add test

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

@@ -1,4 +1,4 @@
-/** @import { Derived, Source } from './types.js' */
+/** @import { Source } from './types.js' */
 import { DEV } from 'esm-env';
 import {
 	PROPS_IS_BINDABLE,
@@ -22,6 +22,7 @@ import { safe_equals } from './equality.js';
 import * as e from '../errors.js';
 import { BRANCH_EFFECT, DESTROYED, LEGACY_DERIVED_PROP, ROOT_EFFECT } from '../constants.js';
 import { proxy } from '../proxy.js';
+import { teardown } from './effects.js';
 
 /**
  * @param {((value?: number) => number)} fn
@@ -292,8 +293,6 @@ export function prop(props, key, flags, fallback) {
 
 	/** @type {() => V} */
 	var getter;
-	/** @type {Derived} */
-	var derived_getter;
 	if (runes) {
 		getter = () => {
 			var value = /** @type {V} */ (props[key]);
@@ -305,7 +304,7 @@ export function prop(props, key, flags, fallback) {
 	} else {
 		// Svelte 4 did not trigger updates when a primitive value was updated to the same value.
 		// Replicate that behavior through using a derived
-		derived_getter = with_parent_branch(() =>
+		var derived_getter = with_parent_branch(() =>
 			(immutable ? derived : derived_safe_equal)(() => /** @type {V} */ (props[key]))
 		);
 		derived_getter.f |= LEGACY_DERIVED_PROP;
@@ -350,12 +349,22 @@ 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);
+
+	teardown(() => {
+		// 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 (getter() === undefined) {
+			from_child = true;
+		}
+	});
+
 	var current_value = with_parent_branch(() =>
 		derived(() => {
 			var parent_value = getter();
 			var child_value = get(inner_current_value);
 
-			if (from_child || (derived_getter !== undefined && (derived_getter.f & DESTROYED) !== 0)) {
+			if (from_child) {
 				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}
+