webpack.server.config.js

var fs = require('fs')
var path = require('path')

module.exports = {

  entry: [
    'babel-polyfill',
    path.resolve(__dirname, 'server.js')
  ],

  output: {
    filename: 'server.bundle.js'
  },

  target: 'node',

  // keep node_module paths out of the bundle
  externals: fs.readdirSync(path.resolve(__dirname, 'node_modules')).concat([
    'react-dom/server'
  ]).reduce(function (ext, mod) {
    ext[mod] = 'commonjs ' + mod
    return ext
  }, {}),

  node: {
    __filename: false,
    __dirname: false
  },

  module: {
    loaders: [
      { test: /\.js$/, 
        exclude: /node_modules/, 
        loader: 'babel-loader',
        query: {
          presets:[ 'es2015', 'react', 'stage-2' ]
        } 
      }
    ]
  }

}