Simple XMLHttpRequest helper function

ajax.js

function ajax(method, url) {
  var x = (function () {
    return window.XMLHttpRequest
      ? new XMLHttpRequest()
      : window.ActiveXObject
      ? new ActiveXObject("Microsoft.XMLHTTP")
      : null;
  })();

  x.open(method, url, true);

  return {
    header: function (header, value) {
      x.setRequestHeader(header, value);
      return this;
    },
    success: function (callback) {
      x.onreadystatechange = function () {
        x.readyState === 4 && x.responseType === 200 && callback(x);
      };
      return this;
    },
    send: x.send,
  };
}