手写 AJAX

平时其实很少用到，但是面试可能会问到相关问题，故而写博客来方便自己记忆和查看。怎么实现 AJAX，或者手写 AJAX，代码如下：

const ajax = (method, url, data, 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();
}