I'm pretty comfortable with JavaScript, Python and C#. You would think I would love TypeScript, but I specifically mainly use JS when its native to the browser, I hate the nightmare that is maintaining JS tools and packages. I can understand why someone might want to avoid the ceremony of setting up a JS project and just use as much OOTB as possible.
They are using tsc though. It’s all type-checked. In my experience using tsc on JavaScript files is a good experience. But you don’t get the full feature set.