Svelte 5 bug - unexpected behaviour of Date from svelte/reactivity about svelte HOT 2 CLOSED

The svelte/reactivity classes allow for internal modifications (e.g. due to functions calls) to be detected, if you re-assign, you still need $state. It works at the top level only because Svelte is not in runes mode and it falls back to top level let declarations being reactive automatically (REPL with runes mode).

let modified_at = $state(new Date());

If you only ever re-assign, you don't need svelte/reactivity.

Example making use of its reactivity:

<svelte:options runes />
<script>
  import { Date } from 'svelte/reactivity';

  const modified_at = new Date();

  function update() {
    console.log('update');
    modified_at.setTime(Date.now());
  }
</script>

<p>
  <!-- Need to call a function instead of coercing a conversion -->
  {modified_at.toString()}
</p>

<button onclick={update}> Update </button>

from svelte.

modified_at.setTime(Date.now());

Yes this is what I ended up with.

It works at the top level only because Svelte is not in runes mode and it falls back to top level let declarations being reactive automatically (REPL with runes mode).

I see, that make lots of sense. Thanks for explanation. It was confusing to me.
I suppose it will be documented later on how to use svelte/reactivity classes, so I am closing this issue.

from svelte.