`$inspect` can cause an error in `$derived` based on `data`

[brunnerh]

Describe the bug

$inspect can somehow cause a $derived to operate on data belonging to the next page when navigating. When the $inspect is removed, the problem disappears.

Reproduction

StackBlitz

Code

// /some-route/+page.server.js
export function load() {
  return {
    items: [
      { id: 1, name: 'Item 1' },
      { id: 2, name: 'Item 2' },
      { id: 3, name: 'Item 3' },
      { id: 4, name: 'Item 4' },
      { id: 5, name: 'Item 5' },
    ],
  };
}

<!-- /some-route/+page.svelte -->
<script lang="ts">
  import List from './list.svelte';

  let { data } = $props();

  const { items } = $derived(data);

  const even = $derived(items.filter((item) => item.id % 2 === 0));
</script>

<List items={even} />

<!-- /some-route/list.svelte -->
<script lang="ts">
  const { items } = $props();

  $inspect(items); // <- remove this
</script>

{#each items as item}
  <div>
    <a href="/">{item.id}</a>
  </div>
{/each}

Logs

+page.svelte:13 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'filter')

	in <unknown>
	in layout.svelte
	in root.svelte

    at +page.svelte:13:49
    at update_reaction (chunk-DJJYHU3C.js?v=ad8341b0:2754:23)
    at execute_derived (chunk-DJJYHU3C.js?v=ad8341b0:1240:15)
    at update_derived (chunk-DJJYHU3C.js?v=ad8341b0:1257:15)
    at is_dirty (chunk-DJJYHU3C.js?v=ad8341b0:2681:11)
    at flush_inspect_effects (chunk-DJJYHU3C.js?v=ad8341b0:1398:9)
    at internal_set (chunk-DJJYHU3C.js?v=ad8341b0:1386:7)
    at set (chunk-DJJYHU3C.js?v=ad8341b0:1337:10)
    at Object.set (chunk-SRUB5KV5.js?v=ad8341b0:829:11)
    at Object.assign (<anonymous>)

System Info

(StackBlitz)

  System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.0.1 
    @sveltejs/kit: ^2.22.0 => 2.25.1 
    @sveltejs/vite-plugin-svelte: ^6.0.0 => 6.1.0 
    svelte: ^5.25.0 => 5.36.13 
    vite: ^7.0.4 => 7.0.5

Severity

annoyance

Additional Information

This seems like more of a Svelte issue, but I was unable to create a clean reproduction showing the difference between having the $inspect and not having it without SvelteKit.

[eltigerchino]

There was a similar issue about this before (related to event listeners) and I think it's caused by data being updated before we navigate and code on the page before still trying to access the old properties.

[zhihengGet]

just want to say i encountered same bug, data is updated before page is fully updated causing "x is undefined" error. 2.22.0 works for me. latest does not work. somwhere in bewteen failed (nvm somehow it failed again... lmc...)
(i have async turned on svelte)