fix: make `$state` component exports settable (#12345)

* fix: make `$state` component exports settable

fixes #11983

* failing test

* fix

---------

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

.changeset/tidy-lizards-happen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make `$state` component exports settable

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1010,6 +1010,9 @@ const runes_scope_tweaker = {
 			name: node.local.name,
 			alias: node.exported.name
 		});
+
+		const binding = state.scope.get(node.local.name);
+		if (binding) binding.reassigned = true;
 	},
 	ExportNamedDeclaration(node, { next, state }) {
 		if (!node.declaration || state.ast_type !== 'instance') {

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

@@ -198,14 +198,38 @@ export function client_component(source, analysis, options) {
 	}
 
 	/** @type {Array<ESTree.Property | ESTree.SpreadElement>} */
-	const component_returned_object = analysis.exports.map(({ name, alias }) => {
+	const component_returned_object = analysis.exports.flatMap(({ name, alias }) => {
+		const binding = instance_state.scope.get(name);
 		const expression = serialize_get_binding(b.id(name), instance_state);
+		const getter = b.get(alias ?? name, [b.return(expression)]);
+
+		if (expression.type === 'Identifier') {
+			if (binding?.declaration_kind === 'let' || binding?.declaration_kind === 'var') {
+				return [
+					getter,
+					b.set(alias ?? name, [b.stmt(b.assignment('=', expression, b.id('$$value')))])
+				];
+			} else if (!options.dev) {
+				return b.init(alias ?? name, expression);
+			}
+		}
 
-		if (expression.type === 'Identifier' && !options.dev) {
-			return b.init(alias ?? name, expression);
+		if (binding?.kind === 'state' || binding?.kind === 'frozen_state') {
+			return [
+				getter,
+				b.set(alias ?? name, [
+					b.stmt(
+						b.call(
+							'$.set',
+							b.id(name),
+							b.call(binding.kind === 'state' ? '$.proxy' : '$.freeze', b.id('$$value'))
+						)
+					)
+				])
+			];
 		}
 
-		return b.get(alias ?? name, [b.return(expression)]);
+		return getter;
 	});
 
 	const properties = [...analysis.instance.scope.declarations].filter(

packages/svelte/tests/runtime-runes/samples/exports-3/_config.js

@@ -4,10 +4,9 @@ import { test } from '../../test';
 export default test({
 	test({ assert, target }) {
 		assert.htmlEqual(target.innerHTML, `0 0 <button>0 / 0</button>`);
-		const [btn] = target.querySelectorAll('button');
+		const btn = target.querySelector('button');
 
-		btn?.click();
-		flushSync();
+		flushSync(() => btn?.click());
 		assert.htmlEqual(target.innerHTML, '1 2 <button>1 / 2</button>');
 	}
 });

packages/svelte/tests/runtime-runes/samples/exports-3/main.svelte

@@ -1,5 +1,5 @@
 <script>
-	import Sub from './sub.svelte'
+	import Sub from './sub.svelte';
 	let sub = $state();
 </script>
 

packages/svelte/tests/runtime-runes/samples/exports-4/_config.js

@@ -0,0 +1,12 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		assert.htmlEqual(target.innerHTML, `0 0 <button>0 / 0</button>`);
+		const btn = target.querySelector('button');
+
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, '1 2 <button>1 / 2</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/exports-4/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Sub from './sub.svelte';
+	let sub = $state();
+</script>
+
+<Sub bind:this={sub} />
+<button on:click={() => sub.count++}>{sub?.count} / {sub?.doubled}</button>

packages/svelte/tests/runtime-runes/samples/exports-4/sub.svelte

@@ -0,0 +1,9 @@
+<script>
+	let count = $state(0);
+	let doubled = $derived(count * 2);
+
+	export { count, doubled };
+</script>
+
+{count}
+{doubled}