My solution in Clojure(Script) for https://github.com/josevalim/nested-data-structure-traversal

solution.cljs

(def data 
  [{:title "Getting started" 
    :reset_lesson_position false 
    :lessons [{:name "Welcome"} 
              {:name "Installation"}]} 
   {:title "Basic operator" 
    :reset_lesson_position false 
    :lessons [{:name "Addition / Subtraction"} 
              {:name "Multiplication / Division"}]} 
   {:title "Advanced topics" 
    :reset_lesson_position true 
    :lessons [{:name "Mutability"} 
              {:name "Immutability"}]}])

(def lesson-position (atom 0))
(defn lessons [{:keys [lessons reset_lesson_position]}]  
  (when reset_lesson_position (reset! lesson-position 0)) 
  (into [] (map #(assoc % :position (swap! lesson-position inc))) lessons))
(into [] (map-indexed #(assoc %2 :position (inc %1) :lessons (lessons %2))) data)