before.cljs

(defn new-contact []
  (let [val (r/atom "")]
    (fn []
      [:div
       [:input {:type "text"
                :value @val
                :on-change #(reset! val (-> % .-target .-value))}]])))

; Works fine

with-refactor.cljs

(defn update-input [input]
  (reset! val (-> input .-target .-value)))

(defn new-contact []
  (let [val (r/atom "")]
    (fn []
      [:div
       [:input {:type "text"
                :value @val
                :on-change update-input}]])))

; Gives error: 
;   Uncaught Error: No protocol method IReset.-reset! defined for type function: function val(map_entry){
;   return cljs.core._val.call(null,map_entry); }