简写一个AJAX

const ajax = (method, url, data, success, fail) => { 
  var request = new XMLHttpRequest()
  request.open(method, url);  //设置get和路径('GET', '/XXX')
  request.onreadystatechange = function () { 
    if(request.readyState === 4) { 
      if(request.status >= 200 && request.status < 300 || request.status === 304) { 
          success(request) 
        }else{ 
          fail(request) 
         } 
      } 
   };
  request.send(); //发送消息体