fix: reuse proxy between objects (#9821)

* chore: reuse proxy between objects

* lint

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/smart-parents-swim.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reuse existing proxy when object has multiple references

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

@@ -17,7 +17,7 @@ import {
 	object_keys
 } from '../utils.js';
 
-/** @typedef {{ s: Map<string | symbol, import('../types.js').SourceSignal<any>>; v: import('../types.js').SourceSignal<number>; a: boolean, i: boolean }} Metadata */
+/** @typedef {{ s: Map<string | symbol, import('../types.js').SourceSignal<any>>; v: import('../types.js').SourceSignal<number>; a: boolean, i: boolean, p: StateObject }} Metadata */
 /** @typedef {Record<string | symbol, any> & { [STATE_SYMBOL]: Metadata }} StateObject */
 
 export const STATE_SYMBOL = Symbol('$state');
@@ -35,15 +35,23 @@ const is_frozen = Object.isFrozen;
  * @returns {T}
  */
 export function proxy(value, immutable = true) {
-	if (typeof value === 'object' && value != null && !is_frozen(value) && !(STATE_SYMBOL in value)) {
+	if (typeof value === 'object' && value != null && !is_frozen(value)) {
+		if (STATE_SYMBOL in value) {
+			return /** @type {T} */ (value[STATE_SYMBOL].p);
+		}
+
 		const prototype = get_prototype_of(value);
 
 		// TODO handle Map and Set as well
 		if (prototype === object_prototype || prototype === array_prototype) {
-			define_property(value, STATE_SYMBOL, { value: init(value, immutable), writable: false });
+			const proxy = new Proxy(value, handler);
+			define_property(value, STATE_SYMBOL, {
+				value: init(value, proxy, immutable),
+				writable: false
+			});
 
 			// @ts-expect-error not sure how to fix this
-			return new Proxy(value, handler);
+			return proxy;
 		}
 	}
 
@@ -102,15 +110,17 @@ export function unstate(value) {
 
 /**
  * @param {StateObject} value
+ * @param {StateObject} proxy
  * @param {boolean} immutable
  * @returns {Metadata}
  */
-function init(value, immutable) {
+function init(value, proxy, immutable) {
 	return {
 		s: new Map(),
 		v: source(0),
 		a: is_array(value),
-		i: immutable
+		i: immutable,
+		p: proxy
 	};
 }
 

packages/svelte/tests/runtime-runes/samples/proxy-shared/_config.js

@@ -0,0 +1,30 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<button>0</button>
+		<button>0</button>
+	`,
+
+	async test({ assert, target }) {
+		const [btn1, btn2] = target.querySelectorAll('button');
+
+		await btn1?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>1</button>
+				<button>1</button>
+			`
+		);
+
+		await btn2?.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>2</button>
+				<button>2</button>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/proxy-shared/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	let obj = { count: 0 };
+
+	let a = $state(obj);
+	let b = $state(obj);
+</script>
+
+<button onclick={() => a.count += 1}>
+	{a.count}
+</button>
+
+<button onclick={() => b.count += 1}>
+	{b.count}
+</button>