From 97d77d6918642721d0bba5cbd3dfac70d88d1bb7 Mon Sep 17 00:00:00 2001
From: Mathias Picker <48158184+MathiasWP@users.noreply.github.com>
Date: Wed, 20 May 2026 15:27:36 +0200
Subject: [PATCH] perf: store setters cache as Set instead of Array

The per-element-type setters cache (`setters_cache`) in `attributes.js`
stored arrays and searched them with `Array.prototype.includes`, which is
O(n). Element prototypes have 30-50 setters, and the cache is consulted
on every spread-attribute assignment and on every non-string attribute
write. Switching the cache to `Set<string>` makes each lookup O(1)
without changing any behaviour.
---
 .changeset/setters-cache-set.md                      |  5 +++++
 .../src/internal/client/dom/elements/attributes.js   | 12 ++++++------
 2 files changed, 11 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/setters-cache-set.md

diff --git a/.changeset/setters-cache-set.md b/.changeset/setters-cache-set.md
new file mode 100644
index 0000000000..2013446bc5
--- /dev/null
+++ b/.changeset/setters-cache-set.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: store setters cache as `Set` instead of `Array`
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index a193a70dd5..67f73d1e3f 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -201,7 +201,7 @@ export function set_attribute(element, attribute, value, skip_warning) {
 
 	if (value == null) {
 		element.removeAttribute(attribute);
-	} else if (typeof value !== 'string' && get_setters(element).includes(attribute)) {
+	} else if (typeof value !== 'string' && get_setters(element).has(attribute)) {
 		// @ts-ignore
 		element[attribute] = value;
 	} else {
@@ -252,7 +252,7 @@ export function set_custom_element_data(node, prop, value) {
 			// customElements may not be available in browser extension contexts
 			!customElements ||
 			customElements.get(node.getAttribute('is') || node.nodeName.toLowerCase())
-				? get_setters(node).includes(prop)
+				? get_setters(node).has(prop)
 				: value && typeof value === 'object')
 		) {
 			// @ts-expect-error
@@ -463,7 +463,7 @@ function set_attributes(
 				}
 			} else if (
 				is_default ||
-				(setters.includes(name) && (is_custom_element || typeof value !== 'string'))
+				(setters.has(name) && (is_custom_element || typeof value !== 'string'))
 			) {
 				// @ts-ignore
 				element[name] = value;
@@ -572,7 +572,7 @@ function get_attributes(element) {
 	);
 }
 
-/** @type {Map<string, string[]>} */
+/** @type {Map<string, Set<string>>} */
 var setters_cache = new Map();
 
 /** @param {Element} element */
@@ -580,7 +580,7 @@ function get_setters(element) {
 	var cache_key = element.getAttribute('is') || element.nodeName;
 	var setters = setters_cache.get(cache_key);
 	if (setters) return setters;
-	setters_cache.set(cache_key, (setters = []));
+	setters_cache.set(cache_key, (setters = new Set()));
 
 	var descriptors;
 	var proto = element; // In the case of custom elements there might be setters on the instance
@@ -599,7 +599,7 @@ function get_setters(element) {
 				key !== 'textContent' &&
 				key !== 'innerText'
 			) {
-				setters.push(key);
+				setters.add(key);
 			}
 		}