Add a .js file and routes

If file saved as /public/js/menu.js and baseUrl equal http://domain.com/
<?= base_URL('js/menu.js'); ?> return http://domain.com/js/menu.js

This is a working option. If you get the wrong address in the source code, it means an error in the configuration. Your problem does not mean that the framework is bad, but just ignorance of its work. In 20 years, you could look at what is output to the source code and find a solution

Simple CI 4 project for beginners codeigniter-expenses ( topic )