A few months ago I started a job where I inherited a JS code base that is around 250,000 lines. It was one big class, with several sub classes, that did everything. Some files were 30k lines long.

No frameworks, no reactivity. If you click a button, you had to update everything on the screen with event listeners manually.

Took the guy years to write it. It's like a monk got locked in a cell for years with a basic book of javascript.

I started by refactoring into web components, because I had to do it piecemeal. It's been a big help, and I've cut 50k lines of code so far. But the real point was to just learn everything the old code was doing before I start a rewrite.