fix: make each items reassignable (#12257)

* chore: make each items reassignable

* add failing test

* update test

* transform reassigned getters inside each block

* Update .changeset/thirty-guests-flow.md

* comment

* add note

* tidy up

* Update packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js

Author: Rich-Harris

.changeset/thirty-guests-flow.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make each items reassignable in legacy mode

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

@@ -186,8 +186,23 @@ export function EachBlock(node, context) {
 	if (invalidate_store) sequence.push(invalidate_store);
 
 	if (node.context.type === 'Identifier') {
+		const binding = /** @type {Binding} */ (context.state.scope.get(node.context.name));
+
 		child_state.transform[node.context.name] = {
-			read: (flags & EACH_ITEM_REACTIVE) !== 0 ? get_value : (node) => node,
+			read: (node) => {
+				if (binding.reassigned) {
+					// we need to do `array[$$index]` instead of `$$item` or whatever
+					// TODO 6.0 this only applies in legacy mode, reassignments are
+					// forbidden in runes mode
+					return b.member(
+						each_node_meta.array_name ? b.call(each_node_meta.array_name) : collection,
+						index,
+						true
+					);
+				}
+
+				return (flags & EACH_ITEM_REACTIVE) !== 0 ? get_value(node) : node;
+			},
 			assign: (_, value) => {
 				uses_index = true;
 

packages/svelte/src/compiler/phases/3-transform/server/visitors/EachBlock.js

@@ -21,7 +21,7 @@ export function EachBlock(node, context) {
 	state.init.push(b.const(array_id, b.call('$.ensure_array_like', collection)));
 
 	/** @type {Statement[]} */
-	const each = [b.const(/** @type {Pattern} */ (node.context), b.member(array_id, index, true))];
+	const each = [b.let(/** @type {Pattern} */ (node.context), b.member(array_id, index, true))];
 
 	if (index.name !== node.index && node.index != null) {
 		each.push(b.let(node.index, index));

packages/svelte/tests/runtime-legacy/samples/component-binding-each-reassigned/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	export let value;
+
+	value += 1;
+</script>
+
+<p>{value}</p>

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

@@ -0,0 +1,19 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<p>2, 3, 4</p>
+		<p>2</p>
+		<p>3</p>
+		<p>4</p>
+		<p>2, 3, 4</p>
+	`,
+
+	ssrHtml: `
+		<p>1, 2, 3</p>
+		<p>2</p>
+		<p>3</p>
+		<p>4</p>
+		<p>1, 2, 3</p>
+	`
+});

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

@@ -0,0 +1,13 @@
+<script>
+	import Child from './Child.svelte';
+
+	let numbers = [1, 2, 3];
+</script>
+
+<p>{numbers.join(', ')}</p>
+
+{#each numbers as n}
+	<Child bind:value={n} />
+{/each}
+
+<p>{numbers.join(', ')}</p>

packages/svelte/tests/snapshot/samples/each-string-template/_expected/server/index.svelte.js

@@ -6,7 +6,7 @@ export default function Each_string_template($$payload) {
 	$$payload.out += `<!--[-->`;
 
 	for (let $$index = 0, $$length = each_array.length; $$index < $$length; $$index++) {
-		const thing = each_array[$$index];
+		let thing = each_array[$$index];
 
 		$$payload.out += `<!---->${$.escape(thing)}, `;
 	}