From f1d9afe32f84ffd138ab2a67906b30305cd3e4e7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 24 Mar 2024 18:03:56 -0400
Subject: [PATCH] changeset (#10906)

---
 .changeset/many-rockets-give.md               |  5 +++++
 .../3-transform/client/visitors/template.js   | 19 ++++++++++++-------
 .../src/internal/client/dom/blocks/if.js      | 10 ++++++++--
 3 files changed, 25 insertions(+), 9 deletions(-)
 create mode 100644 .changeset/many-rockets-give.md

diff --git a/.changeset/many-rockets-give.md b/.changeset/many-rockets-give.md
new file mode 100644
index 0000000000..b26d8ef64f
--- /dev/null
+++ b/.changeset/many-rockets-give.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: more efficient if block compiler output
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js
index 7c783c6f9e..873fa44a70 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js
@@ -2573,15 +2573,20 @@ export const template_visitors = {
 		const args = [
 			context.state.node,
 			b.thunk(/** @type {import('estree').Expression} */ (context.visit(node.test))),
-			b.arrow([b.id('$$anchor')], consequent),
-			node.alternate
-				? b.arrow(
-						[b.id('$$anchor')],
-						/** @type {import('estree').BlockStatement} */ (context.visit(node.alternate))
-					)
-				: b.literal(null)
+			b.arrow([b.id('$$anchor')], consequent)
 		];
 
+		if (node.alternate || node.elseif) {
+			args.push(
+				node.alternate
+					? b.arrow(
+							[b.id('$$anchor')],
+							/** @type {import('estree').BlockStatement} */ (context.visit(node.alternate))
+						)
+					: b.literal(null)
+			);
+		}
+
 		if (node.elseif) {
 			// We treat this...
 			//
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 84b294ccbb..dde4de4dbc 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -12,11 +12,17 @@ import {
  * @param {Comment} anchor
  * @param {() => boolean} get_condition
  * @param {(anchor: Node) => import('#client').Dom} consequent_fn
- * @param {null | ((anchor: Node) => import('#client').Dom)} alternate_fn
+ * @param {null | ((anchor: Node) => import('#client').Dom)} [alternate_fn]
  * @param {boolean} [elseif] True if this is an `{:else if ...}` block rather than an `{#if ...}`, as that affects which transitions are considered 'local'
  * @returns {void}
  */
-export function if_block(anchor, get_condition, consequent_fn, alternate_fn, elseif = false) {
+export function if_block(
+	anchor,
+	get_condition,
+	consequent_fn,
+	alternate_fn = null,
+	elseif = false
+) {
 	hydrate_block_anchor(anchor);
 
 	/** @type {import('#client').Effect | null} */