My experience is that you always use a type system. If the language doesn't include one, you'll end up making a bunch of extra tests and control code that in practice is constraining your data to resemble a type. It's a worse system: more verbose, harder to refactor and it's way easier to miss corner cases.

It's either that or assuming that your data will contain exactly what you want to, ignoring every possibility outside the happy path, which is a recipe for disaster.

That said, I have to say that modern JS produces the cleanest looking and less verbose code I've ever worked with so far. I wish there was a way to work with types without adding verbosity and harming readability.