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/

解决方法： 
1.package.json中设置：

"scripts": {    
     "dev": "webpack --mode development",  // 开发环境
     "build": "webpack --mode production",  // 生产环境
  },

2.webpack.config.js中设置：

module.exports = {    
    entry: './src/app.js',    
    output: {        
        path: path.resolve(__dirname, 'dist'),
        filename: 'main.js'
    },
    mode: 'development' // 设置mode
}