development.js

const webpack = require('webpack');
const PORT = process.env.PORT || 3000;

module.exports = {
  devtool: 'eval',

  entry: [
    'babel-polyfill',
    `webpack-dev-server/client?http://localhost:${PORT}`,
    'webpack/hot/only-dev-server',
  ],

  devServer: {
    colors: true,
    historyApiFallback: true,
    hot: true,
    inline: true,
    port: PORT,
    progress: true,
    stats: 'errors-only'
  },

  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    new webpack.DefinePlugin({
      'process.env': {
        'NODE_ENV': JSON.stringify('development')
      }
    }),
  ]
};

webpack.config.js

/* eslint-disable prefer-es6-imports */

const merge = require('webpack-merge');
const commonConfig = require('./build/common');
const devConfig = require('./build/development');
const prodConfig = require('./build/production');

const NODE_ENV = process.env.NODE_ENV;

if (!NODE_ENV || !NODE_ENV.length) {
  throw new Error('Either not specified or invalid `NODE_ENV`');
}

const isDev = /dev/i.test(NODE_ENV);
const isProd = /prod/i.test(NODE_ENV);

if(isDev) {
  module.exports = merge(commonConfig, devConfig);
} else if(isProd) {
  module.exports = merge(commonConfig, prodConfig);
} else {
  console.error('"%s" is not a valid `NODE_ENV` value', NODE_ENV);
}