fix: handle component binding mutation (#10786)

* fix: handle component binding mutation

https://github.com/sveltejs/svelte/issues/10359#issuecomment-1991885046

* alternative approach to mutating props (#10788)

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

---------

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

.changeset/witty-readers-provide.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: handle component binding mutation

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

@@ -470,12 +470,14 @@ export function serialize_set_binding(node, context, fallback, options) {
 				if (binding.kind === 'prop') {
 					return b.call(
 						left,
-						b.assignment(
-							node.operator,
-							/** @type {import('estree').Pattern} */ (visit(node.left)),
-							value
-						),
-						b.literal(true)
+						b.sequence([
+							b.assignment(
+								node.operator,
+								/** @type {import('estree').Pattern} */ (visit(node.left)),
+								value
+							),
+							b.call(left)
+						])
 					);
 				} else {
 					return b.call(

packages/svelte/src/internal/client/reactivity/props.js

@@ -210,7 +210,7 @@ export function prop(props, key, flags, initial) {
 
 	if (!immutable) current_value.equals = safe_equals;
 
-	return function (/** @type {V} */ value, mutation = false) {
+	return function (/** @type {V} */ value) {
 		var current = get(current_value);
 
 		// legacy nonsense — need to ensure the source is invalidated when necessary
@@ -226,9 +226,9 @@ export function prop(props, key, flags, initial) {
 		}
 
 		if (arguments.length > 0) {
-			if (mutation || (immutable ? value !== current : safe_not_equal(value, current))) {
+			if (!current_value.equals(value)) {
 				from_child = true;
-				set(inner_current_value, mutation ? current : value);
+				set(inner_current_value, value);
 				get(current_value); // force a synchronisation immediately
 			}
 

packages/svelte/tests/runtime-legacy/samples/component-binding-deep2/Widget.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let value;
+</script>
+
+<input bind:value={value.name}>

packages/svelte/tests/runtime-legacy/samples/component-binding-deep2/_config.js

@@ -0,0 +1,32 @@
+import { ok, test } from '../../test';
+
+export default test({
+	html: `
+		<input>
+		<p>foo</p>
+	`,
+
+	ssrHtml: `
+		<input value=foo>
+		<p>foo</p>
+	`,
+
+	async test({ assert, component, target, window }) {
+		const event = new window.MouseEvent('input');
+		const input = target.querySelector('input');
+		ok(input);
+
+		input.value = 'blah';
+		await input.dispatchEvent(event);
+		await Promise.resolve();
+
+		assert.deepEqual(component.deep, { name: 'blah' });
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<input>
+			<p>blah</p>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/component-binding-deep2/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Widget from './Widget.svelte';
+
+	export let deep = {
+		name: 'foo'
+	};
+</script>
+
+<Widget bind:value={deep}/>
+
+<p>{deep.name}</p>