From 4a6316818cd586f32dd4d82a78c64cf9bd7f0734 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sat, 30 Mar 2024 09:46:53 -0400
Subject: [PATCH] feat: faster HTML tags (#10986)

* feat: faster HTML tags

* move code to where it is used
---
 .changeset/swift-poets-carry.md               |  5 ++
 .../src/internal/client/dom/blocks/html.js    | 48 ++++++++++++++++++-
 .../src/internal/client/dom/reconciler.js     | 36 --------------
 3 files changed, 51 insertions(+), 38 deletions(-)
 create mode 100644 .changeset/swift-poets-carry.md

diff --git a/.changeset/swift-poets-carry.md b/.changeset/swift-poets-carry.md
new file mode 100644
index 0000000000..0cb65a9389
--- /dev/null
+++ b/.changeset/swift-poets-carry.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: faster HTML tags
diff --git a/packages/svelte/src/internal/client/dom/blocks/html.js b/packages/svelte/src/internal/client/dom/blocks/html.js
index 31fcc3d2e2..ff96617885 100644
--- a/packages/svelte/src/internal/client/dom/blocks/html.js
+++ b/packages/svelte/src/internal/client/dom/blocks/html.js
@@ -1,7 +1,8 @@
 import { derived } from '../../reactivity/deriveds.js';
 import { render_effect } from '../../reactivity/effects.js';
 import { get } from '../../runtime.js';
-import { reconcile_html, remove } from '../reconciler.js';
+import { hydrate_nodes, hydrating } from '../hydration.js';
+import { create_fragment_from_html, remove } from '../reconciler.js';
 
 /**
  * @param {Element | Text | Comment} anchor
@@ -13,10 +14,53 @@ export function html(anchor, get_value, svg) {
 	let value = derived(get_value);
 
 	render_effect(() => {
-		var dom = reconcile_html(anchor, get(value), svg);
+		var dom = html_to_dom(anchor, get(value), svg);
 
 		if (dom) {
 			return () => remove(dom);
 		}
 	});
 }
+
+/**
+ * Creates the content for a `@html` tag from its string value,
+ * inserts it before the target anchor and returns the new nodes.
+ * @template V
+ * @param {Element | Text | Comment} target
+ * @param {V} value
+ * @param {boolean} svg
+ * @returns {Element | Comment | (Element | Comment | Text)[]}
+ */
+function html_to_dom(target, value, svg) {
+	if (hydrating) return hydrate_nodes;
+
+	var html = value + '';
+	if (svg) html = `<svg>${html}</svg>`;
+
+	// Don't use create_fragment_with_script_from_html here because that would mean script tags are executed.
+	// @html is basically `.innerHTML = ...` and that doesn't execute scripts either due to security reasons.
+	/** @type {DocumentFragment | Element} */
+	var node = create_fragment_from_html(html);
+
+	if (svg) {
+		node = /** @type {Element} */ (node.firstChild);
+	}
+
+	if (node.childNodes.length === 1) {
+		var child = /** @type {Text | Element | Comment} */ (node.firstChild);
+		target.before(child);
+		return child;
+	}
+
+	var nodes = /** @type {Array<Text | Element | Comment>} */ ([...node.childNodes]);
+
+	if (svg) {
+		while (node.firstChild) {
+			target.before(node.firstChild);
+		}
+	} else {
+		target.before(node);
+	}
+
+	return nodes;
+}
diff --git a/packages/svelte/src/internal/client/dom/reconciler.js b/packages/svelte/src/internal/client/dom/reconciler.js
index 18fac51f96..5b9f246ed3 100644
--- a/packages/svelte/src/internal/client/dom/reconciler.js
+++ b/packages/svelte/src/internal/client/dom/reconciler.js
@@ -1,4 +1,3 @@
-import { hydrate_nodes, hydrating } from './hydration.js';
 import { is_array } from '../utils.js';
 
 /** @param {string} html */
@@ -23,38 +22,3 @@ export function remove(current) {
 		current.remove();
 	}
 }
-
-/**
- * Creates the content for a `@html` tag from its string value,
- * inserts it before the target anchor and returns the new nodes.
- * @template V
- * @param {Element | Text | Comment} target
- * @param {V} value
- * @param {boolean} svg
- * @returns {Element | Comment | (Element | Comment | Text)[]}
- */
-export function reconcile_html(target, value, svg) {
-	if (hydrating) {
-		return hydrate_nodes;
-	}
-	var html = value + '';
-	// Even if html is the empty string we need to continue to insert something or
-	// else the element ordering gets out of sync, resulting in subsequent values
-	// not getting inserted anymore.
-	var frag_nodes;
-	if (svg) {
-		html = `<svg>${html}</svg>`;
-	}
-	// Don't use create_fragment_with_script_from_html here because that would mean script tags are executed.
-	// @html is basically `.innerHTML = ...` and that doesn't execute scripts either due to security reasons.
-	var content = create_fragment_from_html(html);
-	if (svg) {
-		content = /** @type {DocumentFragment} */ (/** @type {unknown} */ (content.firstChild));
-	}
-	var clone = content.cloneNode(true);
-	frag_nodes = [...clone.childNodes];
-	frag_nodes.forEach((node) => {
-		target.before(node);
-	});
-	return /** @type {Array<Text | Comment | Element>} */ (frag_nodes);
-}