diff --git a/src/leaf/map/fullscreen/dom-node-class/join-class-names.ts b/src/leaf/map/fullscreen/dom-node-class/join-class-names.ts
new file mode 100644
index 0000000..6f905e0
--- /dev/null
+++ b/src/leaf/map/fullscreen/dom-node-class/join-class-names.ts
@@ -0,0 +1,3 @@
+export function joinClassNames(classNames: string[]): string {
+  return classNames.join(' ')
+}