TypeScript is a structural typing model vs nominal one, such as Java, which we are more used to. Not that one is more sound than the other, just different approaches.

I actually like the structural system a lot, and found that combined typescript expressiveness, typing a code base with TS is much more intent driven and requires much less contortionism that a class-for-everything nominal system such as Java. In fact, we ported one of our java backend to node/TS, 40% less code, and as typed if not more. Also, there are simple ways to bring some of the nominal characteristics when needed, but in a language like JS where object literal is a intrasic part of the language, a structural typing system is great fit.

So, yes, their are different approach, but not sure soundness is a good description of those differences.

Now, if TypeScript is used to code JavaScript ”a la” Java, then yes, it could be seen as a loss of functionality.