diff --git a/CHANGELOG.md b/CHANGELOG.md
index eebe784b..fbd9c25a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 - Fixed handling of false values in entity cache (PR #198, thx [Brandon Bloom](https://github.com/brandonbloom))
 - Fixed issue when string values were interpreted as lookup refs (#214)
 - Speed up rschema calculation (#192, thx [Andre R.](https://github.com/rauhs))
+- Optimize generated JS code by declaring fn arities (#197)
 
 # 0.15.5
 
diff --git a/src/datascript/arrays.cljc b/src/datascript/arrays.cljc
index 3f93997f..480a9bc4 100644
--- a/src/datascript/arrays.cljc
+++ b/src/datascript/arrays.cljc
@@ -8,12 +8,13 @@
 (defn- if-cljs [env then else]
   (if (:ns env) then else))
 
-#?(:cljs (def make-array cljs.core/make-array)
+#?(:cljs (defn ^array make-array [size] (js/Array. size))
    :clj  (defn make-array ^{:tag "[[Ljava.lang.Object;"}
            [size]
            (clojure.core/make-array java.lang.Object size)))
 
-#?(:cljs (def into-array cljs.core/into-array)
+#?(:cljs (defn ^array into-array [aseq]
+           (reduce (fn [a x] (.push a x) a) (js/Array.) aseq))
    :clj  (defn into-array ^{:tag "[[Ljava.lang.Object;"}
            [aseq]
            (clojure.core/into-array java.lang.Object aseq)))
@@ -80,6 +81,8 @@
   #?(:cljs (.sort arr cmp)
      :clj  (doto arr (java.util.Arrays/sort cmp))))
 
-(def array?
-  #?(:cljs cljs.core/array?
-     :clj  (fn array? [^Object x] (-> x .getClass .isArray))))
+#?(:cljs (defn ^boolean array? [x]
+           (if (identical? *target* "nodejs")
+             (.isArray js/Array x)
+             (instance? js/Array x)))
+   :clj  (defn array? [^Object x] (-> x .getClass .isArray)))
\ No newline at end of file
diff --git a/src/datascript/btset.cljc b/src/datascript/btset.cljc
index cbd7331d..81cd8507 100644
--- a/src/datascript/btset.cljc
+++ b/src/datascript/btset.cljc
@@ -413,7 +413,11 @@
 
 ;; BTSet
 
-(declare btset-conj btset-disj btset-iter)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared btset-conj [set key cmp])
+(defn ^:declared btset-disj [set key cmp])
+(defn ^:declared btset-iter [set])
+
 (def ^:const uninitialized-hash #?(:cljs nil :clj -1))
 
 (deftype BTSet [root shift cnt comparator meta #?(:cljs ^:mutable __hash
@@ -687,9 +691,18 @@
   ^long [^BTSet set ^long path]
   (-prev-path (.-root set) path (.-shift set)))
 
-
-
-(declare iter riter iter-first iter-next iter-chunk iter-chunked-next iter-rseq iter-reduce)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871 
+(defn ^:declared iter [set left right])
+(defn ^:declared iter-first [iter])
+(defn ^:declared iter-next [iter])
+(defn ^:declared iter-chunk [iter])
+(defn ^:declared iter-chunked-next [iter])
+(defn ^:declared iter-rseq [iter])
+(defn ^:declared iter-reduce ([iter f]) ([iter f start]))
+(defn ^:declared riter [set left right])
+(defn ^:declared riter-first [riter])
+(defn ^:declared riter-next [ri])
+(defn ^:declared riter-rseq [riter])
 
 (defn btset-iter
   "Iterator that represents whole set"
@@ -837,8 +850,6 @@
 
 ;; reverse iteration
 
-(declare riter-first riter-next riter-rseq)
-
 (deftype ReverseIter [set ^long left ^long right keys ^long idx]
   #?@(:cljs [
     ISeqable
diff --git a/src/datascript/core.cljc b/src/datascript/core.cljc
index 31907fad..baf6475f 100644
--- a/src/datascript/core.cljc
+++ b/src/datascript/core.cljc
@@ -14,22 +14,38 @@
 
 ;; SUMMING UP
 
+(defn ^:declared q [q & inputs])
 (def ^:export  q dq/q)
+
+(defn ^:declared entity [db eid])
 (def ^:export  entity de/entity)
+
 (defn ^:export entity-db [^Entity entity]
   {:pre [(de/entity? entity)]}
   (.-db entity))
 
+(defn ^:declared datom ([e a v]) ([e a v tx]) ([e a v tx added]))
 (def ^:export datom db/datom)
 
+(defn ^:declared pull [db selector eid])
 (def ^:export pull dp/pull)
+
+(defn ^:declared pull-many [db selector eids])
 (def ^:export pull-many dp/pull-many)
+
+(defn ^:declared touch [e])
 (def ^:export touch de/touch)
 
+(defn ^:declared empty-db ([]) ([schema]))
 (def ^:export empty-db db/empty-db)
+
+(defn ^:declared init-db ([datoms]) ([datoms schema]))
 (def ^:export init-db db/init-db)
 
+(defn ^:declared datom? [x])
 (def ^:export datom? db/datom?)
+
+(defn ^:declared db? [x])
 (def ^:export db? db/db?)
 
