webpack.common.js

const path = require('path');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');

module.exports = {
  entry: ['./src/index.js'],
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: 'index.js',
    libraryTarget: 'commonjs2' // THIS IS THE MOST IMPORTANT LINE! :mindblow: I wasted more than 2 days until realize this was the line most important in all this guide.
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: "styles.css",
      disable: process.env.NODE_ENV === "development"
    })
  ],
  module: {
    rules: [
      {
        test: /\.js$/,
        include: path.resolve(__dirname, 'src'),
        exclude: /(node_modules|bower_components|dist)/,
        use: {
          loader: 'babel-loader'
        }
      },
      {
        test: /\.less$|\.css$/,
        use: [
          {
            loader: MiniCssExtractPlugin.loader,
            options: {
              hmr: process.env.NODE_ENV === 'development',
            }
          },
          {
            loader: "css-loader",
          },
          {
            loader: "less-loader",
            options: {
              javascriptEnabled: true
            }
          }],
      }
    ]
  },
  externals: {
    // this line is just to use the React dependency of our
    // parent-testing-project instead of using our own React.
    'react': 'commonjs react'
  }
};