CLJS-1649: Possible issue with in cljs.reader or cljs.core/PersistentHashMap.

We are storing last 256 computed hash-string values in a JS object. JS objects only accept strings as keys. So its not able to distinguish between a `nil` and a `"null"` if either is present in the cache already. We are already assuming that `nil` hashes to 0. So don't check for it in the cache or try to compute its hash.

Author: ducky427

src/main/cljs/cljs/core.cljs

@@ -871,10 +871,12 @@
   (when (> string-hash-cache-count 255)
     (set! string-hash-cache (js-obj))
     (set! string-hash-cache-count 0))
-  (let [h (aget string-hash-cache k)]
-    (if (number? h)
-      h
-      (add-to-string-hash-cache k))))
+  (if (nil? k)
+    0
+    (let [h (aget string-hash-cache k)]
+      (if (number? h)
+        h
+        (add-to-string-hash-cache k)))))
 
 (defn hash
   "Returns the hash code of its argument. Note this is the hash code

src/test/cljs/cljs/core_test.cljs

@@ -3165,6 +3165,10 @@
                         (+ acc x)))
           [2 4 6 :stop 8 10]))))
 
+(deftest test-nil-hashing-cljs-1649
+  (is (zero? (hash-string nil)))
+  (is (not (zero? (hash-string "null")))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))