fix

Author: Rich-Harris

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

@@ -81,7 +81,17 @@ function serialize_assignment(node, context, fallback) {
 	}
 
 	if (left === node.left) {
-		return b.call('$.store_set', b.id(name), /** @type {Expression} */ (context.visit(node.right)));
+		const value =
+			node.operator === '='
+				? /** @type {Expression} */ (context.visit(node.right))
+				: // turn something like x += 1 into x = x + 1
+					b.binary(
+						/** @type {BinaryOperator} */ (node.operator.slice(0, -1)),
+						serialize_get_binding(node.left, context.state),
+						/** @type {Expression} */ (context.visit(node.right))
+					);
+
+		return b.call('$.store_set', b.id(name), value);
 	}
 
 	return b.call(
@@ -92,31 +102,11 @@ function serialize_assignment(node, context, fallback) {
 		b.assignment(
 			node.operator,
 			/** @type {Pattern} */ (context.visit(node.left)),
-			get_assignment_value(node, context)
+			/** @type {Expression} */ (context.visit(node.right))
 		)
 	);
 }
 
-/**
- * @param {AssignmentExpression} node
- * @param {Pick<import('zimmerframe').Context<SvelteNode, ServerTransformState>, 'visit' | 'state'>} context
- */
-function get_assignment_value(node, { state, visit }) {
-	if (node.left.type === 'Identifier') {
-		const operator = node.operator;
-		return operator === '='
-			? /** @type {Expression} */ (visit(node.right))
-			: // turn something like x += 1 into x = x + 1
-				b.binary(
-					/** @type {BinaryOperator} */ (operator.slice(0, -1)),
-					serialize_get_binding(node.left, state),
-					/** @type {Expression} */ (visit(node.right))
-				);
-	}
-
-	return /** @type {Expression} */ (visit(node.right));
-}
-
 /**
  * @param {string} name
  */

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

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: '<p>3</p>'
+});

packages/svelte/tests/runtime-runes/samples/store-assignments/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	const count = writable(0);
+
+	$count += 1;
+	$count += 1;
+	$count += 1;
+</script>
+
+<p>{$count}</p>