webpack.config.js

const path = require('path');

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.
  },
  devtool: 'inline-source-map',
  module: {
    rules: [
      {
        test: /\.js$/,
        include: path.resolve(__dirname, 'src'),
        exclude: /(node_modules|bower_components|dist)/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: ['env'],
            plugins: [
              'transform-object-rest-spread',
              'transform-react-jsx',
              'transform-class-properties'
            ]
          }
        }
      },
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader'
        ]
      }
    ]
  },
  externals: {
    react: 'commonjs react' // this line is just to use the React dependency of our parent-testing-project instead of using our own React.
  }
};