feat: better generated each block code in SSR mode (#13060)

* chore: tidy up each block code

* groundwork

* cache length

* changeset

* regenerate

.changeset/red-maps-nail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: better generated each block code in SSR mode

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

@@ -147,7 +147,7 @@ export function EachBlock(node, context) {
 	// which needs a reference to the index
 	const index =
 		each_node_meta.contains_group_binding || !node.index ? each_node_meta.index : b.id(node.index);
-	const item = each_node_meta.item;
+	const item = node.context.type === 'Identifier' ? node.context : b.id('$$item');
 
 	let uses_index = each_node_meta.contains_group_binding;
 	let key_uses_index = false;

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

@@ -32,7 +32,7 @@ export function VariableDeclaration(node, context) {
 			) {
 				if (init != null && is_hoisted_function(init)) {
 					context.state.hoisted.push(
-						b.declaration('const', declarator.id, /** @type {Expression} */ (context.visit(init)))
+						b.const(declarator.id, /** @type {Expression} */ (context.visit(init)))
 					);
 
 					continue;
@@ -205,7 +205,7 @@ export function VariableDeclaration(node, context) {
 
 				if (init != null && is_hoisted_function(init)) {
 					context.state.hoisted.push(
-						b.declaration('const', declarator.id, /** @type {Expression} */ (context.visit(init)))
+						b.const(declarator.id, /** @type {Expression} */ (context.visit(init)))
 					);
 
 					continue;

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

@@ -14,7 +14,6 @@ export function EachBlock(node, context) {
 
 	const each_node_meta = node.metadata;
 	const collection = /** @type {Expression} */ (context.visit(node.expression));
-	const item = each_node_meta.item;
 	const index =
 		each_node_meta.contains_group_binding || !node.index ? each_node_meta.index : b.id(node.index);
 
@@ -22,11 +21,8 @@ export function EachBlock(node, context) {
 	state.init.push(b.const(array_id, b.call('$.ensure_array_like', collection)));
 
 	/** @type {Statement[]} */
-	const each = [b.const(item, b.member(array_id, index, true))];
+	const each = [b.const(/** @type {Pattern} */ (node.context), b.member(array_id, index, true))];
 
-	if (node.context.type !== 'Identifier') {
-		each.push(b.const(/** @type {Pattern} */ (node.context), item));
-	}
 	if (index.name !== node.index && node.index != null) {
 		each.push(b.let(node.index, index));
 	}
@@ -34,8 +30,11 @@ export function EachBlock(node, context) {
 	each.push(.../** @type {BlockStatement} */ (context.visit(node.body)).body);
 
 	const for_loop = b.for(
-		b.let(index, b.literal(0)),
+		b.declaration('let', [
-		b.binary('<', index, b.member(array_id, 'length')),
+			b.declarator(index, b.literal(0)),
+			b.declarator('$$length', b.member(array_id, 'length'))
+		]),
+		b.binary('<', index, b.id('$$length')),
 		b.update('++', index, false),
 		b.block(each)
 	);

packages/svelte/src/compiler/phases/scope.js

@@ -547,9 +547,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 
 				binding.metadata = { inside_rest: is_rest_id };
 			}
-			if (node.context.type !== 'Identifier') {
+
-				scope.declare(b.id('$$item'), 'template', 'synthetic');
-			}
 			// Visit to pick up references from default initializers
 			visit(node.context, { scope });
 
@@ -583,7 +581,6 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 				contains_group_binding: false,
 				array_name: needs_array_deduplication ? state.scope.root.unique('$$array') : null,
 				index: scope.root.unique('$$index'),
-				item: node.context.type === 'Identifier' ? node.context : b.id('$$item'),
 				declarations: scope.declarations,
 				is_controlled: false
 			};

packages/svelte/src/compiler/types/template.d.ts

@@ -405,7 +405,6 @@ export interface EachBlock extends BaseNode {
 		/** Set if something in the array expression is shadowed within the each block */
 		array_name: Identifier | null;
 		index: Identifier;
-		item: Identifier;
 		declarations: Map<string, Binding>;
 		/**
 		 * Optimization path for each blocks: If the parent isn't a fragment and

packages/svelte/src/compiler/utils/builders.js

@@ -176,27 +176,25 @@ export function logical(operator, left, right) {
 
 /**
  * @param {'const' | 'let' | 'var'} kind
- * @param {string | ESTree.Pattern} pattern
+ * @param {ESTree.VariableDeclarator[]} declarations
- * @param {ESTree.Expression} [init]
  * @returns {ESTree.VariableDeclaration}
  */
-export function declaration(kind, pattern, init) {
+export function declaration(kind, declarations) {
-	if (typeof pattern === 'string') pattern = id(pattern);
-
 	return {
 		type: 'VariableDeclaration',
 		kind,
-		declarations: [init ? declarator(pattern, init) : declarator(pattern)]
+		declarations
 	};
 }
 
 /**
- * @param {ESTree.Pattern} id
+ * @param {ESTree.Pattern | string} pattern
  * @param {ESTree.Expression} [init]
  * @returns {ESTree.VariableDeclarator}
  */
-export function declarator(id, init) {
+export function declarator(pattern, init) {
-	return { type: 'VariableDeclarator', id, init };
+	if (typeof pattern === 'string') pattern = id(pattern);
+	return { type: 'VariableDeclarator', id: pattern, init };
 }
 
 /** @type {ESTree.EmptyStatement} */
@@ -491,7 +489,7 @@ const this_instance = {
  * @returns {ESTree.VariableDeclaration}
  */
 function let_builder(pattern, init) {
-	return declaration('let', pattern, init);
+	return declaration('let', [declarator(pattern, init)]);
 }
 
 /**
@@ -500,7 +498,7 @@ function let_builder(pattern, init) {
  * @returns {ESTree.VariableDeclaration}
  */
 function const_builder(pattern, init) {
-	return declaration('const', pattern, init);
+	return declaration('const', [declarator(pattern, init)]);
 }
 
 /**
@@ -509,7 +507,7 @@ function const_builder(pattern, init) {
  * @returns {ESTree.VariableDeclaration}
  */
 function var_builder(pattern, init) {
-	return declaration('var', pattern, init);
+	return declaration('var', [declarator(pattern, init)]);
 }
 
 /**

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

@@ -5,7 +5,7 @@ export default function Each_string_template($$payload) {
 
 	$$payload.out += `<!--[-->`;
 
-	for (let $$index = 0; $$index < each_array.length; $$index++) {
+	for (let $$index = 0, $$length = each_array.length; $$index < $$length; $$index++) {
 		const thing = each_array[$$index];
 
 		$$payload.out += `${$.escape(thing)}, `;

packages/svelte/types/index.d.ts

@@ -1866,7 +1866,6 @@ declare module 'svelte/compiler' {
 			/** Set if something in the array expression is shadowed within the each block */
 			array_name: Identifier | null;
 			index: Identifier;
-			item: Identifier;
 			declarations: Map<string, Binding>;
 			/**
 			 * Optimization path for each blocks: If the parent isn't a fragment and