webpack 4打包

webpack 3中 是以下方式打包：

webpack ./src/index.js ./dist/bundle.js

 但4中该方式会报错：

WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for this value. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/concepts/mode/

ERROR in multi ./src/index.js ./dist/bundle.js
Module not found: Error: Can't resolve './dist/bundle.js' in 'E:\文档\webproject\homework\webpack'
 @ multi ./src/index.js ./dist/bundle.js main[1]

 以上提示未指定模式

在 package.json 中 scripts 中加入以下两行代码即可解决：

"dev": "webpack --mode development",
"build": "webpack --mode production"

然后 

npm run build//(或dev)

 提示 ：原本js 必须是index.js (webpack4规定的 ) 不然也会报错