Webpack’s externals work with local paths too

index.js

// my-app/webpack.config.js
{
  externals: {
    'jquery': 'jQuery',
    /* ↑ With this line, Webpack will replace every
     *    import $ from 'jquery';
     *
     * in your code with approximately
     *    const $ = window.jQuery;
     *
     * This is useful if you already have jQuery in your global environment
     * and don’t want to download it several times.
     * Most people use `externals` this way. */
    
    '../foo/index.js': 'MyFoo',  
    /* ↑ Turns out, aliasing local paths works too!
     * This:
     *   import F from '../foo/index.js';
     * 
     * will be replaced approximatey with 
     *   const F = window.MyFoo;
     *
     * Useful e. g. if you need to share your module between your old code in the global environment
     * and your new code in the current app bundle. */
  }
}