webpack.config.js

const path = require('path');

module.exports = {
  entry: './src/index.js',
  output: {
    path: path.join(__dirname, 'build'),
    filename: 'index.js',
    libraryTarget: 'commonjs2'
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: 'babel-loader'
      }
    ]
  },
  externals : {
    react: 'commonjs react'
  },
};