Svelte 5: TypeScript types for snippets are not removed

[andersekdahl]

Describe the bug

TypeScript types like these:
https://github.com/andersekdahl/svelte5-snippet-repro/blob/main/src/lib/MyComponent.svelte#L4

Are not removed when building, as seen here:
https://github.com/andersekdahl/svelte5-snippet-repro/blob/main/dist/MyComponent.svelte#L3

Reproduction

Created a new library project using sveltekit with the latest svelte 5 release here:
https://github.com/andersekdahl/svelte5-snippet-repro

Running npm run package will build the code and the issue is visible in the dist folder which I also commited to make it easier to spot the problem.

Logs

No response

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (16) x64 11th Gen Intel(R) Core(TM) i9-11950H @ 2.60GHz
    Memory: 11.02 GB / 31.73 GB
  Binaries:
    Node: 20.11.1 - C:\Program Files\nodejs\node.EXE
    npm: 10.2.4 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (122.0.2365.80)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.0.0-next.75

Severity

blocking an upgrade

[brunnerh]

Repository seems to be private.

[andersekdahl]

@brunnerh Wops! Sorry, fixed now.

[tommyminds]

Thanks for the bug report! I'm running into the same issue

[dummdidumm]

This is tricky. The strip happens in @sveltejs/package because historically Svelte was then free of code needing to be preprocessed. But now that Svelte handles TypeScript in the template, just running a TypeScript preprocessor on the code will remove the TS code in the script tag but not in the template. The solution is to

• either keep the lang="ts" tag so that the Svelte parser can recognize TypeScript code, and take the hit of a needless script TypeScript preprocessing
• or to have some way to tell Svelte to remove the TypeScript parts from the template, which probably needs a separate method from svelte/compiler, like transpile
• or it becomes part of preprocess automatically, not sure

I prefer the second or third option, because the first option would mean consumers of the library would be required to install TypeScript and set up the preprocessor for it.

[dummdidumm]

We decided to make this a special-case in @sveltejs/package: lang="ts" will be preserved in it so that the Svelte parser knows how to handle it. As a follow-up we can likely omit preprocessing TypeScript inside Svelte entirely; Svelte itself can handle that now.