Senior probably means they've been doing JS for decades before TS was invented. They got along just fine without it for so many years, why rock the boat now? Or maybe it is more of a Senior person has a ton of work to do and not enough time to do it in so there isn't time left to learn a whole new way of writing JS. Or maybe many of the errors TS is meant to catch, the Senior developer has already learned over the decades not to do that bad thing, so they don't see the need to adopt TS. Or maybe they are just jaded and tired of learning new things.
I've been programming JS for 13 years (professionally, and some more as a hobby) and I'm pretty aware of its warts. And as a firm believer of TDD, most of my code is pretty well tested.
Unit tests have similar advantages to types: they catch errors early and let you refactor your code with confidence.
But unit tests go even further, because they test the _correctness_ of the returned value, not just its type.
And a massive turnoff for me is that the types are compile-time only.
So you see, if your code is tested, and the types are just partial static checks; I feel like it's just not worth it.
I enjoy using types in other languages, for example it feels nice to add type hinting to old PHP codebases. But in PHP, types are much simpler (less work) and more effective.
If they added optional run-time instrumentation like "spec" in Clojure, I would definitely use TS. But for now, I'm out.
You should be testing regardless of the strength of your type system.
But testing with a weak type system isn't going to give you anywhere near the guarantees you'd get from a well-designed type system in addition to testing: https://kevinmahoney.co.uk/articles/tests-vs-types/
Your IDE should be catching any typos. But it's not about typos, it's about understanding the quirky nature of JS and knowing what things not to do. For instance, an inexperienced developer might try to use .sort() to sort an array of numbers. It wouldn't even occur to an experienced developer to try that since they know from vast past experience that it won't work (or, at least, won't do what the inexperience developer is thinking it might do).
