本项目由 Create React App创建
在项目目录，先执行 npm install 安装依赖
app启动开发模式.
可用浏览器打开 http://localhost:3000 to view it in the browser.
热更,能在console看到lint error
Builds the app for production to the build folder.
It correctly bundles React in production mode and optimizes the build for the best performance.
The build is minified and the filenames include the hashes.
Your app is ready to be deployed!
See the section about deployment for more information.