Svelte 5: `fromStore` breaks reactivity when used with `@const`

[kosmotema]

Describe the bug

When using the value obtained from fromStore in the @const tag, reactivity starts to work strangely:

• the first update of the store value updates only the block with @const, but not other blocks of the component
• subsequent updates, on the contrary, do not update the @const block, but update other blocks

This behavior is only observed when fromStore is used. This is not the case when using reactive syntax (like $store, inclu) or manual subscription using runes (like $state + $effect)

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACrVUTW-cMBT8KxaNFEijpe1xy66SS6UeemmO3R5Y80itgo2MTVtZ_Pf6gzWY0N0QpQeEsOcNbzzjp6KSVNBG228qonkN0Ta6b5roNhJ_GvPRdlAJ0N8tkxyblazFnDRif6AHQeqGcYE-U8yhBipQyVmNrjepX9k4guuPE_hXyLEgHTwIxsGXBKtLZV9yKvPqQR5dA4RRX_t0a4ngk0aH__QrE3iWjgJpNkpL9UL2g7t3KCHYWmgz2B-7SPf6XGtWkJJAEW0Fl9Dfehtag3mGEUgzyAqmfij0ixORHytA_aDVlaeW1B2KeeC3xWNGW4HsHtr52vhdMj2Oc716TSuDo2x37jT-3akHtwFw4zBTpzXYaenyShotnj622GTur3pDSmTFmFp156oLJrX-Qtdbng2WnJsI3KAPFpcVpLMyDmJ0Mya0JQUkW6SCqt6UaSo1sDqG1FGolJS9DdqJckLIpDjPuNTdifmcXT7Uq-265EB4eY5SCB1_RnFF8M-dihO02yNlGF2hbIpcQBx3dqNDb9F7Y1Hf78mpxSx1LGfzF1zH_yvqUmKuHNtCVMKZMYmLK1mZkxnbJCszuqCh56Tj6QBbfaKPIF7rPlcw3mYtxcRFsw_XORlAV1CWgEXsAjbQOAFHH6_4RNMlycsGwZKrC-N-PglWOrvEOB8F4Qi4aO73_i_NGI9M6AcAAA==

Logs

No response

System Info

svelte@5.0.0-next.236

Severity

annoyance

[trueadm]

This is because we're creating an effect inside a derived, which means that when the derived runs again, the inner effect is destroyed and the reactivity is lost. If we make $effect.tracking return false for inside a derived, then this also fixes the issue.

[stuartbritt]

I'm seeing really weird reactivity issues when using fromStore to runify the page store.

<script>
 function get_form(form_name: string) {
    const pagestore = fromStore(page);
    if (pagestore.current.form?.[form_name]) {
        return pagestore.current.form[form_name];
    } else if (pagestore.current.data?.[form_name]) {
        return pagestore.current.data[form_name];
    } else throw new Error('Unknown form');
}

    const form2 = $derived(get_form('form2'));
    const form1 = $derived(get_form('form1'));
</script>
<!-- Two forms here, using use:enhance and form actions-->
<pre>{JSON.stringify(form1.field_data.name, undefined, 2)}</pre> <!-- Not reliably reactive-->
<pre>{JSON.stringify(form2.field_data.name, undefined, 2)}</pre> <!-- Not reliably reactive-->
<Monitor {form1} {form2} /> <!-- Also displays a <pre> and reactivity works fine here-->

If I access form1 or form2 directly in the same page, the reactivity is inconsistently broken (it is really, really weird and I can't work out the pattern). If I pass form1 or form2 to a component on the page, then it all works fine. I tried using $derived and getters inside the function to avoid needing $derived in the main code and that had the same issue.

Could it be related?

[7nik]

 function get_form(form_name: string) {
  const pagestore = fromStore(page);
  if (pagestore.current.form?.[form_name]) {
      return pagestore.current.form[form_name];
  } else if (pagestore.current.data?.[form_name]) {
      return pagestore.current.data[form_name];
  } else throw new Error('Unknown form');
}

  const form2 = $derived(get_form('form2'));
  const form1 = $derived(get_form('form1'));

It doesn't look like good code — it re-creates the state and re-subscribes to the store at each change.

It should be like this

 function get_form(form_name: string) {
    const pagestore = fromStore(page);
    if (pagestore.current.form?.[form_name]) {
        return () => pagestore.current.form[form_name];
    } else if (pagestore.current.data?.[form_name]) {
        return () => pagestore.current.data[form_name];
    } else throw new Error('Unknown form');
}

    const form2 = $derived.by(get_form('form2'));
    const form1 = $derived.by(get_form('form1'));

[trueadm]

I think we need to investigate this more before 5.0, something doesn't seem right here.

[stuartbritt]

Here's a simple reproduction of fromStore() behaving strangely. I'm not sure whether it's exactly the same issue as the above, but seems related. REPL

(edited as posted wrong REPL!)

[pauldemarco]

I don't want to hijack this thread with a potentially different issue, but I am seeing fromStore lacking deep reactivity.

I am trying to convert the form store from superforms into a rune so I can pass it to a snippet and bind it to an input field:

const {form} = superform(...);
const formRune = fromStore(form);

{@render children({formRune})}

{#snippet children({formRune})}
<input bind:value={formRune.current.firstName} />
{/snippet}

I am getting binding_property_non_reactive errors.

[trueadm]

Here's a simple reproduction of fromStore() behaving strangely. I'm not sure whether it's exactly the same issue as the above, but seems related. REPL

(edited as posted wrong REPL!)

I have a fix for this here https://github.com/sveltejs/svelte/pull/13087. It doesn't resolve the original issue though, so that might be a separate issue.