/**
* 封装原生ajax
* @param url  请求地址
* @param data 数据
* @param fn   方法
*/
function ajax(url, data, fn) {
   const xhr = new XMLHttpRequest();
   xhr.onreadystatechange = function() {
       if (xhr.readyState === 4 && xhr.status === 200) {
           fn(xhr.responseText)
       }
   };
   const formData = new FormData();
   formData.append('file', data);
   xhr.open("post", url, true);
   xhr.send(formData)
}

ajax('update.php', {data:1},function(data) {
   console.log(data);
});