用promise 封装 ajax（来自牛客）

请使用Promise封装Ajax操作
原始的Ajax操作如下：

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"));
}

解：------------------------------------------------------------------------------------------------------------------------------------------

const ajax = url => {
    return new Promise((resolve, reject) => {
        let req = new XMLHttpRequest();
        req.open("POST", url, true);
        req.onload = () => {
          if(req.readyState === 4 && req.status === 200){
            resolve(req.response);
          } else {
            reject(req.statusText);
          }
        }
        req.onerror = () => {
          reject(Error("网络异常"));
        }
    })
}
 