ES6 promise的一个实例

//定义一个请求news的方法
    function getNews(url) {
        //创建一个promise对象
        let promise = new Promise((resolve, reject) => {
            //初始化promise状态为pending
            //启动异步任务
            let request = new XMLHttpRequest();
            request.onreadystatechange = function () {
                if(request.readyState === 4){
                    if(request.status === 200){
                        let news = request.response;
                        resolve(news);
                    }else{
                        reject('请求失败了。。。');
                    }
                }
            };
            request.responseType = 'json';//设置返回的数据类型
            request.open("GET", url);//规定请求的方法，创建链接
            request.send();//发送
        })
        return promise;
    }

    getNews('http://localhost:3000/news?id=2')
            .then((news) => {
                console.log(news);
                document.write(JSON.stringify(news));
                console.log('http://localhost:3000' + news.commentsUrl);
                return getNews('http://localhost:3000' + news.commentsUrl);
            }, (error) => {
                alert(error);
            })
            .then((comments) => {
                console.log(comments);
                document.write('<br><br><br><br><br>' + JSON.stringify(comments));
            }, (error) => {
                alert(error);
            })