A solution to a poorly defined "problem" :)

nested-data-structure-traversal.cljs

(def sections
 ;convert to Clojure data
 (js->clj
  #js [
       #js {
            "title"                 "Getting started",
            "reset_lesson_position" false,
            "lessons"               [
                                     {"name" "Welcome"},
                                     {"name" "Installation"}
                                     ]
            },

       #js {
            "title"                 "Basic operator",
            "reset_lesson_position" false,
            "lessons"               [
                                     {"name" "Addition / Subtraction"},
                                     {"name" "Multiplication / Division"}
                                     ]
            },

       #js {
            "title"                 "Advanced topics",
            "reset_lesson_position" true,
            "lessons"               [
                                     {"name" "Mutability"},
                                     {"name" "Immutability"}
                                     ]
            }
       ]
  :keywordize-keys true))

;"Solution"
(let [section-cnt (atom 0)
      lesson-cnt  (atom 0)]
 (mapv
  (fn [{:keys [lessons reset_lesson_position] :as section-m}]
   (when (true? reset_lesson_position)
    (reset! lesson-cnt 0))
   (assoc section-m
    :position (swap! section-cnt inc)
    :lessons (mapv #(assoc % :position (swap! lesson-cnt inc)) lessons)))
  sections))