It's such a powerful complement to the "no rules" JS when you layer it on top. You really get to have your cake and eat it, too. I feel like there's another huge step to take in "layering" invariant checking on top of TS for QA/test systems... Imagine like "in all 500 of my tests, make sure this variable is an integer greater than 10 at all times no matter what". I'm waiting for this to show up