problems(js): add “2665. Counter II”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/counter-ii.js

@@ -0,0 +1,19 @@
+/**
+ * @param {integer} init
+ * @return { increment: Function, decrement: Function, reset: Function }
+ */
+var createCounter = function(init) {
+    let n = init;
+    return {
+        increment: () => ++n,
+        decrement: () => --n,
+        reset:     () => (n = init),
+    };
+};
+
+/**
+ * const counter = createCounter(5)
+ * counter.increment(); // 6
+ * counter.reset(); // 5
+ * counter.decrement(); // 4
+ */