The developer who wrote it was a smart guy, but he had never worked on any other JS project. All state was stored in the DOM in custom attributes, .addEventListeners EVERYWHERE... I joke that it was as if you took a monk, gave him a book about javascript, and then locked him in a cell for 10 years.
I started refactoring pieces into web components, and after about 6 months had removed 50k lines of code. Now knowing enough about the app, I started a complete rewrite. The rewrite is about 80% feature parity, and is around 17k lines of code (not counting libraries like Vue/pinia/etc).
So, soon, I shall have removed over 200,000 loc from the project. I feel like then I should retire as I will never top that.