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);
})