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).
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