promise对象

直接放大佬写的：https://es6.ruanyifeng.com/#docs/promise；

这边写一下怎么用promise去封装请求

var urla =  'http://api.xuandan.com/DataApi/index?AppKey=3aw4643qpb&page=1&cid=';

    var de = '请求失败';

    var ajax = function(url){

      return   new Promise(function(resolve,reject){

                var xhr = new XMLHttpRequest();

                xhr.open('get',url);

                xhr.send();

                xhr.onreadystatechange = function(){

                    if(xhr.readyState == 4 && xhr.status == 200){

                        var res = JSON.parse(xhr.responseText);

                        resolve(res)

                    }else if(xhr.readyState !== 4 && xhr.status !== 200){

                        reject(de)

                    }

                }

        })

    }

    ajax(urla).then(res => {

        console.log(res);

    }).catch(error =>{

        console.log(error);

    })