fix: ensure proxy is updated before notifying listeners (#10267)

fixes #10264
fixes #10265

Author: dummdidumm

.changeset/witty-steaks-dream.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure proxy is updated before notifying listeners

packages/svelte/src/internal/client/proxy/proxy.js

@@ -244,26 +244,32 @@ const handler = {
 		const is_array = metadata.a;
 		const not_has = !(prop in target);
 
+		// variable.length = value -> clear all signals with index >= value
 		if (is_array && prop === 'length') {
 			for (let i = value; i < target.length; i += 1) {
 				const s = metadata.s.get(i + '');
 				if (s !== undefined) set(s, UNINITIALIZED);
 			}
 		}
-		if (not_has) {
-			update(metadata.v);
-		}
+
+		// Set the new value before updating any signals so that any listeners get the new value
 		// @ts-ignore
 		target[prop] = value;
 
-		// If we have mutated an array directly, we might need to
-		// signal that length has also changed too.
-		if (is_array && not_has) {
-			const ls = metadata.s.get('length');
-			const length = target.length;
-			if (ls !== undefined && ls.v !== length) {
-				set(ls, length);
+		if (not_has) {
+			// If we have mutated an array directly, we might need to
+			// signal that length has also changed. Do it before updating metadata
+			// to ensure that iterating over the array as a result of a metadata update
+			// will not cause the length to be out of sync.
+			if (is_array) {
+				const ls = metadata.s.get('length');
+				const length = target.length;
+				if (ls !== undefined && ls.v !== length) {
+					set(ls, length);
+				}
 			}
+
+			update(metadata.v);
 		}
 
 		return true;

packages/svelte/tests/runtime-runes/samples/inspect-new-property/_config.js

@@ -0,0 +1,33 @@
+import { test } from '../../test';
+
+/**
+ * @type {any[]}
+ */
+let log;
+/**
+ * @type {typeof console.log}}
+ */
+let original_log;
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	before_test() {
+		log = [];
+		original_log = console.log;
+		console.log = (...v) => {
+			log.push(...v);
+		};
+	},
+	after_test() {
+		console.log = original_log;
+	},
+	async test({ assert, target }) {
+		const [btn] = target.querySelectorAll('button');
+		btn.click();
+		await Promise.resolve();
+
+		assert.deepEqual(log, ['init', {}, 'init', [], 'update', { x: 'hello' }, 'update', ['hello']]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/inspect-new-property/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	let obj = $state({});
+	let array = $state([]);
+
+	$inspect(obj);
+	$inspect(array);
+</script>
+
+<button
+	onclick={() => {
+		obj.x = "hello";
+		array[0] = "hello";
+	}}>
+	add prop
+</button>