More recently, it's been designed so this is the case. Namespaces, enums, and the property constructor shortcut thing were all added relatively early on, before the philosophy of "just JS + types" had been fully defined.

These days, TypeScript will only add new features if they are either JavaScript features that have reached consensus (stage 3 iirc), or exist at the type system only.

There have been attempts to add type hints directly to JavaScript, so that you really could run something like TypeScript in the browser directly (with the types being automatically stripped out), but this causes a lot of additional parsing complexity and so nothing's really come of it yet. There's also the question of how useful it would even be in the end, given you can get much the same effect by using TypeScript's JSDoc-based annotations instead of `.ts` files, if you really need to be able to run your source code directly.