fix: improve each block index handling (#9889)

Author: trueadm

.changeset/wise-dancers-hang.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve each block index handling

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

@@ -2236,6 +2236,12 @@ export const template_visitors = {
 		const item = b.id(each_node_meta.item_name);
 		const binding = /** @type {import('#compiler').Binding} */ (context.state.scope.get(item.name));
 		binding.expression = each_item_is_reactive ? b.call('$.unwrap', item) : item;
+		if (node.index) {
+			const index_binding = /** @type {import('#compiler').Binding} */ (
+				context.state.scope.get(node.index)
+			);
+			index_binding.expression = each_item_is_reactive ? b.call('$.unwrap', index) : index;
+		}
 
 		/** @type {import('estree').Statement[]} */
 		const declarations = [];
@@ -2291,7 +2297,7 @@ export const template_visitors = {
 		const key_function =
 			node.key && ((each_type & EACH_ITEM_REACTIVE) !== 0 || context.state.options.dev)
 				? b.arrow(
-						[node.context.type === 'Identifier' ? node.context : b.id('$$item')],
+						[node.context.type === 'Identifier' ? node.context : b.id('$$item'), index],
 						b.block(
 							declarations.concat(
 								b.return(/** @type {import('estree').Expression} */ (context.visit(node.key)))

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

@@ -1,4 +1,5 @@
-import { untrack } from './runtime.js';
+import { EACH_INDEX_REACTIVE } from '../../constants.js';
+import { source, untrack } from './runtime.js';
 import { is_array } from './utils.js';
 
 /** regex of all html void element names */
@@ -65,7 +66,7 @@ export function validate_dynamic_element_tag(tag_fn) {
 
 /**
  * @param {() => any} collection
- * @param {(item: any) => string} key_fn
+ * @param {(item: any, index: number) => string} key_fn
  * @returns {void}
  */
 export function validate_each_keys(collection, key_fn) {
@@ -78,7 +79,7 @@ export function validate_each_keys(collection, key_fn) {
 		: Array.from(maybe_array);
 	const length = array.length;
 	for (let i = 0; i < length; i++) {
-		const key = key_fn(array[i]);
+		const key = key_fn(array[i], i);
 		if (keys.has(key)) {
 			throw new Error(
 				`Cannot have duplicate keys in a keyed each: Keys at index ${keys.get(

packages/svelte/tests/runtime-legacy/samples/each-block-keyed-index/_config.js

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

packages/svelte/tests/runtime-legacy/samples/each-block-keyed-index/main.svelte

@@ -0,0 +1,3 @@
+{#each {length: 2} as item, i (`${i}`)}
+	<div>{i}</div>
+{/each}