diff --git a/src/cljs/ulysses/utils.cljs b/src/cljs/ulysses/utils.cljs
index 399e75a..26d6349 100644
--- a/src/cljs/ulysses/utils.cljs
+++ b/src/cljs/ulysses/utils.cljs
@@ -22,6 +22,9 @@
 (defn boolean? [x]
   (or (true? x) (false? x)))
 
+(defn lazy-seq? [x]
+  (instance? LazySeq x))
+
 (defn ptr
   "print x, then return it. useful for
   debugging inside threading macros or map"
@@ -120,14 +123,18 @@
       seqable)))
 
 (defn merge-by-id
-  "merge multiple seqables of maps by :id; the rightmost having the highest preference"
-  ([a b]
-   (let [new-ids (map :id b)]
-    (concat
-      (remove #(-> % :id hash-set (some new-ids)) a)
-      b)))
-  ([& seqables]
-   (reduce merge-by-id seqables)))
+  "merge multiple seqables of maps by :id;
+  the rightmost having the highest preference;
+  the result taking on the type of the first"
+  [a & more]
+  (let [list-or-lzs? ((some-fn list? lazy-seq?) a)
+        into-maybe (if list-or-lzs? identity (partial into (empty a)))
+        all (apply (partial concat a) more)]
+    (->> all
+         (group-by :id)
+         (vals)
+         (map last)
+         (into-maybe))))
 
 (defn remove-by-id
   "remove a map by :id within a seqable (of maps)"
diff --git a/test/cljs/ulysses/utils_test.cljs b/test/cljs/ulysses/utils_test.cljs
index 4f381b3..51032a6 100644
--- a/test/cljs/ulysses/utils_test.cljs
+++ b/test/cljs/ulysses/utils_test.cljs
@@ -2,11 +2,11 @@
   (:require [cljs.test :refer-macros [deftest testing are is]]
             [ulysses.utils :as utils]))
 
-(deftest test-noop
+(deftest noop
   (are [x] (nil? (apply utils/noop x))
     [] [1] [1 2]))
 
-(deftest test-boolean?
+(deftest boolean?
   (are [x y] (= x (utils/boolean? y))
    true true
    true false
@@ -18,13 +18,13 @@
 
 ; missing: ptr-first
 
-(deftest test-metas-equal?
+(deftest metas-equal?
   (is (utils/metas-equal? [(with-meta {:a 1} {:key 1})]
                           [(with-meta {:a 2} {:key 1})]))
   (is (not (utils/metas-equal? [(with-meta {:a 1} {:key 1})]
                                [(with-meta {:a 2} {:key 2})]))))
 
-(deftest test-remap-and-variations
+(deftest remap-and-variations
   (let [from {:a 1 :b 2 :c 3}]
     (testing "remap"
       (is (= {:a 2 :b 4 :c 6}
@@ -40,10 +40,10 @@
                (fn [k v] (keyword (str (name k) v)))
                from))))))
 
-(deftest test-hyphenate-keys
+(deftest hyphenate-keys
   (is (= {:a-b 1 :c-d 2} (utils/hyphenate-keys {:a_b 1 :c_d 2}))))
 
-(deftest test-map-subels
+(deftest map-subels
   (testing "when values have ids"
     (let [in [{:id 23 :value 3} {:id 95 :value 4}]
           out (utils/map-subels :div in)
@@ -66,7 +66,7 @@
       (is (= out expected))
       (is (utils/metas-equal? out expected)))))
 
-(deftest test-str->int
+(deftest str->int
   (testing "successful cases"
     (are [x y] (= x (utils/str->int y))
       0 "0"
@@ -83,7 +83,7 @@
       "garbage" "12moregarbage12"
       123.123 nil)))
 
-(deftest test-clj->json
+(deftest clj->json
   (are [x y] (= x (utils/clj->json y))
     "null" nil
     "true" true
@@ -91,7 +91,7 @@
     "[1,2,3]" [1 2 3]
     "[\"one\",\"three\",\"two\"]" (sorted-set :one :two :three)))
 
-(deftest test-json->clj
+(deftest json->clj
   (testing "basic values"
     (are [x y] (= x (utils/json->clj y))
       nil "null"
@@ -107,19 +107,19 @@
     (are [x] (thrown? js/SyntaxError (utils/json->clj x))
       "asdf" "[1,2")))
 
-(deftest test-url-encode-component
+(deftest url-encode-component
   (are [x y] (= x (utils/url-encode-component y))
     "foo" "foo"
     "foo%20bar" "foo bar"
     "%3F%26%3F%26" "?&?&"))
 
-(deftest test-truncate-with-ellipsis
+(deftest truncate-with-ellipsis
   (testing "normal"
     (is (= "foo..." (utils/truncate-with-ellipsis "foo bar" 3))))
   (testing "custom ellipsis"
     (is (= "foo[..]" (utils/truncate-with-ellipsis "foo bar" 3 "[..]")))))
 
-(deftest test-find-by-id
+(deftest find-by-id
   (let [ms [{:id 1} {:id 2} {:id 94}]]
     (testing "found (type int)"
       (are [pf y] (= (pf ms) (utils/find-by-id y ms))
@@ -134,3 +134,25 @@
     (testing "not found"
       (are [x] (nil? (utils/find-by-id x ms))
         33 "33" "1.0" nil))))
+
+(deftest merge-by-id
+  ;; ih test util, ex: (ih [1 2 3]) => [{:id 1} {:id 2} {:id 3}]
+  (let [ih (fn [coll]
+            ((if (list? coll) identity (partial into (empty coll)))
+             (map #(if (map? %) % (hash-map :id %)) coll)))]
+    (testing "two seq args"
+      (are [x a b] (= x (utils/merge-by-id (ih a) (ih b)))
+        [] [] []
+        [] [] (list)
+        [{:id 0} {:id 1} {:id 2}] [0] [1 2]
+        #{{:id 0} {:id 1} {:id 2}} #{0} #{1 2}
+        #{{:id 0} {:id 1} {:id 2}} #{0} [1 2]
+        (list {:id 0} {:id 1 :a true} {:id 2} {:id 3}) (list 0 {:id 1 :a false} 2) [{:id 1 :a true} 2 3]))
+    (testing "many seq args"
+      (are [x many] (= x (apply utils/merge-by-id (map ih many)))
+        [] [[] [] []]
+        [] [[] (list) (list)]
+        [{:id 0}] [[] [0] [] [0]]
+        #{{:id 0} {:id 1} {:id 2}} [#{0} #{1 2} #{0 2 1}]
+        #{{:id 0} {:id 1} {:id 2 :a true}} [#{0} [1 2] #{{:id 2 :a true}}]
+        (list {:id 0} {:id 1} {:id 2 :a true} {:id 3}) [(list 0 1 2) [1 {:id 2 :a true} 3] []]))))