ES6 promise的一个实例

 1 //定义一个请求news的方法
 2     function getNews(url) {
 3         //创建一个promise对象
 4         let promise = new Promise((resolve, reject) => {
 5             //初始化promise状态为pending
 6             //启动异步任务
 7             let request = new XMLHttpRequest();
 8             request.onreadystatechange = function () {
 9                 if(request.readyState === 4){
10                     if(request.status === 200){
11                         let news = request.response;
12                         resolve(news);
13                     }else{
14                         reject('请求失败了。。。');
15                     }
16                 }
17             };
18             request.responseType = 'json';//设置返回的数据类型
19             request.open("GET", url);//规定请求的方法，创建链接
20             request.send();//发送
21         })
22         return promise;
23     }
24 
25     getNews('http://localhost:3000/news?id=2')
26             .then((news) => {
27                 console.log(news);
28                 document.write(JSON.stringify(news));
29                 console.log('http://localhost:3000' + news.commentsUrl);
30                 return getNews('http://localhost:3000' + news.commentsUrl);
31             }, (error) => {
32                 alert(error);
33             })
34             .then((comments) => {
35                 console.log(comments);
36                 document.write('<br><br><br><br><br>' + JSON.stringify(comments));
37             }, (error) => {
38                 alert(error);
39             })