webpack.config.js

var path = require('path');
module.exports = {
  entry: './src/index.js',
  output: {
    path: path.resolve(__dirname, 'build'),
    filename: 'index.js',
    libraryTarget: 'umd'
  },
  module: {
    rules: [
      {
        test: /\.(js|jsx)$/,
        include: path.resolve(__dirname, 'src'),
        exclude: /(node_modules|bower_components|build)/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: [
              'es2015',
              'react',
              'stage-2'
            ]
          }
        }
      }
    ]
  },
  externals: {
    react: {
        commonjs: "react",
        commonjs2: "react",
        amd: "React",
        root: "React"
    },
    "react-dom": {
        commonjs: "react-dom",
        commonjs2: "react-dom",
        amd: "ReactDOM",
        root: "ReactDOM"
    }
  }
};