From 42f5d10a42d8c6a0ac5f99f17f3805ea3048b2c0 Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 29 Jun 2025 15:49:15 +0100
Subject: [PATCH 1/5] Store deriveds old value before updating them for
 consistency with directly assigned sources when reading in teardown functions

---
 .../internal/client/reactivity/deriveds.js    | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index e9cea0df3e64..ee2bf3ed2f81 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -15,7 +15,7 @@ import {
 import { equals, safe_equals } from './equality.js';
 import * as e from '../errors.js';
 import { destroy_effect } from './effects.js';
-import { inspect_effects, set_inspect_effects } from './sources.js';
+import { inspect_effects, old_values, set_inspect_effects } from './sources.js';
 import { get_stack } from '../dev/tracing.js';
 import { tracing_mode_flag } from '../../flags/index.js';
 import { component_context } from '../context.js';
@@ -175,6 +175,23 @@ export function update_derived(derived) {
 	var value = execute_derived(derived);
 
 	if (!derived.equals(value)) {
+		// store old value before updating
+		// so that user_effect teardown functions
+		// can access the previous value.
+		// this is needed because derived updates happen early during
+		// effects dependency resolution (before cleanup),
+		// unlike direct state/derived updates, and this
+		// can happen also during template_effect execution
+		// that also happens before user_effect teardown.
+		//
+		// store old value only if not inside a teardown function
+		// because in legacy mode, bindable props are deriveds
+		// and they are executed during teardown.
+		if (!is_destroying_effect) {
+			var old_value = derived.v;
+			old_values.set(derived, old_value);
+		}
+
 		derived.v = value;
 		derived.wv = increment_write_version();
 	}

From 1fc9f0801c1ec583da9c69986925887377b8fffa Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 29 Jun 2025 16:45:35 +0100
Subject: [PATCH 2/5] Add tests for derived old values in teardown functions

---
 .../derived-cleanup-old-value/_config.js      | 39 +++++++++++++++++++
 .../derived-cleanup-old-value/main.svelte     | 17 ++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js b/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js
new file mode 100644
index 000000000000..4810ba6fb985
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js
@@ -0,0 +1,39 @@
+import { flushSync } from 'svelte';
+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');
+
+		// Initial state: count=1, derived_value=1
+
+		// 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'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte b/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte
new file mode 100644
index 000000000000..6cef12b1e345
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte
@@ -0,0 +1,17 @@
+<script>
+	let count = $state(1);
+	let derived_value = $derived(count * count);
+
+	$effect(() => {
+		console.log(`$effect: ${derived_value}`);
+		return () => {
+			console.log(`$effect teardown: ${derived_value}`);
+		};
+	});
+</script>
+
+<button id="increment" onclick={() => count++}>Increment s</button>
+<button id="overwrite" onclick={() => (derived_value = 7)}>Overwrite derived_value</button>
+
+<p>count: {count}</p>
+<p>derived_value: {derived_value}</p>

From 9d9b64b94eb4a1b700d15da9fa08e793ed6ded06 Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 29 Jun 2025 16:46:58 +0100
Subject: [PATCH 3/5] Add tests for props old values in onDestroy hooks to
 prevent regressions

