[{"_id":0,"text":"Node1","children":[{"text":"Node2","children":[]},{"text":"Node3","children":[{"text":"Node4","children":[{"text":"Node5","children":[]},{"text":"Node6","children":[]}]},{"text":"Node7","children":[]}]}]}]

db.collection.aggregate([{"$set":{"texts":{"$function":{"body":"function(t, n) {function drill(t, n) {if (n.length > 0) {for (let elem of n) {t.push(elem.text); drill(t, elem.children)}} return t};drill(t,n)}","args":[["$text"],"$children"],"lang":"js"}}}}])