event loop

一、以一道题目开始

参考： https://juejin.im/post/5c8a024d51882546be0a3082

https://nodejs.org/zh-cn/docs/guides/event-loop-timers-and-nexttick/

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/EventLoop

http://www.ruanyifeng.com/blog/2018/02/node-event-loop.html

https://zhuanlan.zhihu.com/p/24460769