https://github.com/sveltejs/svelte/issues/1639
Still I'm curious.
Typescript and RxJS (or MobX) with Svelte as the afterthought (https://michel.codes/blogs/ui-as-an-afterthought) would be a nice combo.
Internally Svelte uses TS though, the author claims to love TS also! It just does not mix very well with Svelte.
Stencil.js and lit-element are better picks in this regard IIRC.
Obviously Svelte is a JS to JS transformative compiler - you need to make a TS to JS transformative compiler.
I'm hacking on my next app using TypeScript on the backend (particularly around the data layer) and JS/Svelte on the frontend. I would like TS on Svelte, but the rest of Svelte is compelling enough I'd still use it over older tech like React.