From b93d004d98608a9a7083ffc5bc1b940c8c472b82 Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Mon, 31 Mar 2025 16:48:52 +0200
Subject: [PATCH] fix: ensure `undefined` class still applies scoping class, if
 necessary (#15643)

fixes #15635
---
 .changeset/weak-knives-battle.md                            | 5 +++++
 packages/svelte/src/internal/client/dom/elements/class.js   | 6 +++++-
 .../tests/css/samples/undefined-with-scope/expected.html    | 3 ++-
 .../tests/css/samples/undefined-with-scope/input.svelte     | 3 ++-
 4 files changed, 14 insertions(+), 3 deletions(-)
 create mode 100644 .changeset/weak-knives-battle.md

diff --git a/.changeset/weak-knives-battle.md b/.changeset/weak-knives-battle.md
new file mode 100644
index 0000000000..702053ee9e
--- /dev/null
+++ b/.changeset/weak-knives-battle.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure `undefined` class still applies scoping class, if necessary
diff --git a/packages/svelte/src/internal/client/dom/elements/class.js b/packages/svelte/src/internal/client/dom/elements/class.js
index ecbfcbc010..fc081b8956 100644
--- a/packages/svelte/src/internal/client/dom/elements/class.js
+++ b/packages/svelte/src/internal/client/dom/elements/class.js
@@ -14,7 +14,11 @@ export function set_class(dom, is_html, value, hash, prev_classes, next_classes)
 	// @ts-expect-error need to add __className to patched prototype
 	var prev = dom.__className;
 
-	if (hydrating || prev !== value) {
+	if (
+		hydrating ||
+		prev !== value ||
+		prev === undefined // for edge case of `class={undefined}`
+	) {
 		var next_class_name = to_class(value, hash, next_classes);
 
 		if (!hydrating || next_class_name !== dom.getAttribute('class')) {
diff --git a/packages/svelte/tests/css/samples/undefined-with-scope/expected.html b/packages/svelte/tests/css/samples/undefined-with-scope/expected.html
index 5eecaa9bb2..5548be6e5a 100644
--- a/packages/svelte/tests/css/samples/undefined-with-scope/expected.html
+++ b/packages/svelte/tests/css/samples/undefined-with-scope/expected.html
@@ -1 +1,2 @@
-<p class="svelte-xyz">Foo</p>
\ No newline at end of file
+<p class="svelte-xyz">Foo</p>
+<p class="svelte-xyz">Bar</p>
\ No newline at end of file
diff --git a/packages/svelte/tests/css/samples/undefined-with-scope/input.svelte b/packages/svelte/tests/css/samples/undefined-with-scope/input.svelte
index c68fb40dea..20639600d0 100644
--- a/packages/svelte/tests/css/samples/undefined-with-scope/input.svelte
+++ b/packages/svelte/tests/css/samples/undefined-with-scope/input.svelte
@@ -1,3 +1,4 @@
 <style>p { color: red; }</style>
 
-<p class={undefined}>Foo</p>
\ No newline at end of file
+<p class={undefined}>Foo</p>
+<p class="{undefined}">Bar</p>