storybook/app/react-native/src/server/middleware.js

import fs from 'fs';
import path from 'path';
import { Router } from 'express';
import webpack from 'webpack';
import webpackDevMiddleware from 'webpack-dev-middleware';
import webpackHotMiddleware from 'webpack-hot-middleware';
import baseConfig from './config/webpack.config';
import baseProductionConfig from './config/webpack.config.prod';
import loadConfig from './config';

function getMiddleware(configDir) {
  const middlewarePath = path.resolve(configDir, 'middleware.js');
  if (fs.existsSync(middlewarePath)) {
    let middlewareModule = require(middlewarePath); // eslint-disable-line
    if (middlewareModule.__esModule) { // eslint-disable-line
      middlewareModule = middlewareModule.default;
    }
    return middlewareModule;
  }
  return () => {};
}

export default function({ projectDir, configDir, ...options }) {
  // Build the webpack configuration using the `baseConfig`
  // custom `.babelrc` file and `webpack.config.js` files
  const environment = options.environment || 'DEVELOPMENT';
  const isProd = environment === 'PRODUCTION';
  const currentWebpackConfig = isProd ? baseProductionConfig(options) : baseConfig(options);
  const config = loadConfig(environment, currentWebpackConfig, projectDir, configDir);

  // remove the leading '/'
  let publicPath = config.output.publicPath;
  if (publicPath[0] === '/') {
    publicPath = publicPath.slice(1);
  }

  const compiler = webpack(config);
  const devMiddlewareOptions = {
    noInfo: true,
    publicPath: config.output.publicPath,
    watchOptions: config.watchOptions || {},
  };

  const router = new Router();
  const middlewareFn = getMiddleware(configDir);
  middlewareFn(router);

  router.use(webpackDevMiddleware(compiler, devMiddlewareOptions));

  if (!isProd) {
    router.use(webpackHotMiddleware(compiler));
  }

  router.get('/', (req, res) => {
    res.set('Content-Type', 'text/html');
    res.sendFile(path.join(`${__dirname}/public/index.html`));
  });

  return router;
}