一、问题

react 项目中，通过 ·process.env. 在 .env 或 .env.development 中读取不到变量，undefined

二、解决

• 首先，确保 .env 和 .env.development 文件放在项目根目录（和 package.json 同级)，而不是 src 目录
• 其次，必须以 REACT_APP_ 开头建立自定义环境变量（同理，vue项目是以 VUE_APP_ 开头）
  • bad：API_URL=http://localhost:8080
  • good：REACT_APP_API_URL=http://localhost:8080

• build 读取 .env 中变量
• start 读取 .env.development 中变量

三、拓展学习

Adding Custom Environment Variables | Create React App

• What other .env files can be used?

Note: this feature is available with react-scripts@1.0.0 and higher.

.env：Default.
.env.local：Local overrides. This file is loaded for all environments except test.
.env.development, .env.test, .env.production：Environment-specific settings.
.env.development.local, .env.test.local, .env.production.local：Local overrides of environment-specific settings.

• Files on the left have more priority than files on the right:

npm start: .env.development.local, .env.local, .env.development, .env
npm run build: .env.production.local, .env.local, .env.production, .env
npm test: .env.test.local, .env.test, .env (note .env.local is missing)

These variables will act as the defaults if the machine does not explicitly set them.