpRequest = function (options) {
      var that = this,
        def = {
          async: true,
          cached: true,
          method: 'GET',
          data: null,
          url: '',
          success: function () {},
          failure: function () {},
          prepare: null
        },
        prop;
      for (prop in def) {
        if (def.hasOwnProperty(prop) && options[prop] === undefined) {
          options[prop] = def[prop];
        }
      }
      this.options = options;
      this.request = null;
      if (window.XMLHttpRequest) {
        this.request = new XMLHttpRequest();
      } else if (window.ActiveXObject) {
        try {
          this.request = new ActiveXObject('Msxml2.XMLHTTP');
        } catch (e1) {
          try {
            this.request = new ActiveXObject('Microsoft.XMLHTTP');
          } catch (e2) {}
        }
      }
      if (this.options.cached === false) {
        this.options.url += ((/\?/).test(this.options.url) ? '&' : '?') + (new Date()).getTime();
      }
      if (typeof this.options.prepare === 'function') {
        this.request = this.options.prepare(this.request);
      }
      this.request.onreadystatechange = function () {
        if (that.request.readyState === 4) {
          if (that.request.status === 200) {
            that.options.success(that.request);
          } else {
            that.options.failure(that.request);
          }
        }
      };
    };
  w.HttpRequest = HttpRequest;
  HttpRequest.prototype.send = function () {
    this.request.open(this.options.method, this.options.url, this.options.async);
    if (typeof this.options.data === 'string' && this.options.data.length > 0) {
      this.request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    }
    this.request.send(this.options.data);
  };
}(window));

Usage example:

var xhr = new HttpRequest({
  async: true,
  method: 'POST',
  data: 'foo=bar',
  url: 'api.php',
  success: function (xhr) { console.log(xhr); },
  failure: function (xhr) { console.warn(xhr); }
});