TypeScript warning when using <svelte:element/> with sveltekit:prefetch attribute

[pascalpp]

Describe the bug

I have a Button component that renders a dynamic tag using svelte:element. Sometimes it renders anchor tags, so I have applied sveltekit:prefetch to the element, but TypeScript complains that svelte:element doesn't support that attribute. The attribute still works as expected: when an anchor tag is rendered, the sveltekit:prefetch attribute is present and triggers the desired prefetching behavior on hover. When a different tag (e.g. button) is rendered, the sveltekit:prefetch attribute is also present, but seems to have no adverse effect.

Is there a 'valid' way to use this attribute on this element? Should the type definition of svelte:element be expanded to allow this usage?

Reproduction

I have a REPL explaining the issue, and the use case behind it, here: https://svelte.dev/repl/bfbeb5c43f21461c9cbd0fea3cd680cb?version=3.49.0

Logs

No response

System Info

System:
    OS: macOS 12.5
    CPU: (10) arm64 Apple M1 Pro
    Memory: 107.34 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.15.1 - /usr/local/bin/node
    Yarn: 1.22.19 - ~/.yarn/bin/yarn
    npm: 8.11.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 104.0.5112.79
    Firefox: 101.0.1
    Safari: 15.6
    Safari Technology Preview: 16.0
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.63 
    @sveltejs/kit: next => 1.0.0-next.393 
    svelte: ^3.44.0 => 3.49.0 
    vite: ^3.0.0 => 3.0.3

Severity

annoyance

Additional Information

No response

[elliott-with-the-longest-name-on-github]

Hmmm. I would think this is correct behavior. In pure TypeScript you could do some type narrowing to make this work, but I'm not sure if there's a way to do that in markup.

In the meantime, I bet this would work:

{#if selected === 'a'}
 <a />
{:else}
<svelte:element />
{/if}

That does unfortunately force you to duplicate the wiring up of props, but I'd think it's the most "correct" way to do what you want to do.

[dummdidumm]

I'd say this is purely a TS error which in this case we should fix, we can't know what svelte:element is so we should allow all attributes. Moving this to language-tools.

[pascalpp]

There are kinda two issues here:

• the typescript error
• sveltekit:prefetch being adding where not needed (e.g. on a button)

so I wonder if the sveltekit:prefetch attribute could be modified to accept an expression, e.g.

<svelte:element this={tag} sveltekit:prefetch={tag === 'a'}>

similar to other 'true if present' attributes, like disabled which can be conditionalized with disabled={expression}.

[elliott-with-the-longest-name-on-github]

I'd say this is purely a TS error which in this case we should fix, we can't know what svelte:element is so we should allow all attributes. Moving this to language-tools.

Interesting -- I would've thought the allowable attributes on an unknown element would be the intersection of allowable attributes for every HTML element, not any, but I suppose being permissive is also a valid approach (and makes more sense now that I think about it!). Thanks. 🙂

[dummdidumm]

By "all attributes" I meant what you mean, all valid attributes of all elements + the special ones (that's the problem here) 👍