webpack.common.js

const path = require('path');
const CopyWebpackPlugin = require('copy-webpack-plugin')



module.exports = {
  entry: './src/app.js',
  target: 'web',
  node: {
    fs: 'empty'
  },
  output: {
    path: path.join(__dirname, 'public'),
    filename: 'bundle.js',
    publicPath: '/public/'
  },
  module: {
    rules: [{
      test: /\.js$/,
      include: path.join(__dirname, 'src'),
      exclude: [ /(node_modules|bower_components)/,
                './src/server.js',
                './src/srv-compiled.js'
      ],
      use: {
        loader: 'babel-loader',
        options: {
          presets: ['env']
        }
      }
    },]
  },
  module: {
    rules: [{
        test: /\.scss$/,
        use: [
            "style-loader", // creates style nodes from JS strings
            "css-loader", // translates CSS into CommonJS
            "sass-loader" // compiles Sass to CSS
        ],
    }]
  },
  plugins: [
    new CopyWebpackPlugin([{
        test: /\.(png|jpg|gif)$/,
        from: 'assets/', to: 'public' 
    }
    ]),
  ]
}

webpack.prod.js

const path = require('path');
const webpack = require('webpack');
const merge = require('webpack-merge');
const common = require('./webpack.common.js');
const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const OptimizeCSSAssetsPlugin = require("optimize-css-assets-webpack-plugin");
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

module.exports = merge(common, {
  mode: 'production',
  optimization: {
    minimizer: [
      new UglifyJsPlugin({
        cache: true,
        parallel: true
      }),
      new OptimizeCSSAssetsPlugin({})
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: "[name].css",
      chunkFilename: "[id].css"
    }),
  ],
  module: {
    rules: [
      {
        test: /\.(sa|sc|c)ss$/,
        use: [
          MiniCssExtractPlugin.loader,
          'css-loader',
          'sass-loader',
        ],
      }
    ]
  }
});