Why is this object re-assigned?

[frederikhors]

REPL: https://svelte.dev/repl/953050f84c9d4f528ecfaaaf50defbbf?version=3.55.1

Using the below code I found that the first team is re-assigned, the second nope (see the console).

Why?

I expect to not get re-assigned when the date is already present.

This may seem negligible, but when there are 30+ components on a page it is not at all (performances).

<script>
  import Flatpickr from './flatpickr.svelte';

  // this assigns team 2 times
  let team = {id: "1", start_date: new Date()};

  // this not
  // let team = {id: "1"};

  $: console.log("team assigned:", team);
</script>

[jacobmischka]

I don't think we're doing anything that should cause this, it seems like svelte is doing this on its own somehow. This seems to be a bug in svelte: date properties seem to be reassigned even though they aren't changed at all.

See this example, which doesn't use Flatpickr at all, and does no prop modification whatsoever: https://svelte.dev/repl/8ead02e349fa4884835654f809a77543?version=3.55.1

[frederikhors]

Wow! We found a bug!

Would you mind open an issue on https://github.com/sveltejs/svelte/issues?

[jacobmischka]

I did: sveltejs/svelte#8225

[jacobmischka]

It seems like this is a known bug that might not be fixed until a new major released based on what I gathered from the comments there: sveltejs/svelte#5689