See simple example:

https://typescripttolua.github.io/play/#code/FAMwrgdgxgLglgewgAhAhAKAlMg3sZQmAJwE88DDCokBnBAGwFMA6BhAcwwEYtLkAvtQCGMKAAsMTHPirU6jVuy4AmPlQHBNQA

This results in assignment to global ____returnValue, which is likely not intended.