Fix/@sveltejs/svelte/16114

.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

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 +175,27 @@ 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 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);
+		}
+
 		derived.v = value;
 		derived.wv = increment_write_version();
 	}

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>

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'
+		]);
+	}
+});

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} />

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>

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'
+		]);
+	}
+});

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} />

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'
+		]);
+	}
+});

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>

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>