Basic ajax handler function in JavaScript without jQuery

ajax.js

function ajax(url, method, data, async) {
  method = typeof method !== 'undefined' ? method : 'GET';
  async = typeof async !== 'undefined' ? async : false;

  if (window.XMLHttpRequest) {
    var xhReq = new XMLHttpRequest();
  }
  else {
    var xhReq = new ActiveXObject('Microsoft.XMLHTTP');
  }

  if(typeof data != 'string') {
    var query = [];
    for(var key in data) {
      query.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
    }
    data = query.join('&');
  }

  if(method == 'POST') {
    xhReq.open(method, url, async);
    xhReq.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhReq.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    if(async === true && xhReq.readyState === 4 && xhReq.status === 200) {
      callback(xhReq);
    }
    xhReq.send(data);
  }
  else {
    if(typeof data !== 'undefined' && data !== null) {
      url = url+'?'+data;
    }
    xhReq.open(method, url, async);
    xhReq.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
    xhReq.send(null);

    if(method == 'GET') {
      return xhReq.responseText;
    }
  }

}