Update: Getting a lot of downvotes (yes, I know against HN 'rules' to comment about that), but I don't get why people dislike my response so much and would love feedback on that (similar to how the OP asked for honest feedback).
What I did was pull the source code, load it up in IDEA, convert the .js files to .ts files. Then looked at the errors. Most of what stood out was that the author included a copy of threejs directly and used parts that TS can't infer types on. Not a huge deal, but makes following the code more difficult for sure. I don't think it would take too much time to update it at all.