fix: ensure local prop value is read during teardown

.changeset/clean-sloths-nail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure local prop value is read during teardown

packages/svelte/src/internal/client/reactivity/props.js

@@ -1,4 +1,4 @@
-/** @import { Source } from './types.js' */
+/** @import { Derived, Source } from './types.js' */
 import { DEV } from 'esm-env';
 import {
 	PROPS_IS_BINDABLE,
@@ -12,6 +12,7 @@ import { mutable_source, set, source } from './sources.js';
 import { derived, derived_safe_equal } from './deriveds.js';
 import {
 	active_effect,
+	active_reaction,
 	get,
 	is_signals_recorded,
 	set_active_effect,
@@ -20,7 +21,7 @@ import {
 } from '../runtime.js';
 import { safe_equals } from './equality.js';
 import * as e from '../errors.js';
-import { BRANCH_EFFECT, LEGACY_DERIVED_PROP, ROOT_EFFECT } from '../constants.js';
+import { BRANCH_EFFECT, DESTROYED, LEGACY_DERIVED_PROP, ROOT_EFFECT } from '../constants.js';
 import { proxy } from '../proxy.js';
 
 /**
@@ -348,12 +349,17 @@ export function prop(props, key, flags, fallback) {
 	// The derived returns the current value. The underlying mutable
 	// source is written to from various places to persist this value.
 	var inner_current_value = mutable_source(prop_value);
+
 	var current_value = with_parent_branch(() =>
 		derived(() => {
 			var parent_value = getter();
 			var child_value = get(inner_current_value);
+			var current_derived = /** @type {Derived} */ (active_reaction);
 
-			if (from_child) {
+			// If the getter from the parent returns undefined, switch
+			// to using the local value from inner_current_value instead,
+			// as the parent value might have been torn down
+			if (from_child || (parent_value === undefined && (current_derived.f & DESTROYED) !== 0)) {
 				from_child = false;
 				was_from_child = true;
 				return child_value;

packages/svelte/tests/runtime-runes/samples/props-local-teardown/Component.svelte

@@ -0,0 +1,15 @@
+<script>
+	let { x } = $props();
+
+	$effect(() => {
+		console.log('init')
+
+		x = () => {
+			console.log('teardown')
+		}
+
+		return () => {
+			x();
+		}
+	})
+</script>

packages/svelte/tests/runtime-runes/samples/props-local-teardown/_config.js

@@ -0,0 +1,19 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs, target }) {
+		const [btn1] = target.querySelectorAll('button');
+
+		btn1?.click();
+		flushSync();
+
+		btn1?.click();
+		flushSync();
+
+		btn1?.click();
+		flushSync();
+
+		assert.deepEqual(logs, ['init', 'teardown', 'init', 'teardown']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/props-local-teardown/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	import Component from "./Component.svelte";
+
+	let toggle = $state(true);
+</script>
+
+<button onclick={() => toggle = !toggle}>toggle</button>
+
+{#if toggle}
+	<Component />
+{/if}
+