ex.cljs

(defmutation add-n
  [{:keys [n]}]
  (action [{:keys [state]}]
    (swap! state update :counter + n)))

;; This simulates the app state
(def st (atom {:counter 0}))

;; First arg is the env, second is the mutation name, third is the params
(fulcro.client.mutations/mutate {:state st} `add-n {:n 3})
;; => {:action #object[Function]}

(let [{:keys [action]} (fulcro.client.mutations/mutate {:state st} `add-n {:n 3}) ]
  (action))

;; Now in tests you can verify the state has been updated
@st
;; => {:counter 3}

(defn run-mutation [sym params]
  (let [{:keys [action]} (fulcro.client.mutations/mutate {:state st} sym params)]
    (action)))

;; Same effect as the above code
(run-mutation `add-n {:n 3})