A compiler would tell me what all the types should be and if they are being respected. I'd also write a lot of unit tests to make sure that the code is doing what it is expected to as well as enable refactoring more easily. The code isn't formatted consistently, so that also makes it hard for me to read, I'm kind of OCD about that and having tools like eslint/prettier, which do it automatically, makes this super easy to fix. Linting the code would also point out other issues that the compiler misses. Putting it into CI would ensure that all changes get checked and builds would fail, if there are issues.

So I guess that is what I mean when I say 'clean it up'.