fix: ensure bind:this unmount behavior for members is conditional (#11193)

* fix: ensure bind:this unmount behavior for members is conditional

* revise

.changeset/weak-drinks-speak.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure bind:this unmount behavior for members is conditional

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -917,6 +917,15 @@ function serialize_bind_this(bind_this, context, node) {
 
 	/** @type {import('estree').Expression[]} */
 	const args = [node, b.arrow([b.id('$$value'), ...ids], update), b.arrow([...ids], bind_this_id)];
+	// If we're mutating a property, then it might already be non-existent.
+	// If we make all the object nodes optional, then it avoids any runtime exceptions.
+	/** @type {import('estree').Expression | import('estree').Super} */
+	let bind_node = bind_this_id;
+
+	while (bind_node?.type === 'MemberExpression') {
+		bind_node.optional = true;
+		bind_node = bind_node.object;
+	}
 	if (each_ids.size) {
 		args.push(b.thunk(b.array(Array.from(each_ids.values()).map((id) => id[1]))));
 	}

packages/svelte/tests/runtime-runes/samples/each-bind-this-member-2/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { item = $bindable() } = $props();
+</script>
+
+<div bind:this={item.dom}>
+	{item.text}
+</div>

packages/svelte/tests/runtime-runes/samples/each-bind-this-member-2/_config.js

@@ -0,0 +1,25 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, component }) {
+		const [b1, b2] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			b1.click();
+			b1.click();
+			b1.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>add item</button><button>clear</button><div>Item 1</div><div>Item 2</div><div>Item 3</div>`
+		);
+
+		flushSync(() => {
+			b2.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>add item</button><button>clear</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/each-bind-this-member-2/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	import Child from './Child.svelte';
+	let items = $state([]);
+
+	function add_item() {
+		items.push({
+			id: items.length,
+			text: 'Item ' + (items.length + 1),
+			dom: null,
+		})
+	}
+
+	function clear() {
+		items = [];
+	}
+</script>
+
+<button on:click={add_item}>add item</button>
+<button on:click={clear}>clear</button>
+
+{#each items as item, index (item.id)}
+	<Child bind:item={items[index]} />
+{/each}