fix: correct increment/decrement code generation

fixes #10226

.changeset/honest-buses-add.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: correct increment/decrement code generation

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

@@ -83,8 +83,16 @@ export const global_visitors = {
 			argument.property.type === 'PrivateIdentifier' &&
 			context.state.private_state.has(argument.property.name)
 		) {
-			let fn = node.operator === '++' ? '$.increment' : '$.decrement';
+			let fn = '$.update';
-			return b.call(fn, argument);
+			if (node.prefix) fn += '_pre';
+
+			/** @type {import('estree').Expression[]} */
+			const args = [argument];
+			if (node.operator === '--') {
+				args.push(b.literal(-1));
+			}
+
+			return b.call(fn, ...args);
 		} else {
 			// turn it into an IIFEE assignment expression: i++ -> (() => { const $$value = i; i+=1; return $$value; })
 			const assignment = b.assignment(

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

@@ -511,24 +511,17 @@ const global_visitors = {
 		const { state, next } = context;
 		const argument = node.argument;
 
-		if (argument.type === 'Identifier') {
+		if (argument.type === 'Identifier' && state.scope.get(argument.name)?.kind === 'store_sub') {
-			const binding = state.scope.get(argument.name);
+			let fn = '$.update_store';
-			const is_store = binding?.kind === 'store_sub';
-			const name = is_store ? argument.name.slice(1) : argument.name;
-			// use runtime functions for smaller output
-			if (is_store) {
-				let fn = node.operator === '++' ? '$.increment' : '$.decrement';
 			if (node.prefix) fn += '_pre';
 
-				if (is_store) {
+			/** @type {import('estree').Expression[]} */
-					fn += '_store';
+			const args = [b.id('$$store_subs'), b.literal(argument.name), b.id(argument.name.slice(1))];
-					return b.call(fn, serialize_get_binding(b.id(name), state), b.call('$' + name));
+			if (node.operator === '--') {
-				} else {
+				args.push(b.literal(-1));
-					return b.call(fn, b.id(name));
-				}
-			} else {
-				return next();
 			}
+
+			return b.call(fn, ...args);
 		}
 		return next();
 	}

packages/svelte/src/internal/server/index.js

@@ -407,6 +407,32 @@ export function mutate_store(store_values, store_name, store, expression) {
 	return expression;
 }
 
+/**
+ * @param {Record<string, [any, any, any]>} store_values
+ * @param {string} store_name
+ * @param {import('../client/types.js').Store<number>} store
+ * @param {1 | -1} [d]
+ * @returns {number}
+ */
+export function update_store(store_values, store_name, store, d = 1) {
+	let store_value = store_get(store_values, store_name, store);
+	store.set(store_value + d);
+	return store_value;
+}
+
+/**
+ * @param {Record<string, [any, any, any]>} store_values
+ * @param {string} store_name
+ * @param {import('../client/types.js').Store<number>} store
+ * @param {1 | -1} [d]
+ * @returns {number}
+ */
+export function update_store_pre(store_values, store_name, store, d = 1) {
+	const value = store_get(store_values, store_name, store) + d;
+	store.set(value);
+	return value;
+}
+
 /** @param {Record<string, [any, any, any]>} store_values */
 export function unsubscribe_stores(store_values) {
 	for (const store_name in store_values) {

packages/svelte/tests/runtime-runes/samples/store-increment-decrement/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `0 0 2 2 0`
+});

packages/svelte/tests/runtime-runes/samples/store-increment-decrement/main.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+	import { writable } from "svelte/store";
+
+	let x = writable(0);
+	const a = $x++;
+	const b = ++$x;
+	const c = $x--;
+	const d = --$x;
+</script>
+
+{$x} {a} {b} {c} {d}