How it works

The plugin does its work in two build hooks, buildStart and writeBundle, so tsdown (or rolldown) stays in charge of your .ts files and their declarations.

1. Preprocess. At the start of the build, every .svelte under src is run through your preprocessors, which turns its <script lang="ts"> into JS.
2. Strip the markup. Svelte’s parser reads the file once more, still as TypeScript, and every type-only span in the template is cut out: as and satisfies casts, ! assertions, annotations and generic arguments. lang="ts" is removed from each script. A relative ./x.ts import in the component becomes ./x.js, the built module it sits beside, the same way tsdown rewrites your .ts sources.
3. Emit. The result is emitted as a build asset at the same path in out, and each plain .css file under src is emitted as it is. Emitting assets, rather than writing files, means tsdown’s clean never wipes them.
4. Types. After the bundle is written, svelte2tsx generates declarations into a scratch folder, and only the *.svelte.d.ts files are copied next to the components (with the same ./x.ts → ./x.js rewrite). The scratch folder is removed afterwards.

What ships is a .svelte file with plain JS in its script, no TypeScript in its template, and its styles as written. Your users’ Svelte compiles it like any component in their own app.