Push to array inside SvelteMap not reactive

[tinnyw]

Describe the bug

Hi friends, hope you are well! I am working on an app that deals with complex objects to store state. In one of these objects I modify a map with one of the entries in the map yielding an array for the value which I push to. Updating this array does not cause a reactive state change in the UI that depends on it. This seems unintuitive to me since I thought that objects were deeply reactive with recursive monitoring for state change.

To make this reactive I have to delete the SvelteMap entry of the key that contains the array value and set it back again with the array.

Reproduction

App.svelte

<script>
	import {bob} from "./bob.svelte.js"
	
	let name = $state('world');
	let newKid = 1
</script>

<button onclick={
	() => bob.attribs.set('type', 'friend')}
	>Bob type: {bob.attribs.get('type')}
</button>
<br/>
<br/>
<button onclick={() => {
	// Expecting just the next line that's commented to work
	/*bob.attribs.get('kids').push(newKid)*/
	const kids = bob.attribs.get('kids')
	kids.push(newKid++)
	bob.updateKids()
}}>
	Kids are: 
	{#each bob.attribs.get('kids') as kid}
		<li>{kid}</li>
	{/each}
</button>

bob.svelte.js

import {SvelteMap} from 'svelte/reactivity';

class Bob {
	attribs = $state(new SvelteMap([['type', 'employee'],
																 ['kids', ['a', 'b', 'c']]]));

	updateKids() {
		const kids = this.attribs.get('kids')
		this.attribs.delete('kids')
		this.attribs.set('kids', kids)
	}
}

export const bob = new Bob();

Logs

No response

System Info

Able to replicate this on the Svelte playground and in Ubuntu WSL.

Severity

blocking an upgrade

[brunnerh]

Duplicate of:

• Fetching an object from a SveltMap, modifying it, then setting it back does not have obvious behavior #14386

[paoloricciuti]

Duplicate of:

• Fetching an object from a SveltMap, modifying it, then setting it back does not have obvious behavior #14386

As pointed out we already have a issue for it (which probably is more a doc issue anyway...closing this but thanks for reporting