port CLJ-2949: add req! (throwing get) and tests (#325)

* port CLJ-2949: add req! (throwing get) and tests

Co-authored-by: Fogus <mefogus@gmail.com>

* make REQ_NOT_FOUND private

---------

Co-authored-by: Fogus <mefogus@gmail.com>

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -2072,6 +2072,16 @@ reduces them without incurring seq initialization"
         :else not-found)
       not-found)))
 
+(def ^:private REQ_NOT_FOUND #js {})
+
+(defn req!
+  "Like arity-2 'get', but throws if key not present."
+  [m k]
+  (let [v (get m k REQ_NOT_FOUND)]
+    (if (identical? REQ_NOT_FOUND v)
+      (throw (js/Error. (str_ "Missing required key: " k)))
+      v)))
+
 (declare PersistentHashMap PersistentArrayMap MapEntry)
 
 (defn assoc

src/test/cljs/cljs/core_test.cljs

@@ -2012,4 +2012,13 @@
 
 (deftest test-cljs-3415
   (let [a 1 b 1]
-    (is (thrown? js/Error #{a b}))))
+    (is (thrown? js/Error #{a b}))))
+
+(deftest test-req!
+  (let [m {:a 1, :b 2, :f nil, :g false, nil "nil"}]
+    (is (thrown? js/Error (req! m :e)))
+    (are [x y] (= x y)
+         (req! m :a)  1
+         (req! m nil) "nil"
+         (req! m :b)  2
+         (req! m :f)  nil)))