.cljs

(defn toggle-class [a k class1 class2]
  (println "test")
  (if (= (@a k) class1)
      (swap! a assoc k class2)
      (swap! a assoc k class1)))

(def local-state (r/atom {:class "note note-off"}))
(toggle-class local-state :class "note note-off" "note note-on") ;; works here
(@local-state :class)

(defn render-note []
  (fn [text key]
    (let [local-state (r/atom {:class "note note-off"})]
      [:div {:key key
             :on-click (fn [_] (toggle-class local-state :class "note note-off" "note note-on")) ;; doesn't work here
             :class (@local-state :class)}
       (str text (@local-state :class))])))

(defn render-notes []
    [:div {:id "notes-container" :class "notes-container"}
     (doall
      (for [n (range 1 65)]
        ((render-note) n n)))])

(r/render-component [render-notes]
                    (. js/document (getElementById "app")))