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：

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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"));

        }

    })

}