1、安装create-react-app：npm/cnpm installl create-react-app -g
2、创建项目：create-react-app my-first-app
3、此时项目里包含一个registerServiceWorker.js文件，作用是什么呢？
service worker是在后台运行的一个线程，可以用来处理离线缓存、消息推送、后台自动更新等任务。registerServiceWorker就是为react项目注册了一个service worker，用来做资源的缓存，这样你下次访问时，就可以更快的获取资源。而且因为资源被缓存，所以即使在离线的情况下也可以访问应用（此时使用的资源是之前缓存的资源）。注意，registerServiceWorker注册的service worker 只在生产环境中生效（process.env.NODE_ENV === 'production'），所以开发的时候，可以注释掉，当然生产模式下，也可以不用这个功能。
// In production, we register a service worker to serve assets from local cache.
// This lets the app load faster on subsequent visits in production, and gives
// it offline capabilities. However, it also means that developers (and users)
// will only see deployed updates on the "N+1" visit to a page, since previously
// cached resources are updated in the background.
// To learn more about the benefits of this model, read https://goo.gl/KwvDNy.
// This link also includes instructions on opting out of this behavior.
参考:
1、https://segmentfault.com/q/1010000010667462