Two select boxes linked together in such a way that when a user selects a value in the first select the options in the second one change accordingly.
Implemented in react, vue.js and svelte web frameworks as an example for my medium.com article.
To see it in action, run static-server in the corresponding public directory.
Simply clicking the index.html won't work because of the way javascripts does the importing nowadays.
To build a project see the README.md files in the corresponding directories.