手写ajax

const ajax = (method,url,success,fail) => { 
  var request = new XMLHttpRequest();
  request.open(method, url);
  request.onreadystatechange = function () {
    if (request.readyState === 4) {
      if (
        (request.status >= 200 && request.status < 300) ||
        request.status === 304
      ) {
        success(request);
      } else {
        fail(request);
      }
    }
  };
  request.send();
}