Yeah, that can be confusing. On the other hand, if you write unit / functional tests then testing for invalid inputs one of the first things you'll probably do. Your comment makes me think a fuzzer to test all those falsey values could be useful.
In general, this argument is self-defeating: "I don't need a type checker because I write unit tests." Obviously that means you need a type checker, because then you don't have to write 70-90% of your unit tests. Unit tests require time and energy to produce, run, and maintain. A type-checking compiler can remove much of this burden from the developer.
A lot of JavaScript is UI code though, so unit tests might not be possible, and a suite of comprehensive functional tests with something like Selenium gets dog slow very fast, in my experience.
