If you don't mind, can I ask why you use plain JavaScript rather than TypeScript?

The product is truly remarkable, so what technologies used under the hood doesn't matter, but I think TypeScript is natural choice, especially if you 1) start a project from scratch in 2023, 2) have complex local states like choosing equipment and body parts, and proceeding exercises step by step until completed in a single page, and 3) have complex data structure (like exercises, histories, and seamless connection between locally stored data and remotely stored data used only after registering).

edit: typo