Type safety is easy in a dynamic language. Stop mutating variables and stop mixing types. Suddenly that whole class of errors evaporates. No need to test things that are impossible.

And for those times you want some magic in your life, nothing is stopping you, no need for generics, just proceed with caution and test accordingly.

Which, if we're doing TDD, involves no waiting at all, since it was already written before the implementation. Ideally anyway.

At the end of the day those type exist in JavaScript. If you access ‘first_name’ instead of ‘firstname’ it will not work. The difference is that typescript would let you know before running the code instead of getting an obscure runtime error.