diff --git a/test/clojure/core_test/defonce.cljc b/test/clojure/core_test/defonce.cljc
new file mode 100644
index 0000000..3f14067
--- /dev/null
+++ b/test/clojure/core_test/defonce.cljc
@@ -0,0 +1,27 @@
+(ns clojure.core-test.defonce
+  (:require [clojure.test :refer [are deftest is testing]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer) [when-var-exists]]))
+
+(when-var-exists defonce
+  (deftest test-defonce
+
+    #?(:cljs    "shadow-cljs treats defonce like def (https://github.com/thheller/shadow-cljs/issues/1185)"
+       :default (testing "defining var only once"
+                  (is (some? (defonce one 1)))
+                  (is (nil? (defonce one 2)))
+                  (is (nil? (defonce one (assert false "should not be evaluated"))))
+                  (is (= 1 one))))
+
+    (testing "bad shape"
+      (are [name] (thrown? #?(:cljs js/Error :default Exception) (eval '(defonce name nil)))
+                  nil
+                  :k
+                  's
+                  0
+                  37.5
+                  true
+                  false
+                  '()
+                  []
+                  #{}
+                  {}))))