feat: shorter each blocks (#10937)

Author: Rich-Harris

.changeset/three-lions-visit.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: more efficient each block compiler output

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

@@ -2314,12 +2314,6 @@ export const template_visitors = {
 		// TODO should use context.visit?
 		const children = create_block(node, 'each_block', node.body.nodes, context);
 
-		const else_block = node.fallback
-			? b.arrow(
-					[b.id('$$anchor')],
-					/** @type {import('estree').BlockStatement} */ (context.visit(node.fallback))
-				)
-			: b.literal(null);
 		const key_function = node.key
 			? b.arrow(
 					[node.context.type === 'Identifier' ? node.context : b.id('$$item'), index],
@@ -2337,40 +2331,46 @@ export const template_visitors = {
 			declarations.push(b.let(node.index, index));
 		}
 
+		let callee = '$.each_indexed';
+
+		/** @type {import('estree').Expression[]} */
+		const args = [];
+
 		if ((each_type & EACH_KEYED) !== 0) {
 			if (context.state.options.dev && key_function.type !== 'Literal') {
 				context.state.init.push(
 					b.stmt(b.call('$.validate_each_keys', b.thunk(collection), key_function))
 				);
 			}
 
-			context.state.after_update.push(
-				b.stmt(
-					b.call(
-						'$.each_keyed',
-						context.state.node,
-						each_node_meta.array_name ? each_node_meta.array_name : b.thunk(collection),
-						b.literal(each_type),
-						key_function,
-						b.arrow([b.id('$$anchor'), item, index], b.block(declarations.concat(children))),
-						else_block
-					)
-				)
+			callee = '$.each_keyed';
+
+			args.push(
+				context.state.node,
+				each_node_meta.array_name ? each_node_meta.array_name : b.thunk(collection),
+				b.literal(each_type),
+				key_function,
+				b.arrow([b.id('$$anchor'), item, index], b.block(declarations.concat(children)))
 			);
 		} else {
-			context.state.after_update.push(
-				b.stmt(
-					b.call(
-						'$.each_indexed',
-						context.state.node,
-						each_node_meta.array_name ? each_node_meta.array_name : b.thunk(collection),
-						b.literal(each_type),
-						b.arrow([b.id('$$anchor'), item, index], b.block(declarations.concat(children))),
-						else_block
-					)
+			args.push(
+				context.state.node,
+				each_node_meta.array_name ? each_node_meta.array_name : b.thunk(collection),
+				b.literal(each_type),
+				b.arrow([b.id('$$anchor'), item, index], b.block(declarations.concat(children)))
+			);
+		}
+
+		if (node.fallback) {
+			args.push(
+				b.arrow(
+					[b.id('$$anchor')],
+					/** @type {import('estree').BlockStatement} */ (context.visit(node.fallback))
 				)
 			);
 		}
+
+		context.state.after_update.push(b.stmt(b.call(callee, ...args)));
 	},
 	IfBlock(node, context) {
 		context.state.template.push('<!>');

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -197,10 +197,10 @@ function each(anchor, get_collection, flags, get_key, render_fn, fallback_fn, re
  * @param {number} flags
  * @param {null | ((item: V) => string)} get_key
  * @param {(anchor: null, item: V, index: import('#client').MaybeSource<number>) => void} render_fn
- * @param {null | ((anchor: Node | null) => void)} fallback_fn
+ * @param {null | ((anchor: Node | null) => void)} [fallback_fn]
  * @returns {void}
  */
-export function each_keyed(anchor, get_collection, flags, get_key, render_fn, fallback_fn) {
+export function each_keyed(anchor, get_collection, flags, get_key, render_fn, fallback_fn = null) {
 	each(anchor, get_collection, flags, get_key, render_fn, fallback_fn, reconcile_tracked_array);
 }
 
@@ -210,10 +210,10 @@ export function each_keyed(anchor, get_collection, flags, get_key, render_fn, fa
  * @param {() => V[]} get_collection
  * @param {number} flags
  * @param {(anchor: null, item: V, index: import('#client').MaybeSource<number>) => void} render_fn
- * @param {null | ((anchor: Node | null) => void)} fallback_fn
+ * @param {null | ((anchor: Node | null) => void)} [fallback_fn]
  * @returns {void}
  */
-export function each_indexed(anchor, get_collection, flags, render_fn, fallback_fn) {
+export function each_indexed(anchor, get_collection, flags, render_fn, fallback_fn = null) {
 	each(anchor, get_collection, flags, null, render_fn, fallback_fn, reconcile_indexed_array);
 }
 

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

@@ -10,18 +10,12 @@ export default function Each_string_template($$anchor, $$props) {
 	var fragment = $.comment($$anchor);
 	var node = $.first_child(fragment);
 
-	$.each_indexed(
-		node,
-		() => ['foo', 'bar', 'baz'],
-		1,
-		($$anchor, thing, $$index) => {
-			var text = $.space_frag($$anchor);
+	$.each_indexed(node, () => ['foo', 'bar', 'baz'], 1, ($$anchor, thing, $$index) => {
+		var text = $.space_frag($$anchor);
 
-			$.render_effect(() => $.set_text(text, `${$.stringify($.unwrap(thing))}, `));
-			return $.close($$anchor, text);
-		},
-		null
-	);
+		$.render_effect(() => $.set_text(text, `${$.stringify($.unwrap(thing))}, `));
+		return $.close($$anchor, text);
+	});
 
 	$.close_frag($$anchor, fragment);
 	$.pop();