How to start with clj + shadow-cljs + rum

It is still not working. The first render with Shadow-Cljs works but when the file gets saved, shadow-cljs reloads and reexecutes the file and then the result is:

  45 | (rum/defc item [data]
  46 |   [:div.item
  47 |    [:div.picture (picture "https://t1p.de/adga")]
  48 |    [:div.title (:title data)]])
------------------------------^-------------------------------------------------
Use of undeclared Var projectname.core/data
--------------------------------------------------------------------------------

All I do, is the following in src/cljs/projectname.cljs:

(ns projectname.core
  (:require [rum.core :as rum]))

(def item_data
  {:title "testitem"
   :main-picture-url "https://t1p.de/adga"})

(rum/defc picture [url]
  [:img {:src url :loading "lazy" :alt "Productpicture"}])

(rum/defc item [data]
  [:div.item
   [:div.picture (picture "https://t1p.de/adga")] ;;(:main-picture-url data))]
   [:div.title (:title data)]])

(defonce item_comp (item item_data))

(rum/mount item_comp (js/document.getElementById "item-container"))

the defonce is already a desperate attempt to fix this… :frowning: