thing.cljs

postwalk comes from clojure.walk/postwalk (namespace might be cljs.walk in cljs, you'll have to check)
may need to look at prewalk, i didn't test so dunno. also didn't pay attention to which (pre or post) is more efficient in this case (if they both work)

(postwalk
    (fn [item]
        (let [t (:type item)]
            (if (or (not (map? item))
                    (nil? t)
                    (= t filter-type))
                item
                nil)))
    nested-maps)