The 'mode' option has not been set, webpack will fallback to 'production' for this value.绝对行，改不了来找我

在学习webpack的过程中，打包过程出现这个错误The ‘mode’ option has not been set, webpack will fallback to ‘production’ for this value.一开始我的命令是写：webpack ./src/main.js ./dist/bundle.js

原因是webpack4.0以上版本更改了命令，要写成webpack .\src\main.js -o ./dist/bundle.js才是正确的