WIP

Author: Rich-Harris

packages/svelte/src/internal/client/runtime.js

@@ -26,7 +26,7 @@ import {
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
-import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
+import { destroy_derived_effects, execute_derived, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 
 import { tracing_mode_flag } from '../flags/index.js';
@@ -764,7 +764,7 @@ export function get(signal) {
 		}
 	}
 
-	if (is_derived) {
+	if (is_derived && !is_destroying_effect) {
 		derived = /** @type {Derived} */ (signal);
 
 		if (check_dirtiness(derived)) {
@@ -805,8 +805,19 @@ export function get(signal) {
 		}
 	}
 
-	if (is_destroying_effect && old_values.has(signal)) {
-		return old_values.get(signal);
+	if (is_destroying_effect) {
+		if (old_values.has(signal)) {
+			return old_values.get(signal);
+		}
+
+		if (is_derived) {
+			derived = /** @type {Derived} */ (signal);
+
+			var value = execute_derived(derived);
+			old_values.set(derived, value);
+
+			return value;
+		}
 	}
 
 	return signal.v;

packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js

@@ -3,37 +3,20 @@ import { test } from '../../test';
 
 export default test({
 	async test({ assert, logs, target }) {
-		/** @type {HTMLButtonElement | null} */
-		const increment_btn = target.querySelector('#increment');
-		/** @type {HTMLButtonElement | null} */
-		const overwrite_btn = target.querySelector('#overwrite');
+		const [increment] = target.querySelectorAll('button');
 
-		// Initial state: count=1, derived_value=1
+		flushSync(() => increment.click());
+		flushSync(() => increment.click());
+		flushSync(() => increment.click());
 
-		// Click to increment count: count=2, derived_value=4
-		flushSync(() => {
-			increment_btn?.click();
-		});
-
-		// Click to increment count: count=3, derived_value=9
-		flushSync(() => {
-			increment_btn?.click();
-		});
-
-		// Click to overwrite derived_value: count=3, derived_value=7
-		flushSync(() => {
-			overwrite_btn?.click();
-		});
-
-		// Should log old value during cleanup (4) and new value during setup (9)
 		assert.deepEqual(logs, [
-			'$effect: 1',
-			'$effect teardown: 1',
-			'$effect: 4',
-			'$effect teardown: 4',
-			'$effect: 9',
-			'$effect teardown: 9',
-			'$effect: 7'
+			'count: 1',
+			'squared: 1',
+			'count: 2',
+			'squared: 4',
+			'count: 3',
+			'squared: 9',
+			'count: 4'
 		]);
 	}
 });

packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte

@@ -1,17 +1,20 @@
 <script>
 	let count = $state(1);
-	let derived_value = $derived(count * count);
+	let squared = $derived(count * count);
 
 	$effect(() => {
-		console.log(`$effect: ${derived_value}`);
+		console.log(`count: ${count}`);
+
 		return () => {
-			console.log(`$effect teardown: ${derived_value}`);
+			console.log(`squared: ${squared}`);
 		};
 	});
 </script>
 
-<button id="increment" onclick={() => count++}>Increment s</button>
-<button id="overwrite" onclick={() => (derived_value = 7)}>Overwrite derived_value</button>
+<button onclick={() => count++}>increment</button>
 
 <p>count: {count}</p>
-<p>derived_value: {derived_value}</p>
+
+{#if count % 2 === 0}
+	<p id="squared">squared: {squared}</p>
+{/if}