`undefined` not omitted in `<svelte:head>` with `page` store

[hyunbinseo]

Describe the bug

<script>
  import { page } from '$app/state';
</script>

<svelte:head>
  <!-- non-defined key -->
  <title>{page.data?.a}</title>
</svelte:head>

In client-side rendering (browser)

<title>undefined</title>

In pre-rendered HTML

<title></title>

Reproduction

https://github.com/hyunbinseo/svelte-kit-14548

System Info

System:
  OS: Windows 11 10.0.26100
  CPU: (8) x64 Intel(R) Core(TM) Ultra 7 258V
  Memory: 7.42 GB / 31.48 GB
Binaries:
  Node: 24.9.0 - ~\AppData\Local\fnm_multishells\42696_1759035529459\node.EXE
  Yarn: 1.22.22 - ~\AppData\Local\fnm_multishells\42696_1759035529459\yarn.CMD
  npm: 11.6.0 - ~\AppData\Local\fnm_multishells\42696_1759035529459\npm.CMD
  pnpm: 10.17.1 - ~\AppData\Local\fnm_multishells\42696_1759035529459\pnpm.CMD
  Deno: 2.4.0 - ~\.deno\bin\deno.EXE
Browsers:
  Edge: Chromium (140.0.3485.54)
npmPackages:
  @sveltejs/adapter-auto: ^6.1.0 => 6.1.0
  @sveltejs/kit: ^2.43.5 => 2.43.5
  @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
  svelte: ^5.39.6 => 5.39.6
  vite: ^7.1.7 => 7.1.7

Severity

annoyance

Additional Information

Checked on Firefox@143.0.1 and Chrome@140.0.7339.208

[Ocean-OS]

This is a bug in Svelte, not Kit. document.title coerces undefined to the string "undefined".

[hyunbinseo]

Thanks for pointing out. This seems to be the problematic part:

$.template_effect(() => $.document.title = thing.thing);

I originally made an MRE in an non-{} example and forgot about the changes.

<script>
  const title = undefined;
</script>

<svelte:head>
  <title>{title}</title>
</svelte:head>

$.document.title = ''; // works as expected

[Ocean-OS]

This should be fixed in sveltejs/svelte#16863, could someone please close this?