study promise mdn-promise promises-aplus 相关博客 http://bruce-xu.github.io/blogs/js/promise http://fex.baidu.com/blog/2015/07/we-have-a-problem-with-promises/ http://jser.me/2016/04/24/Promise%E7%9A%84%E9%94%99%E8%AF%AF%E5%A4%84%E7%90%86.html