The markup strip on its own

strip_markup_ts is the second step of the plugin’s build, exported for tools that ship Svelte source another way. Run it on the output of Svelte’s preprocess(), when the script is already JS but the template still has types:

import { preprocess } from 'svelte/compiler';
import { strip_markup_ts } from '@ogygia/tsdown-plugin-svelte';

const { code } = await preprocess(source, preprocessors, { filename });
const plain_js = strip_markup_ts(code, filename);

It parses the file with Svelte’s own parser, as TypeScript, and cuts out every type-only span in the template: as and satisfies casts, ! assertions, annotations and generic arguments. It also removes lang="ts" from each script. Strings and comments that only look like types are left alone, because nothing is matched by pattern. filename is passed to the parser, for its error messages.

The lang="ts" attribute is how it knows the template is TypeScript: preprocess() leaves it on the script, and a file without it is returned unchanged. Everything that is not a type stays byte for byte as it was.