关于前端:普通的ajax请求和使用promise的ajax请求

一般的ajax申请：

    //创立一个对象    let xhr = new XMLHttpRequest();    //申明申请的形式和门路    xhr.open('get',url);    //发送申请    xhr.send();        xhr.onreadystatechange = function(){        if(xhr.readyState == 4 && xhr.status == 200){            console.log(JSON.parse(xhr.responseText));                }    }

应用promise的ajax申请：

    let getJSON = fucntion(url){        let promise = new Promise(function(resolve,reject){            let client = new XMLHttpRequest();            client.open('get',url);            client.onreadystatechange = handler;            client.responseType = 'json';            client.setRequestHeader('Accept','application/json');            client.send();                        function handler(){                if(this.readyState !== 4)return;                if(this.status === 200){                    resolve(this.response);                }else{                    reject(new Error(this.statusText));                }            }        });        return promise;    }    getJSON('url').then(function(){        console.log('内容是：' + json);    },function(){        console.log('出错了',err);    })