Keep your core webpack config simple and mutate it with ES.whatever Object Rest/Spread

webpack.mutate.js

import webpack from 'webpack';
import config from './webpack.config';
import { mapObject } from './web/util/util';

export const original = config;

/**
 * Development config. Inspired by
 * https://github.com/gaearon/react-transform-boilerplate/blob/master/webpack.config.dev.js
 */
export const development = {
  ...config,
  debug: true,
  devtool: 'source-map',
  entry: mapObject(config.entry, ([ name, sources ]) => {
    const newSources = [
      'eventsource-polyfill', // necessary for hot reloading with IE
      'webpack-hot-middleware/client',
      ...sources,
    ];
    return [ name, newSources ];
  }),
  output: {
    ...config.output,
    pathinfo: true,
  },
  plugins: [
    ...(config.plugins || []),
    new webpack.HotModuleReplacementPlugin(),
    new webpack.NoErrorsPlugin(),
  ],
};

/**
 * Production config. Inspired by
 * https://github.com/gaearon/react-transform-boilerplate/blob/master/webpack.config.prod.js
 */
export const production = {
  ...config,
  plugins: [
    ...(config.plugins || []),
    new webpack.optimize.OccurenceOrderPlugin(),
    new webpack.DefinePlugin({
      'process.env': {
        NODE_ENV: JSON.stringify('production'),
      },
    }),
    new webpack.optimize.UglifyJsPlugin({
      compressor: {
        warnings: false,
      },
    }),
  ],
};