1、安裝create-react-app：npm/cnpm installl create-react-app -g
2、創建項目：create-react-app my-first-app
3、此時項目里包含一個registerServiceWorker.js文件，作用是什么呢？
service worker是在后台運行的一個線程，可以用來處理離線緩存、消息推送、后台自動更新等任務。registerServiceWorker就是為react項目注冊了一個service worker，用來做資源的緩存，這樣你下次訪問時，就可以更快的獲取資源。而且因為資源被緩存，所以即使在離線的情況下也可以訪問應用（此時使用的資源是之前緩存的資源）。注意，registerServiceWorker注冊的service worker 只在生產環境中生效（process.env.NODE_ENV === 'production'），所以開發的時候，可以注釋掉，當然生產模式下，也可以不用這個功能。
// 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.
參考:
1、https://segmentfault.com/q/1010000010667462