creact-react-app創建的項目是無法手動指定NODE_ENV的，NODE_ENV在你選擇執行命令時已經確定下來，npm start時，NODE_ENV='development';npm run build 時，NODE_ENV='production'，你無法覆蓋默認選項，下面是creact-react-app官網的原話：

When you run npm start, it is always equal to 'development', when you run npm test it is always equal to 'test', and when you run npm run build to make a production bundle, it is always equal to 'production'. You cannot override NODE_ENV manually. This prevents developers from accidentally deploying a slow development build to production.

如果你非要在npm start時區分環境，只能自己配置webpack，這時候NODE_ENV是可以被修改的。