完整错误提示：

错误分析：这句警告意思是module文件夹中没有express框架相关依赖包。

解决方案：只需要运行命令自行安装express依赖包即可。

PS D:\WEB前端\案例\前端工程化：ES6模块化和webpack打包\webpack_study> npm install express
npm WARN webpack_study@1.0.0 No description
npm WARN webpack_study@1.0.0 No repository field.

+ express@4.17.1
added 50 packages from 37 contributors in 2.751s

成功运行：

PS D:\WEB前端\案例\前端工程化：ES6模块化和webpack打包\webpack_study> nodemon .\app.js
[nodemon] 2.0.7
[nodemon] to restart at any time, enter `rs`
[nodemon] watching path(s): *.*
[nodemon] watching extensions: js,mjs,json
[nodemon] starting `node .\app.js`
服务器连接！