page.params from $app/state is not reactive where as from $app/stores is reactive

[mkmoisen]

Describe the bug

I apologize if this is something simple, I have read the documentation on $app/state/page and believe that this should work the same as it does for $app/stores/state but it does not.

It seems that $app/state/page lacks the reactivity that $app/stores/page has.

/foos
    [fooId]
        +page.svelte

<script>
    import { page } from '$app/state';
    
    $: fooId = page.params.fooId;
</script>

<h1>{fooId}</h1>

<a href="/foos/1">1</a>
<a href="/foos/2">2</a>

Clicking on the link here will no longer cause the page.params.fooId to change. The URL is correctly updated when clicking on the link, but $fooId = page.params.fooId does not react.

This works works fine when using { page } from '$app/stores':

    import { page } from '$app/stores';
    $: fooId = $page.params.fooId;

Reproduction

/foos
    [fooId]
        +page.svelte

<script>
    import { page } from '$app/state';
    
    $: fooId = page.params.fooId;
</script>

    <h1>{fooId}</h1>

    <a href="/foos/1">1</a>
    <a href="/foos/2">2</a>

Logs

No response

System Info

npmPackages:
    @sveltejs/adapter-static: ^3.0.6 => 3.0.8
    @sveltejs/kit: ^2.0.0 => 2.15.2
    @sveltejs/vite-plugin-svelte: ^4.0.0 => 4.0.4
    svelte: ^5.0.0 => 5.16.6
    vite: ^5.4.11 => 5.4.11

Severity

blocking an upgrade

Additional Information

No response

[mkmoisen]

This does work when using svelte5 $derived runes:

<script>
    import { page } from '$app/state';
    
    let fooId = $derived(page.params.fooId);
</script>

<h1>{fooId}</h1>

<a href="/foos/1">1</a>
<a href="/foos/2">2</a>

And this works too:

<script>
    import { page } from '$app/stores';
    
    let fooId = $derived($page.params.fooId);
</script>

<h1>{fooId}</h1>

<a href="/foos/1">1</a>
<a href="/foos/2">2</a>

But why won't it work using $ reactivity with /app/stores/page?

[brunnerh]

$app/state uses rune-based reactivity while $: uses legacy compile-time-based reactivity.
They do not work together because the $app/state is more fine-grained, the generated output would only observe the top level object, i.e. page which itself does not change.

$.legacy_pre_effect(() => (page), () => {
	$.set(fooId, page.params.fooId);
});

(If you force runes mode via <svelte:options runes /> you will also get an error because $: is not supported in the new system.)

$app/state is meant to be used with runes, so you should use $derived here.

[mkmoisen]

@brunnerh

That makes sense, thank you.

Would it be worthwhile to update the sveltekit $app/state/page documentation to give some examples of how it should be used differently compared to $app/stores/page?

https://svelte.dev/docs/kit/$app-state#page

Another question, I see that that $app/stores/page is deprecated. This would have the effect of encouraging us to move to Svelte5 style runes.

Is svelte planning on getting rid of the legacy $ reactivity? If so then the deprecation of $app/stores/page makes sense. But if not, then perhaps $app/stores/page should not be deprecated?

[brunnerh]

Would it be worthwhile to update the sveltekit $app/state/page documentation to give some examples of how it should be used differently compared to $app/stores/page?

Possibly.

Is svelte planning on getting rid of the legacy $ reactivity?

Yes, it will eventually be removed in some later major version of Svelte.

[jdhuntington]

Would it be worthwhile to update the sveltekit $app/state/page documentation to give some examples of how it should be used differently compared to $app/stores/page?

Would heartily endorse it if at all possible! I'm new to svelte and updating a page based on URL params is the first interesting thing I've tried to do. I've spent many hours trying to figure out why the most basic thing isn't working. I suspect this is a thing on the critical path for beginners and some docs here would have high ROI.