fix: improve member expression mutation logic (#9625)

* fix: improve member expression mutation logic

* cleanup

* Update .changeset/moody-frogs-exist.md

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/moody-frogs-exist.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve member expression mutation logic

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

@@ -2170,17 +2170,15 @@ export const template_visitors = {
 
 		/**
 		 * @param {import('estree').Pattern} expression_for_id
-		 * @param {import('estree').Expression} expression_for_other
 		 * @returns {import('#compiler').Binding['mutation']}
 		 */
-		const create_mutation = (expression_for_id, expression_for_other) => {
+		const create_mutation = (expression_for_id) => {
 			return (assignment, context) => {
 				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);
 				}
-
 				const left = object(assignment.left);
 				const value = get_assignment_value(assignment, context);
 				const invalidate = b.call(
@@ -2193,11 +2191,7 @@ export const template_visitors = {
 					return context.state.analysis.runes ? assign : b.sequence([assign, invalidate]);
 				} else {
 					const original_left = /** @type {import('estree').MemberExpression} */ (assignment.left);
-					const left = b.member(
-						expression_for_other,
-						context.visit(original_left).property,
-						original_left.computed
-					);
+					const left = context.visit(original_left);
 					const assign = b.assignment(assignment.operator, left, value);
 					return context.state.analysis.runes ? assign : b.sequence([assign, invalidate]);
 				}
@@ -2223,8 +2217,7 @@ export const template_visitors = {
 					each_node_meta.array_name ? b.call(each_node_meta.array_name) : collection,
 					index,
 					true
-				),
-				binding.expression
+				)
 			);
 		} else {
 			const unwrapped = binding.expression;
@@ -2252,8 +2245,7 @@ export const template_visitors = {
 
 				binding.expression = b.call(name);
 				binding.mutation = create_mutation(
-					/** @type {import('estree').Pattern} */ (path.update_expression(unwrapped)),
-					binding.expression
+					/** @type {import('estree').Pattern} */ (path.update_expression(unwrapped))
 				);
 			}
 		}

packages/svelte/tests/runtime-runes/samples/member-mutation/_config.js

@@ -0,0 +1,19 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>person.name.first = "dave"</button><h3>JSON output</h3><div>[{"name":{"first":"rob"}}]</div>`,
+
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => {
+			button?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>person.name.first = "dave"</button><h3>JSON output</h3><div>[{"name":{"first":"dave"}}]</div>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/member-mutation/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	let people = $state([{name:{first:'rob'}}]);
+</script>
+
+{#each people as person}
+	<button on:click={()=>{
+		person.name.first = "dave";
+		people = people;
+	}}>person.name.first = "dave"</button>
+{/each}
+
+<h3>JSON output</h3>
+{#each people as person}
+	<div>{JSON.stringify(people)}</div>
+{/each}