fix: array.lastIndexOf without second argument

.changeset/clever-stingrays-shout.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: `array.lastIndexOf` without second argument

packages/svelte/src/internal/client/dev/equality.js

@@ -28,10 +28,18 @@ export function init_array_prototype_warnings() {
 	};
 
 	array_prototype.lastIndexOf = function (item, from_index) {
-		const index = lastIndexOf.call(this, item, from_index);
+		// we need to specify this.length - 1 because it's probably using something like
+		// `arguments` inside so passing undefined is different from not passing anything
+		const index = lastIndexOf.call(this, item, from_index ?? this.length - 1);
 
 		if (index === -1) {
-			const test = lastIndexOf.call(get_proxied_value(this), get_proxied_value(item), from_index);
+			// we need to specify this.length - 1 because it's probably using something like
+			// `arguments` inside so passing undefined is different from not passing anything
+			const test = lastIndexOf.call(
+				get_proxied_value(this),
+				get_proxied_value(item),
+				from_index ?? this.length - 1
+			);
 
 			if (test !== -1) {
 				w.state_proxy_equality_mismatch('array.lastIndexOf(...)');

packages/svelte/tests/runtime-runes/samples/array-lastindexof/_config.js

@@ -0,0 +1,9 @@
+import { ok, test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, logs }) {
+		assert.equal(logs[0], logs[1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/array-lastindexof/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	const arr = [0, 1, 2];
+	console.log(arr.lastIndexOf(2));
+	console.log(arr.lastIndexOf(2, arr.length - 1));
+</script>