A naive ajax function

ajax.js

// @credit to http://stackoverflow.com/questions/8567114/how-to-make-an-ajax-call-without-jquery
function ajax(options) {
  if (!options) {
    throw new Error('Ajax: options is missing');
  }
  if (!options.url) {
    throw new Error('Ajax: options.url is missing');
  }
  var async = options.async === undefined ? true : (!!options.async);
  var type = options.type || 'GET';
  var url = options.url;
  var data = options.data ? options.data : null;
  var contentType = 'application/x-www-form-urlencoded; charset=UTF-8';
  var xhr;

  if (window.XMLHttpRequest) {
    xhr =new XMLHttpRequest();
  } else { // <= IE6
    xhr = new ActiveXObject('Microsoft.XMLHTTP');
  }

  xhr.open(type, url, async);
  xhr.setRequestHeader('Content-Type', contentType);

  xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
      if (xhr.status === 200) {
        if (typeof options.success === 'function') {
          options.success.call(null, xhr.responseText);
        }
      } else {
        if (typeof options.error === 'function') {
          options.error.call(null, xhr, xhr.status);
        }
      }
    }
  }

  xhr.send(data);
}