webpack.config.js

const webpack = require('webpack')
const path = require('path')

module.exports = {

  entry: {
    // the app bundle
    main: path.join(__dirname, 'app.js'),
    // the vendor bundle, white-list stuff here
    vendor: [ 'react', 'react-dom' ]
  },

  output: {
    path: path.join(__dirname, 'build'),
    // will save "main.js" and "vendor.js"
    filename: `[name].js`,
    pubicPath: 'build/'
  },

  plugins: [
    // this will find anything common between any of your bundles
    // not found in the whitelist you specified and put them in
    // vendor too!
    new webpack.optimize.CommonsChunkPlugin('vendor', `vendor.js`),
  ]

}