---
 .../Component.svelte                          | 11 ++++
 .../_config.js                                | 63 +++++++++++++++++++
 .../main.svelte                               | 52 +++++++++++++++
 .../Component.svelte                          | 14 +++++
 .../_config.js                                | 63 +++++++++++++++++++
 .../main.svelte                               | 52 +++++++++++++++
 .../Component.svelte                          | 11 ++++
 .../_config.js                                | 49 +++++++++++++++
 .../main.svelte                               | 41 ++++++++++++
 9 files changed, 356 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/Component.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/Component.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/Component.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/Component.svelte b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/Component.svelte
new file mode 100644
index 000000000000..40d6b1c2fbb4
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/Component.svelte
@@ -0,0 +1,11 @@
+<script>
+	import { onDestroy } from 'svelte';
+
+	let { checked = $bindable(), count = $bindable() } = $props();
+
+	onDestroy(() => {
+		console.log(`${count} ${checked}`);
+	});
+</script>
+
+<p>Count: {count}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/_config.js b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/_config.js
new file mode 100644
index 000000000000..07ea5af153a6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/_config.js
@@ -0,0 +1,63 @@
+import { flushSync } from 'svelte';
+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 toggle_btn = target.querySelector('#toggle');
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 0');
+		}
+
+		// Click increment: count=1
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 1');
+		}
+
+		// Click increment again: count=2, components with count < 2 should unmount and log old values
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 2');
+		}
+
+		// Toggle show to hide components that depend on show
+		flushSync(() => {
+			toggle_btn?.click();
+		});
+
+		// Should log old values during cleanup from the six components guarded by `count < 2`:
+		// 1. Component with bind: "1 true"
+		// 2. Component with spread: "1 true"
+		// 3. Component with normal props: "1 true"
+		// 4. Runes dynamic component with bind: "1 true"
+		// 5. Runes dynamic component with spread: "1 true"
+		// 6. Runes dynamic component with normal props: "1 true"
+		// Then from the four components guarded by `show`:
+		// 7. Component with bind (show): "2 true" (old value of checked)
+		// 8. Runes dynamic component with bind (show): "2 true"
+		// 9. Runes dynamic component with spread (show): "2 true"
+		// 10. Runes dynamic component with normal props (show): "2 true"
+		assert.deepEqual(logs, [
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'2 true',
+			'2 true',
+			'2 true',
+			'2 true'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/main.svelte b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/main.svelte
new file mode 100644
index 000000000000..592c46ff429f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-no-setter/main.svelte
@@ -0,0 +1,52 @@
+<script>
+	import Component from './Component.svelte';
+
+	let show = $state(true);
+	let count = $state(0);
+	let spread = $derived({ checked: show, count });
+
+	// Runes dynamic component pattern
+	let DynComponent1 = $derived(count < 2 ? Component : undefined);
+	let DynComponent2 = $derived(show ? Component : undefined);
+</script>
+
+<button id="increment" onclick={() => count++}>Increment count</button>
+<button id="toggle" onclick={() => (show = !show)}>Toggle show</button>
+
+<!-- count with bind -->
+{#if count < 2}
+	<Component bind:count bind:checked={show} />
+{/if}
+
+<!-- spread syntax -->
+{#if count < 2}
+	<Component {...spread} />
+{/if}
+
+<!-- normal prop -->
+{#if count < 2}
+	<Component {count} checked={show} />
+{/if}
+
+<!-- prop only accessed in destroy -->
+{#if show}
+	<Component bind:count bind:checked={show} />
+{/if}
+
+<!-- runes dynamic component pattern -->
+<DynComponent1 bind:count bind:checked={show} />
+
+<!-- runes dynamic component pattern with spread -->
+<DynComponent1 {...spread} />
+
+<!-- runes dynamic component pattern with normal props -->
+<DynComponent1 {count} checked={show} />
+
+<!-- runes dynamic component pattern (show) -->
+<DynComponent2 bind:count bind:checked={show} />
+
+<!-- runes dynamic component pattern with spread (show) -->
+<DynComponent2 {...spread} />
+
+<!-- runes dynamic component pattern with normal props (show) -->
+<DynComponent2 {count} checked={show} />
diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/Component.svelte b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/Component.svelte
new file mode 100644
index 000000000000..cc47017946ee
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/Component.svelte
@@ -0,0 +1,14 @@
+<script>
+	import { onDestroy } from 'svelte';
+
+	let { checked = $bindable(), count = $bindable() } = $props();
+
+	onDestroy(() => {
+		console.log(`${count} ${checked}`);
+	});
+</script>
+
+<p>Count: {count}</p>
+
+<button onclick={() => count--}>Decrement count</button>
+<button onclick={() => (checked = !checked)}>Toggle checked</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/_config.js b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/_config.js
new file mode 100644
index 000000000000..07ea5af153a6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/_config.js
@@ -0,0 +1,63 @@
+import { flushSync } from 'svelte';
+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 toggle_btn = target.querySelector('#toggle');
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 0');
+		}
+
+		// Click increment: count=1
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 1');
+		}
+
+		// Click increment again: count=2, components with count < 2 should unmount and log old values
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 2');
+		}
+
+		// Toggle show to hide components that depend on show
+		flushSync(() => {
