special formsince v0.0-1236Edit
Parser code @ clojurescript:src/main/clojure/cljs/analyzer.cljc
(defmethod parse 'letfn*
  [op env [_ bindings & exprs :as form] name _]
  (when-not (and (vector? bindings) (even? (count bindings)))
    (throw (error env "bindings must be vector of even number of elements")))
  (let [n->fexpr (into {} (map (juxt first second) (partition 2 bindings)))
        names    (keys n->fexpr)
        context  (:context env)
        ;; first pass to collect information for recursive references
        [meth-env bes]
        (reduce (fn [[{:keys [locals] :as env} bes] n]
                  (let [ret-tag (-> n meta :tag)
                        fexpr (no-warn (analyze env (n->fexpr n)))
                        be (cond->
                             {:name n
                              :op :binding
                              :fn-var true
                              :line (get-line n env)
                              :column (get-col n env)
                              :local :letfn
                              :shadow (handle-symbol-local n (locals n))
                              :variadic? (:variadic? fexpr)
                              :max-fixed-arity (:max-fixed-arity fexpr)
                              :method-params (map :params (:methods fexpr))}
                             ret-tag (assoc :ret-tag ret-tag))]
                    [(assoc-in env [:locals n] be)
                     (conj bes be)]))
                [env []] names)
        meth-env (assoc meth-env :context :expr)
        ;; the real pass
        [meth-env bes]
        (reduce (fn [[meth-env bes] {:keys [name shadow] :as be}]
                  (let [env (assoc-in meth-env [:locals name] shadow)
                        fexpr (analyze env (n->fexpr name))
                        be' (assoc be
                              :init fexpr
                              :variadic? (:variadic? fexpr)
                              :max-fixed-arity (:max-fixed-arity fexpr)
                              :method-params (map :params (:methods fexpr))
                              :children [:init])]
                    [(assoc-in env [:locals name] be')
                     (conj bes be')]))
          [meth-env []] bes)
        expr (-> (analyze (assoc meth-env :context (if (= :expr context) :return context)) `(do ~@exprs))
                 (assoc :body? true))]
    {:env env :op :letfn :bindings bes :body expr :form form
     :children [:bindings :body]}))

Emitting code @ clojurescript:src/main/clojure/cljs/compiler.cljc
(defmethod emit* :letfn
  [{expr :body :keys [bindings env]}]
  (let [context (:context env)]
    (when (= :expr context) (emits "(function (){"))
    (doseq [{:keys [init] :as binding} bindings]
      (emitln "var " (munge binding) " = " init ";"))
    (emits expr)
    (when (= :expr context) (emits "})()"))))