webpack.config.js

var path = require('path');
module.exports = {
    mode: 'production',
    entry: './index.js',
    output: {
        path: path.resolve(__dirname, 'build'),
        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.
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                include: path.resolve(__dirname, 'src'),
                exclude: /(node_modules|bower_components|build)/,
                use: {
                    loader: 'babel-loader',
                    options: {
                        presets: ['env']
                    }
                }
            }
        ]
    },
    externals: {
        'react': 'commonjs react'
        // this line is just to use the React dependency of our 
        // parent-testing-project instead of using our own React.
    }
};