webpack.config.js

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

module.exports = {
  mode: "production",
  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.
  },
  module: {
    rules: [
      {
        test: /\.(js|jsx)$/,
        exclude: /node_modules/,
        use: ["babel-loader"]
      },
      {
        test: /\.css$/,
        include: path.resolve(__dirname, "src"),
        exclude: /node_modules/,
        use: ["style-loader", "css-loader"]
      },
      {
        test: /\.(jpe?g|png|gif|mp3|svg|ico)$/,
        use: {
          loader: "file-loader",
          options: {}
        }
      }
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: "style.css"
    })
  ],
  externals: {
    react: {
      commonjs: "react",
      commonjs2: "react",
      amd: "React",
      root: "React"
    },
  }
};