When I am writing plain JS I often end up having to write loads of tests to check that I don't to wrong things with types. Did I remember to consider null/undefined? What if I sent in something totally different?

With TS I don't need to spend as much time with these types of tests. I just let the compiler do it for it. If my function says in it's signature the argument can't be null I don't need to do null-checks.