手写 AJAX

const ajax = (method, url, data, success, fail) => {
  var request = new XMLHttpRequest()
  request.open(method, url);//设置请求方式和地址
  request.onreadystatechange = function () {//设置回调
    if(request.readyState === 4) {//状态码是4请求成功
      if(request.status >= 200 && request.status < 300 || request.status === 304) {
        success(request)
      }else{
        fail(request)
      }
    }
  };
  request.send();//发送请求
}