Svelte 5 Component Library - How to export types for property interfaces

[sdgoldstein]

I'm working on a reusable Svelte 5 component library. When I try to import and use the library, VSCode complains about the types of all the attributes:

Type 'string' is not assignable to type 'never'.ts(2322)

This is how I'm trying to use the component:

<Label for="name_input_id">Name</Label>

This is how I'm defining the prop interface in Label.svelte:

<script module lang="ts">
    import type { ParentComponentProps } from "../../component.js";

    export interface LabelProps extends ParentComponentProps {
        for: string;
    }
</script>

My build command is using sveltekit to build and package the library:

    "build": "vite build && npm run package",
    "package": "svelte-kit sync && svelte-package -i src/main -o build/dist && publint",

The resulting Label.svelte.d.ts file seems to remove my interface defintion:

import { SvelteComponentTyped } from "svelte";
declare const __propDef: {
    props: Record<string, never>;
    events: {
        [evt: string]: CustomEvent<any>;
    };
    slots: {};
};
type LabelProps_ = typeof __propDef.props;
export type LabelEvents = typeof __propDef.events;
export type LabelSlots = typeof __propDef.slots;
export default class Label extends SvelteComponentTyped<LabelProps_, LabelEvents, LabelSlots> {
}
export {};

I'm using the following lib versions:
Svelte - 5.0.0-next.244
@sveltejs/package - 2.3.4
@sveltejs/kit - 2.5.24

Specific questions:

1. Is it possible to export an interface from a .svelte file and expect an appropriate .d.ts file to contain the interface? Or, do I need to place the interface in a separate .ts file and import it into the svelte file?
2. Is this behavior the result of the library versions I'm using? If so, what versions should I be using?
3. Is the issue in the build command/process?
4. Anything that I haven't mentioned/included that might be causing this?

Thanks!

[timephy]

same problem here

[brunnerh]

Not sure whether this should work.

Personally, I would probably not export props interfaces but resolve the type where it's needed via the generic helper types from svelte that you put a component type into (for props that should be ComponentProps).

E.g.

<script lang="ts">
  import Label from '$lib/Label.svelte';
  import type { ComponentProps } from 'svelte';

  let props: ComponentProps<typeof Label> = {
    // ...
  };
</script>

[ryanatkn]

Related, generics cannot currently be used with a standalone props type declaration because of the export limitation.

<script lang="ts" generics="T">

  // works:
  const {v}: {v: T} = $props();

  // fails: Return type of exported function has or is using private name 'Props'.ts(4060)
  interface Props {
    v: T;
  }
  const {v}: Props = $props();

  // fails: Modifiers cannot appear here. If this is a declare statement,
  // move it into <script context="module">..</script>ts(1184)
  export interface Props {

And generics can't be referenced or declared in the module script.