support type annotations in javascript as jsdoc comments

[DetachHead]

Describe the problem

while attempting to do a type cast while working around sveltejs/svelte#4701, i noticed that jsdoc comment types aren't supported

Describe the proposed solution

support type casts in jsdoc comments:

<Dropdown items="{/** @type {Foo} */ (items)}" />

Alternatives considered

• wait for [Proposal] Run js expressions in markup template through Svelte script preprocessor code svelte#4701 so i can use an actual type cast ie.

  <Dropdown items="{items as Foo}" />

• suppress the type error using this workaround Support @ts-ignore comments in template markup #1026 (comment)

Importance

would make my life easier

[jasonlyu123]

In non typescript components, i.e. no lang="ts" or something similar. This works in some areas. But it mostly doesn't work in the new transformation.

In typescript component. I don't think this will ever be supported. In the current implementation. The whole svelte component is transformed into a single .tsx file or a .ts file in the new transformation. This means there's no way it can partly be treated as js.
In this case, I would suggest using a function in the script to type assert instead.

<script>
function castAsFoo(_items) {
  return _items as Foo
}
</script>

<Dropdown items={castAsFoo(items)} />

Or use control flow to narrow the type

<script>
function isFoo(_items): items is Foo {
  return 'foo' in items
}
</script>

{#if isFoo(items)}
<Dropdown items={castAsFoo(items)} />
{/if}

{#if  'foo' in items}
<Dropdown items={castAsFoo(items)} />
{/if}