webpack打包报错The ‘mode‘ option has not been set,

 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.

配置中的 WARNING
'mode' 选项尚未设置，webpack 将针对此值回退到 'production'。
将 'mode' 选项设置为 'development' 或 'production' 以启用每个环境的默认值。
您还可以将其设置为 'none' 以禁用任何默认行为。

解决办法： 

1.在package.json文件

  "scripts": {
    "dev": "webpack --mode development",
    "build": "webpack --mode production"
  },

2.在webpack.config.js

module.exports={
    mode: 'development' ,//设置mode开发环境还是生产环境
}