fix: improve handling of object property deletions (#10456)

* fix: improve handling of object property deletions

* revert

* revert

* revert

Author: trueadm

.changeset/orange-crews-rescue.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve handling of object property deletions

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

@@ -157,7 +157,7 @@ const state_proxy_handler = {
 		}
 		if (s !== undefined) set(s, UNINITIALIZED);
 
-		if (prop in target) update(metadata.v);
+		if (boolean) update(metadata.v);
 
 		return boolean;
 	},

packages/svelte/tests/runtime-runes/samples/state-delete-property/_config.js

@@ -0,0 +1,20 @@
+import { test } from '../../test';
+
+export default test({
+	html: '<button>set</button><button>delete</button><p>a,b,c</p><p>{"a":1,"b":2,"c":3}</p>',
+	async test({ assert, target }) {
+		const [btn, bt2] = target.querySelectorAll('button');
+
+		await btn?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>set</button><button>delete</button><p>a,b,c</p><p>{"a":1,"b":2,"c":3}</p>`
+		);
+
+		await bt2?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>set</button><button>delete</button><p>a,c</p><p>{"a":1,"c":3}</p>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-delete-property/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let numbers = $state({ a: 1, b: 2, c: 3 });
+</script>
+
+<button onclick={() => { numbers.b = 2; }}>set</button>
+<button onclick={() => { delete numbers.b; }}>delete</button>
+<p>{Object.keys(numbers)}</p>
+<p>{JSON.stringify(numbers)}</p>