Yes, precisely so. Type definitions are as good as the programmers who wrote them, and since humans are imperfect they make mistakes. After you have defined you

Yes, precisely so. Type definitions are as good as the programmers who wrote them, and since humans are imperfect they make mistakes. After you have defined your types wrong, there are devious bugs in your code that you have no way of knowing (until you step on that mine and the program hopefully crashes). Although every language by design is mostly bug-free, it's that in JS/TS there is a much greater human factor (similar to C/C++) that just adds a large error margin for the correctness of the application. Especially when there are very loose runtime guarantees for type-safety, there is always a chance for something ugly to happen.