ajax.js

(function(w) {
  "use strict";
  
  function ajax(method, url, callback, data) {
    return (function(x) {
      x.onreadystatechange = function() {
        x.readyState ^ 4 || callback(this);
      };
      x.open(method, url, callback);
      x.send(data);
      return x;
    })(new XMLHttpRequest());
  }
  w.ajax = ajax;
})(window);