请使用Promise封装Ajax操作
原始的Ajax操作如下：

原型：

var onSuccess = function(result){}; //成功的回调
var onFail = function(error){}; //失败的回调
var req = new XMLHttpRequest();
req.open("POST", "www.baidu.com", true);
req.onload = function(){
  if(req.readyState === 4 && req.status === 200){
    onSuccess(req.response);
  } else {
    onFail(req.statusText);
  }
}
req.onerror = function(){
  onFail(Error("网络异常"));
}

封装后：

return new Promise(function(resolve, reject){
   var req = new XMLHttpRequest();
   req.open("POST", "<a href="http://www.baidu.com" target="_blank">www.baidu.com", true);
   req.onload = function(){
     if(req.readyState === 4 && req.status === 200){
       resolve(req.response);
     } else {
       reject(req.statusText);
     }
   }
   req.onerror = function(){
     reject(Error("网络异常"));
   }
 });</a>