+			toggle_btn?.click();
+		});
+
+		// Should log old values during cleanup from the six components guarded by `count < 2`:
+		// 1. Component with bind: "1 true"
+		// 2. Component with spread: "1 true"
+		// 3. Component with normal props: "1 true"
+		// 4. Runes dynamic component with bind: "1 true"
+		// 5. Runes dynamic component with spread: "1 true"
+		// 6. Runes dynamic component with normal props: "1 true"
+		// Then from the four components guarded by `show`:
+		// 7. Component with bind (show): "2 true" (old value of checked)
+		// 8. Runes dynamic component with bind (show): "2 true"
+		// 9. Runes dynamic component with spread (show): "2 true"
+		// 10. Runes dynamic component with normal props (show): "2 true"
+		assert.deepEqual(logs, [
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'1 true',
+			'2 true',
+			'2 true',
+			'2 true',
+			'2 true'
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/main.svelte b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/main.svelte
new file mode 100644
index 000000000000..592c46ff429f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/bindable-props-cleanup-with-setter/main.svelte
@@ -0,0 +1,52 @@
+<script>
+	import Component from './Component.svelte';
+
+	let show = $state(true);
+	let count = $state(0);
+	let spread = $derived({ checked: show, count });
+
+	// Runes dynamic component pattern
+	let DynComponent1 = $derived(count < 2 ? Component : undefined);
+	let DynComponent2 = $derived(show ? Component : undefined);
+</script>
+
+<button id="increment" onclick={() => count++}>Increment count</button>
+<button id="toggle" onclick={() => (show = !show)}>Toggle show</button>
+
+<!-- count with bind -->
+{#if count < 2}
+	<Component bind:count bind:checked={show} />
+{/if}
+
+<!-- spread syntax -->
+{#if count < 2}
+	<Component {...spread} />
+{/if}
+
+<!-- normal prop -->
+{#if count < 2}
+	<Component {count} checked={show} />
+{/if}
+
+<!-- prop only accessed in destroy -->
+{#if show}
+	<Component bind:count bind:checked={show} />
+{/if}
+
+<!-- runes dynamic component pattern -->
+<DynComponent1 bind:count bind:checked={show} />
+
+<!-- runes dynamic component pattern with spread -->
+<DynComponent1 {...spread} />
+
+<!-- runes dynamic component pattern with normal props -->
+<DynComponent1 {count} checked={show} />
+
+<!-- runes dynamic component pattern (show) -->
+<DynComponent2 bind:count bind:checked={show} />
+
+<!-- runes dynamic component pattern with spread (show) -->
+<DynComponent2 {...spread} />
+
+<!-- runes dynamic component pattern with normal props (show) -->
+<DynComponent2 {count} checked={show} />
diff --git a/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/Component.svelte b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/Component.svelte
new file mode 100644
index 000000000000..1a95f1edf55c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/Component.svelte
@@ -0,0 +1,11 @@
+<script>
+	import { onDestroy } from 'svelte';
+
+	let { checked, count } = $props();
+
+	onDestroy(() => {
+		console.log(`${count} ${checked}`);
+	});
+</script>
+
+<p>Count: {count}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/_config.js b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/_config.js
new file mode 100644
index 000000000000..3d73bfd1a90c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/_config.js
@@ -0,0 +1,49 @@
+import { flushSync } from 'svelte';
+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 toggle_btn = target.querySelector('#toggle');
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 0');
+		}
+
+		// Click increment: count=1
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 1');
+		}
+
+		// Click increment again: count=2, components with count < 2 should unmount and log old values
+		flushSync(() => {
+			increment_btn?.click();
+		});
+
+		for (const p of target.querySelectorAll('p')) {
+			assert.equal(p.innerHTML, 'Count: 2');
+		}
+
+		// Toggle show to hide components that depend on show
+		flushSync(() => {
+			toggle_btn?.click();
+		});
+
+		// Should log old values during cleanup from the four components guarded by `count < 2`:
+		// 1. Component with normal props: "1 true"
+		// 2. Component with spread: "1 true"
+		// 3. Runes dynamic component with normal props: "1 true"
+		// 4. Runes dynamic component with spread: "1 true"
+		// Then from the three components guarded by `show`:
+		// 5. Component with normal props (show): "2 true" (old value of checked)
+		// 6. Runes dynamic component with normal props (show): "2 true"
+		// 7. Runes dynamic component with spread (show): "2 true"
+		assert.deepEqual(logs, ['1 true', '1 true', '1 true', '1 true', '2 true', '2 true', '2 true']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/main.svelte b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/main.svelte
new file mode 100644
index 000000000000..8bc6abfa4d6b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/regular-props-cleanup-old-value/main.svelte
@@ -0,0 +1,41 @@
+<script>
+	import Component from './Component.svelte';
+
+	let show = $state(true);
+	let count = $state(0);
+	let spread = $derived({ checked: show, count });
+
+	// Runes dynamic component pattern
+	let DynComponent1 = $derived(count < 2 ? Component : undefined);
+	let DynComponent2 = $derived(show ? Component : undefined);
+</script>
+
+<button id="increment" onclick={() => count++}>Increment count</button>
+<button id="toggle" onclick={() => (show = !show)}>Toggle show</button>
+
+<!-- normal props -->
+{#if count < 2}
+	<Component {count} checked={show} />
+{/if}
+
+<!-- spread syntax -->
+{#if count < 2}
+	<Component {...spread} />
+{/if}
+
+<!-- prop only accessed in destroy -->
+{#if show}
+	<Component {count} checked={show} />
+{/if}
+
+<!-- runes dynamic component pattern with normal props -->
+<DynComponent1 {count} checked={show} />
+
+<!-- runes dynamic component pattern with spread -->
+<DynComponent1 {...spread} />
+
+<!-- runes dynamic component pattern (show) with normal props -->
+<DynComponent2 {count} checked={show} />
+
+<!-- runes dynamic component pattern with spread (show) -->
+<DynComponent2 {...spread} />