 (def ^:export ^:const tx0 db/tx0)
@@ -81,6 +97,7 @@
   {:pre [(db/db? db)]}
   (db/-index-range db attr start end))
 
+(defn ^:declared entid [db eid])
 (def ^:export entid db/entid)
 
 ;; Conn
diff --git a/src/datascript/db.cljc b/src/datascript/db.cljc
index aa849c52..20485e54 100644
--- a/src/datascript/db.cljc
+++ b/src/datascript/db.cljc
@@ -26,7 +26,9 @@
           data (last fragments)]
       `(throw (ex-info (str ~@(map (fn [m#] (if (string? m#) m# (list 'pr-str m#))) msgs)) ~data)))))
 
-(defn seqable? [x]
+(defn #?@(:clj  [^Boolean seqable?]  
+          :cljs [^boolean seqable?])
+  [x]
   (and (not (string? x))
   #?(:cljs (or (cljs.core/seqable? x)
                (da/array? x))
@@ -115,7 +117,13 @@
 
 ;; ----------------------------------------------------------------------------
 
-(declare hash-datom equiv-datom seq-datom val-at-datom nth-datom assoc-datom)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn- ^:declared hash-datom [d])
+(defn- ^:declared equiv-datom [a b])
+(defn- ^:declared seq-datom [d])
+(defn- ^:declared nth-datom ([d i]) ([d i nf]))
+(defn- ^:declared assoc-datom [d k v])
+(defn- ^:declared val-at-datom [d k nf])
 
 (deftype Datom [e a v tx added]
   #?@(:cljs
@@ -368,7 +376,16 @@
 
 ;; ----------------------------------------------------------------------------
 
-(declare hash-db hash-fdb equiv-db empty-db pr-db resolve-datom validate-attr components->pattern indexing?)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn- ^:declared hash-db [db])
+(defn- ^:declared hash-fdb [db])
+(defn- ^:declared equiv-db [a b])
+(defn- ^:declared empty-db ([]) ([schema]))
+#?(:cljs (defn ^:declared pr-db [db w opts]))
+(defn- ^:declared resolve-datom [db e a v t])
+(defn- ^:declared validate-attr [attr at])
+(defn- ^:declared components->pattern [db index cs])
+(defn ^:declared indexing? [db attr])
 
 (defrecord-updatable DB [schema eavt aevt avet max-eid max-tx rschema hash]
   #?@(:cljs
@@ -681,7 +698,10 @@
 
 ;; ----------------------------------------------------------------------------
 
-(declare entid-strict entid-some ref?)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared entid-strict [db eid])
+(defn ^:declared entid-some [db eid])
+(defn ^:declared ref? [db attr])
 
 (defn- resolve-datom [db e a v t]
   (when a (validate-attr a (list 'resolve-datom 'db e a v t)))
@@ -985,7 +1005,8 @@
          ~expr
          (cond-let ~@rest)))))
 
-(declare transact-tx-data)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared transact-tx-data [report es])
 
 (defn retry-with-tempid [report es tempid upserted-eid]
   (if (contains? (:tempids report) tempid)
diff --git a/src/datascript/impl/entity.cljc b/src/datascript/impl/entity.cljc
index 7de87e5b..1c1ca33f 100644
--- a/src/datascript/impl/entity.cljc
+++ b/src/datascript/impl/entity.cljc
@@ -3,7 +3,12 @@
   (:require [#?(:cljs cljs.core :clj clojure.core) :as c]
             [datascript.db :as db]))
 
-(declare entity ->Entity equiv-entity lookup-entity touch)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared entity [db eid])
+(defn ^:declared ->Entity [db eid touched cache])
+(defn- ^:declared equiv-entity [this that])
+(defn- ^:declared lookup-entity ([this attr]) ([this attr not-found]))
+(defn ^:declared touch [e])
 
 (defn- entid [db eid]
   (when (or (number? eid)
diff --git a/src/datascript/js.cljs b/src/datascript/js.cljs
index 9e8fa4c5..9216a3a5 100644
--- a/src/datascript/js.cljs
+++ b/src/datascript/js.cljs
@@ -17,7 +17,7 @@
        (reduce-kv
          (fn [m k v] (assoc m k (walk/postwalk keywordize v))) {})))
 
