AJAX = Asynchronous JavaScript and XML（异步的 JavaScript 和 XML）

    1. const ajax = (method, url, data, success, fail) => {
    2.   var request = new XMLHttpRequest()
    3.   request.open(method, url);
    4.   // onreadystatechange 准备状态变化
    5.   request.onreadystatechange = function () {
    6.     // 表示下载完成
    7.     if(request.readyState === 4) {
    8.       // request.status === 304 没有变更
    9.       if(request.status >= 200 && request.status < 300 || request.status === 304) {
    10.         success(request)
    11.       }else{
    12.         fail(request)
    13.       }
    14.     }
    15.   };
    16.   request.send();
    17. }