create-react-app/packages/react-dev-utils/redirectServedPathMiddleware.js

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
'use strict';

const path = require('path');

module.exports = function createRedirectServedPathMiddleware(servedPath) {
  // remove end slash so user can land on `/test` instead of `/test/`
  servedPath = servedPath.slice(0, -1);
  return function redirectServedPathMiddleware(req, res, next) {
    if (
      servedPath === '' ||
      req.url === servedPath ||
      req.url.startsWith(servedPath)
    ) {
      next();
    } else {
      const newPath = path.join(servedPath, req.path !== '/' ? req.path : '');
      res.redirect(newPath);
    }
  };
};