Generics support

[ChrRubin]

Would it be possible to introduce TypeScript generic type parameters support? TS recognizes @template tags from JSDoc annotations for type parameters, if that would be of any help.

From what I've tested, sveld would currently generate prop and slot typings with the type parameter(s) included, but because the exported class and interface do not have a type parameter list, it is invalid.

[metonym]

I like this idea.

Could you illustrate a use case?

One that comes to mind would be a generic for a prop, but I'm curious how it'd look for events/slots.

import type { TypeaheadProps } from "svelte-typeahead/src/Typeahead.svelte";

let extract: TypeaheadProps<{ id: number }>["extract"] = (item) => item;

[ChrRubin]

Generic events and slots would be very useful when they are tied to generic-typed props.

Using svelte-virtual-scroll-list as an example for slots:

<script lang="ts">
  import VirtualScroll from 'svelte-virtual-scroll-list';

  let items = [{ id: 0, text: 'zero'}, { id: 1, text: 'one'}];
</script>

<VirtualScroll data={items} key="id" let:data>
  <div>{data.id}</div>
  <div>{data.text}<div>
</VirtualScroll>

Since sveld currently doesn't support generics, the typings for the data prop and slot prop would be any[] and any respectively, which is not ideal in a TypeScript environment (the slot prop is currently generated as any[] due to them sharing the same name, but that's a separate issue).

For events, generics can be used when the dispatched detail object is based on another generic-typed prop(s):

<script>
  import { createEventDispatcher } from "svelte";

  /**
   * @template T
   * @type {T[]}
   */
  export let items = [];

  /**
   * @event {T} bar
   */
  const dispatch = createEventDispatcher();

  function foo(i) {
    dispatch('bar', items[i])
  }
</script>