fix: ensure $state.snapshot clones holey arrays correctly

.changeset/clean-bobcats-fail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure $state.snapshot clones holey arrays correctly

packages/svelte/src/internal/shared/clone.js

@@ -54,30 +54,33 @@ export function snapshot(value, skip_warning = false) {
  */
 function clone(value, cloned, path, paths, original = null) {
 	if (typeof value === 'object' && value !== null) {
-		const unwrapped = cloned.get(value);
+		var unwrapped = cloned.get(value);
 		if (unwrapped !== undefined) return unwrapped;
 
 		if (value instanceof Map) return /** @type {Snapshot<T>} */ (new Map(value));
 		if (value instanceof Set) return /** @type {Snapshot<T>} */ (new Set(value));
 
 		if (is_array(value)) {
-			const copy = /** @type {Snapshot<any>} */ ([]);
+			var copy = /** @type {Snapshot<any>} */ (Array(value.length));
 			cloned.set(value, copy);
 
 			if (original !== null) {
 				cloned.set(original, copy);
 			}
 
-			for (let i = 0; i < value.length; i += 1) {
-				copy.push(clone(value[i], cloned, DEV ? `${path}[${i}]` : path, paths));
+			for (var i = 0; i < value.length; i += 1) {
+				var element = value[i];
+				if (i in value) {
+					copy[i] = clone(element, cloned, DEV ? `${path}[${i}]` : path, paths);
+				}
 			}
 
 			return copy;
 		}
 
 		if (get_prototype_of(value) === object_prototype) {
 			/** @type {Snapshot<any>} */
-			const copy = {};
+			copy = {};
 			cloned.set(value, copy);
 
 			if (original !== null) {

packages/svelte/tests/runtime-runes/samples/state-snapshot-array-holes/_config.js

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

packages/svelte/tests/runtime-runes/samples/state-snapshot-array-holes/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let arr = []
+	arr[5] = true
+
+	let state = $state([])
+	state[5] = true
+</script>
+
+<div>{2 in $state.snapshot(state)}</div>
+<div>{5 in $state.snapshot(state)}</div>