-(declare entities->clj)
+(defn- ^:declared entities->clj [entities])
 
 (defn- entity->clj [e]
   (cond (map? e)
@@ -90,15 +90,23 @@
 (defn ^:export entity [db eid]
   (d/entity db (js->clj eid)))
 
-(def ^:export touch       d/touch)
-(def ^:export entity_db   d/entity-db)
-(def ^:export filter      d/filter)
-(def ^:export is_filtered d/is-filtered)
+(defn ^:export ^:declared touch [e])
+(def  ^:export            touch d/touch)
+
+(defn ^:export ^:declared entity_db [entity])
+(def  ^:export            entity_db d/entity-db)
+
+(defn ^:export ^:declared filter [db pred])
+(def  ^:export            filter d/filter)
+
+(defn ^:export ^:declared is-filtered [x])
+(def  ^:export            is_filtered d/is-filtered)
 
 (defn ^:export create_conn [& [schema]]
   (d/create-conn (schema->clj schema)))
 
-(def ^:export conn_from_db d/conn-from-db)
+(defn ^:export ^:declared conn-from-db [db])
+(def  ^:export            conn_from_db d/conn-from-db)
 
 (defn ^:export conn_from_datoms
   ([datoms]        (conn_from_db (init_db datoms)))
@@ -128,9 +136,11 @@
       (callback report))
     db))
 
-(def ^:export listen d/listen!)
+(defn ^:export ^:declared listen ([conn callback]) ([conn key callback]))
+(def  ^:export            listen d/listen!)
 
-(def ^:export unlisten d/unlisten!)
+(defn ^:export ^:declared unlisten [conn key])
+(def  ^:export            unlisten d/unlisten!)
 
 (defn ^:export resolve_tempid [tempids tempid]
   (aget tempids (str tempid)))
diff --git a/src/datascript/lru.cljc b/src/datascript/lru.cljc
index a290268b..ef670adb 100644
--- a/src/datascript/lru.cljc
+++ b/src/datascript/lru.cljc
@@ -1,6 +1,8 @@
 (ns datascript.lru)
 
-(declare assoc-lru cleanup-lru)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared assoc-lru [lru k v])
+(defn ^:declared cleanup-lru [lru])
 
 #?(:cljs
     (deftype LRU [key-value gen-key key-gen gen limit]
diff --git a/src/datascript/parser.cljc b/src/datascript/parser.cljc
index c0716041..ec99130b 100644
--- a/src/datascript/parser.cljc
+++ b/src/datascript/parser.cljc
@@ -7,7 +7,11 @@
 
 ;; utils
 
-(declare collect-vars-acc)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn- ^:declared collect-vars-acc [acc form])
+(defn ^:declared parse-clause [form])
+(defn ^:declared parse-clauses [clauses])
+(defn ^:declared parse-binding [form])
 
 (defprotocol ITraversable
   (-collect      [_ pred acc])
@@ -178,8 +182,6 @@
 (deftrecord BindTuple  [bindings])
 (deftrecord BindColl   [binding])
 
-(declare parse-binding)
-
 (defn parse-bind-ignore [form]
   (when (= '_ form)
     (with-source (BindIgnore.) form)))
@@ -406,7 +408,6 @@
 (deftrecord Or        [source rule-vars clauses])
 (deftrecord And       [clauses])
 
-(declare parse-clause parse-clauses)
 
 (defn parse-pattern-el [form]
   (or (parse-placeholder form)
diff --git a/src/datascript/pull_api.cljc b/src/datascript/pull_api.cljc
index 92d3a06f..20056f7c 100644
--- a/src/datascript/pull_api.cljc
+++ b/src/datascript/pull_api.cljc
@@ -62,8 +62,8 @@
   [key frame]
   (some-> (:kvps frame) persistent! (get key)))
 
-(def ^:private recursion-result
-  (partial single-frame-result ::recursion))
+(defn- recursion-result [frame]
+  (single-frame-result ::recursion frame))
 
 (defn- recursion-frame
   [parent eid]
diff --git a/src/datascript/pull_parser.cljc b/src/datascript/pull_parser.cljc
index 1cd6a373..295eaf74 100644
--- a/src/datascript/pull_parser.cljc
+++ b/src/datascript/pull_parser.cljc
@@ -55,7 +55,8 @@
           spec (reduce aggregate-specs init specs)]
       [:subpattern (update spec :attrs persistent!)])))
 
