理解 `event loop`，相关文章： * [https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/](https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/) * [Event Loop](https://jsblog.insiderattack.net/event-loop-and-the-big-picture-nodejs-event-loop-part-1-1cb67a182810) * [On problems with threads in node.js](https://www.future-processing.pl/blog/on-problems-with-threads-in-node-js/)