From 5ed74b24bad2272f94169178db357f52d3e30d5d Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 29 Jun 2025 16:47:33 +0100
Subject: [PATCH 4/5] Add changeset

---
 .changeset/light-rivers-jump.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/light-rivers-jump.md

diff --git a/.changeset/light-rivers-jump.md b/.changeset/light-rivers-jump.md
new file mode 100644
index 000000000000..9258401e8a86
--- /dev/null
+++ b/.changeset/light-rivers-jump.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Store deriveds old value before updating them for consistency with directly assigned sources when reading in teardown functions

From b213de2caf344daa7d377fd1a5ee723565635804 Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 29 Jun 2025 17:34:55 +0100
Subject: [PATCH 5/5] Update comment

---
 .../svelte/src/internal/client/reactivity/deriveds.js     | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index ee2bf3ed2f81..5e4e08cc6537 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -185,8 +185,12 @@ export function update_derived(derived) {
 		// that also happens before user_effect teardown.
 		//
 		// store old value only if not inside a teardown function
-		// because in legacy mode, bindable props are deriveds
-		// and they are executed during teardown.
+		// because we only need to save the old values before
+		// the cleanup is triggered othewise accessing
+		// a derived during cleanup will return the incorrect
+		// value in case the derived wasn't in the deps of the effect,
+		// or the teardown was executed because the component was
+		// destroyed.
 		if (!is_destroying_effect) {
 			var old_value = derived.v;
 			old_values.set(derived, old_value);