HashMapIter

(deftype HashMapIter [nil-val root-iter ^:mutable seen]
  Object
  (hasNext [_]
    (or (not ^boolean seen) ^boolean (.hasNext root-iter)))
  (next [_]
    (if-not ^boolean seen
      (do
        (set! seen true)
        (MapEntry. nil nil-val nil))
      (.next root-iter)))
  (remove [_] (js/Error. "Unsupported operation")))