fix: ensure state update expressions are serialised correctly (#12109)

Fixes #12103

Author: trueadm

.changeset/gorgeous-boxes-design.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure state update expressions are serialised correctly

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

@@ -119,10 +119,11 @@ export function serialize_get_binding(node, state) {
  * @param {import('estree').AssignmentExpression} node
  * @param {import('zimmerframe').Context<import('#compiler').SvelteNode, State>} context
  * @param {() => any} fallback
+ * @param {boolean} prefix
  * @param {{skip_proxy_and_freeze?: boolean}} [options]
  * @returns {import('estree').Expression}
  */
-export function serialize_set_binding(node, context, fallback, options) {
+export function serialize_set_binding(node, context, fallback, prefix, options) {
 	const { state, visit } = context;
 
 	const assignee = node.left;
@@ -146,7 +147,9 @@ export function serialize_set_binding(node, context, fallback, options) {
 			const value = path.expression?.(b.id(tmp_id));
 			const assignment = b.assignment('=', path.node, value);
 			original_assignments.push(assignment);
-			assignments.push(serialize_set_binding(assignment, context, () => assignment, options));
+			assignments.push(
+				serialize_set_binding(assignment, context, () => assignment, prefix, options)
+			);
 		}
 
 		if (assignments.every((assignment, i) => assignment === original_assignments[i])) {
@@ -411,6 +414,15 @@ export function serialize_set_binding(node, context, fallback, options) {
 						)
 					);
 				}
+			} else if (
+				node.right.type === 'Literal' &&
+				(node.operator === '+=' || node.operator === '-=')
+			) {
+				return b.update(
+					node.operator === '+=' ? '++' : '--',
+					/** @type {import('estree').Expression} */ (visit(node.left)),
+					prefix
+				);
 			} else {
 				return b.assignment(
 					node.operator,

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

@@ -32,7 +32,7 @@ export const global_visitors = {
 		next();
 	},
 	AssignmentExpression(node, context) {
-		return serialize_set_binding(node, context, context.next);
+		return serialize_set_binding(node, context, context.next, false);
 	},
 	UpdateExpression(node, context) {
 		const { state, next, visit } = context;
@@ -98,7 +98,12 @@ export const global_visitors = {
 				/** @type {import('estree').Pattern} */ (argument),
 				b.literal(1)
 			);
-			const serialized_assignment = serialize_set_binding(assignment, context, () => assignment);
+			const serialized_assignment = serialize_set_binding(
+				assignment,
+				context,
+				() => assignment,
+				node.prefix
+			);
 			const value = /** @type {import('estree').Expression} */ (visit(argument));
 			if (serialized_assignment === assignment) {
 				// No change to output -> nothing to transform -> we can keep the original update expression

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

@@ -798,7 +798,9 @@ function serialize_inline_component(node, component_name, context) {
 				const assignment = b.assignment('=', attribute.expression, b.id('$$value'));
 				push_prop(
 					b.set(attribute.name, [
-						b.stmt(serialize_set_binding(assignment, context, () => context.visit(assignment)))
+						b.stmt(
+							serialize_set_binding(assignment, context, () => context.visit(assignment), false)
+						)
 					])
 				);
 			}
@@ -1026,7 +1028,7 @@ function serialize_bind_this(bind_this, context, node) {
 	const bind_this_id = /** @type {import('estree').Expression} */ (context.visit(bind_this));
 	const ids = Array.from(each_ids.values()).map((id) => b.id('$$value_' + id[0]));
 	const assignment = b.assignment('=', bind_this, b.id('$$value'));
-	const update = serialize_set_binding(assignment, context, () => context.visit(assignment));
+	const update = serialize_set_binding(assignment, context, () => context.visit(assignment), false);
 
 	for (const [binding, [, , expression]] of each_ids) {
 		// reset expressions to what they were before
@@ -2400,7 +2402,7 @@ export const template_visitors = {
 				if (assignment.left.type !== 'Identifier' && assignment.left.type !== 'MemberExpression') {
 					// serialize_set_binding turns other patterns into IIFEs and separates the assignments
 					// into separate expressions, at which point this is called again with an identifier or member expression
-					return serialize_set_binding(assignment, context, () => assignment);
+					return serialize_set_binding(assignment, context, () => assignment, false);
 				}
 				const left = object(assignment.left);
 				const value = get_assignment_value(assignment, context);
@@ -2438,7 +2440,7 @@ export const template_visitors = {
 				: b.id(node.index);
 		const item = each_node_meta.item;
 		const binding = /** @type {import('#compiler').Binding} */ (context.state.scope.get(item.name));
-		binding.expression = (id) => {
+		binding.expression = (/** @type {import("estree").Identifier} */ id) => {
 			const item_with_loc = with_loc(item, id);
 			return b.call('$.unwrap', item_with_loc);
 		};
@@ -2762,6 +2764,7 @@ export const template_visitors = {
 				assignment,
 				context,
 				() => /** @type {import('estree').Expression} */ (visit(assignment)),
+				false,
 				{
 					skip_proxy_and_freeze: true
 				}

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

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, logs }) {
+		assert.deepEqual(logs, [1, 1, 1, 1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-update/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let x = $state(0);
+	let o = $state({ x: 0 });
+
+	console.log(++x);
+	console.log(x++);
+	console.log(++o.x);
+	console.log(o.x++);
+</script>