fix: keep default values of props a proxy after reassignment (#11860)

* fix: keep default values of props a proxy after reassignment

* fix: make this work for non bindable props too

* chore: more comprehensive test

* chore: cast away

* chore: better variable name and check

* chore: fix lint

.changeset/empty-horses-tell.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: keep default values of props a proxy after reassignment

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

@@ -292,7 +292,13 @@ export function serialize_set_binding(node, context, fallback, options) {
 
 	const serialize = () => {
 		if (left === node.left) {
-			if (binding.kind === 'prop' || binding.kind === 'bindable_prop') {
+			const is_initial_proxy =
+				binding.initial !== null &&
+				should_proxy_or_freeze(
+					/**@type {import("estree").Expression}*/ (binding.initial),
+					context.state.scope
+				);
+			if ((binding.kind === 'prop' || binding.kind === 'bindable_prop') && !is_initial_proxy) {
 				return b.call(left, value);
 			} else if (is_store) {
 				return b.call('$.store_set', serialize_get_binding(b.id(left_name), state), value);
@@ -318,6 +324,18 @@ export function serialize_set_binding(node, context, fallback, options) {
 							? b.call('$.freeze', value)
 							: value
 					);
+				} else if (
+					(binding.kind === 'prop' || binding.kind === 'bindable_prop') &&
+					is_initial_proxy
+				) {
+					call = b.call(
+						left,
+						context.state.analysis.runes &&
+							!options?.skip_proxy_and_freeze &&
+							should_proxy_or_freeze(value, context.state.scope)
+							? serialize_proxy_reassignment(value, left_name, state)
+							: value
+					);
 				} else {
 					call = b.call('$.set', b.id(left_name), value);
 				}

packages/svelte/tests/runtime-runes/samples/props-default-reactivity/Counter.svelte

@@ -1,12 +1,20 @@
 <script>
-	/** @type {{ object?: { count: number }}} */
-	let { object = $bindable({ count: 0 }) } = $props();
+	/** @type {{ object?: { count: number }, non_bindable?: { count: number }}} */
+	let { object = $bindable({ count: 0 }), non_bindable = { count: 0 } } = $props();
 </script>
 
-<button onclick={() => object.count += 1}>
+<button onclick={() => (object.count += 1)}>
 	mutate: {object.count}
 </button>
 
-<button onclick={() => object = { count: object.count + 1 } }>
+<button onclick={() => (object = { count: object.count + 1 })}>
 	reassign: {object.count}
 </button>
+
+<button onclick={() => (non_bindable.count += 1)}>
+	mutate: {non_bindable.count}
+</button>
+
+<button onclick={() => (non_bindable = { count: non_bindable.count + 1 })}>
+	reassign: {non_bindable.count}
+</button>

packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js

@@ -5,10 +5,12 @@ export default test({
 	html: `
 		<button>mutate: 0</button>
 		<button>reassign: 0</button>
+		<button>mutate: 0</button>
+		<button>reassign: 0</button>
 	`,
 
 	async test({ assert, target }) {
-		const [btn1, btn2] = target.querySelectorAll('button');
+		const [btn1, btn2, btn3, btn4] = target.querySelectorAll('button');
 
 		flushSync(() => {
 			btn1?.click();
@@ -19,6 +21,8 @@ export default test({
 			`
 				<button>mutate: 1</button>
 				<button>reassign: 1</button>
+				<button>mutate: 0</button>
+				<button>reassign: 0</button>
 			`
 		);
 
@@ -31,6 +35,64 @@ export default test({
 			`
 				<button>mutate: 2</button>
 				<button>reassign: 2</button>
+				<button>mutate: 0</button>
+				<button>reassign: 0</button>
+			`
+		);
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
+				<button>mutate: 0</button>
+				<button>reassign: 0</button>
+			`
+		);
+
+		flushSync(() => {
+			btn3?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
+				<button>mutate: 1</button>
+				<button>reassign: 1</button>
+			`
+		);
+
+		flushSync(() => {
+			btn4?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
+				<button>mutate: 2</button>
+				<button>reassign: 2</button>
+			`
+		);
+
+		flushSync(() => {
+			btn3?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
 			`
 		);
 	}