-(declare parse-pattern)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared parse-pattern [pattern])
 
 (def ^:private wildcard? #{'* :* "*"})
 
diff --git a/src/datascript/query.cljc b/src/datascript/query.cljc
index 74d7ffe8..d7e5534b 100644
--- a/src/datascript/query.cljc
+++ b/src/datascript/query.cljc
@@ -19,7 +19,10 @@
 ;; ----------------------------------------------------------------------------
 
 (def ^:const lru-cache-size 100)
-(declare built-ins)
+
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared -collect ([context symbols]) ([acc rels symbols]))
+(defn ^:declared -resolve-clause [context clause])
 
 ;; Records
 
@@ -234,8 +237,6 @@
   (let [rules (if (string? rules) (edn/read-string rules) rules)] ;; for datascript.js interop
     (group-by ffirst rules)))
 
-(def bindable-to-seq? db/seqable?)
-
 (defn empty-rel [binding]
   (let [vars (->> (dp/collect-vars-distinct binding)
                         (map :symbol))]
@@ -256,7 +257,7 @@
   BindColl
   (in->rel [binding coll]
     (cond
-      (not (bindable-to-seq? coll))
+      (not (db/seqable? coll))
         (raise "Cannot bind value " coll " to collection " (dp/source binding)
                {:error :query/binding, :value coll, :binding (dp/source binding)})
       (empty? coll)
@@ -268,7 +269,7 @@
   BindTuple
   (in->rel [binding coll]
     (cond
-      (not (bindable-to-seq? coll))
+      (not (db/seqable? coll))
         (raise "Cannot bind value " coll " to tuple " (dp/source binding)
                {:error :query/binding, :value coll, :binding (dp/source binding)})
       (< (count coll) (count (:bindings binding)))
@@ -494,9 +495,6 @@
                     (second clause)
                     (first clause)))))
 
-(declare -collect)
-(declare -resolve-clause)
-
 (def rule-seqid (atom 0))
 
 (defn expand-rule [clause context used-args]
diff --git a/src/datascript/query_v3.cljc b/src/datascript/query_v3.cljc
index 88f7159c..b94bcfbb 100644
--- a/src/datascript/query_v3.cljc
+++ b/src/datascript/query_v3.cljc
@@ -21,7 +21,13 @@
       [datascript.db  Datom]
       [datascript.btset Iter])))
 
-(declare resolve-clauses collect-rel-xf collect-to)
+;; using defn instead of declare because of http://dev.clojure.org/jira/browse/CLJS-1871
+(defn ^:declared resolve-clauses [context clauses])
+(defn ^:declared collect-rel-xf [syms-indexed rel])
+(defn ^:declared collect-to
+  ([context syms acc])
+  ([context syms acc xfs])
+  ([context syms acc xfs specimen]))
 
 (def ^:const lru-cache-size 100)
 
@@ -468,10 +474,6 @@
 
 ;; Bindings
 
-
-(def bindable-to-seq? db/seqable?)
-
-
 (defn- bind! [tuples binding source indexes]
   (condp instance? binding
 
@@ -485,7 +487,7 @@
         tuples)
 
     BindColl
-      (if (not (bindable-to-seq? source))
+      (if (not (db/seqable? source))
         (db/raise "Cannot bind value " source " to collection " (dp/source binding)
                   {:error :query/binding, :value source, :binding (dp/source binding)})
         (let [inner-binding (:binding binding)]
@@ -500,7 +502,7 @@
 
     BindTuple
     (let [bindings (:bindings binding)]
-      (when-not (bindable-to-seq? source)
+      (when-not (db/seqable? source)
         (db/raise "Cannot bind value " source " to tuple " (dp/source binding)
                   {:error :query/binding, :value source, :binding (dp/source binding)}))
       (when (< (count source) (count bindings))
@@ -579,7 +581,7 @@
 
 
 (defn- matches-pattern? [idxs tuple] ;; TODO handle repeated vars
-;;   (when-not (bindable-to-seq? tuple)
+;;   (when-not (db/seqable? tuple)
 ;;     (db/raise "Cannot match pattern " (dp/source clause) " because tuple is not a collection: " tuple
 ;;            {:error :query/where, :value tuple, :binding (dp/source clause)}))
 ;;   (when (< (count tuple) (count (:pattern clause)))
@@ -595,7 +597,7 @@
 
 
 (defn resolve-pattern-coll [coll clause]
-  (when-not (bindable-to-seq? coll)
+  (when-not (db/seqable? coll)
     (db/raise "Cannot match by pattern " (dp/source clause) " because source is not a collection: " coll
        {:error :query/where, :value coll, :binding (dp/source clause)}))
   (let [pattern (:pattern clause)