ChunkBuffer

typesince v0.0-1424 clojure.lang/ChunkBufferEdit
satisfies ICounted

(ChunkBuffer. buf end)

Source code @ clojurescript:src/main/cljs/cljs/core.cljs
(deftype ChunkBuffer [^:mutable buf ^:mutable end]
  Object
  (add [_ o]
    (aset buf end o)
    (set! end (inc end)))

  (chunk [_]
    (let [ret (ArrayChunk. buf 0 end)]
      (set! buf nil)
      ret))

  ICounted
  (-count [_] end))