Stores can override isCacheableValue + tests

This is usefull ie for redis cache where null is returned when cache data does not exists
In other stores, null can still be stored and undefined used when data does not exist

Each store can implement its own custom method

examples/redis_example/redis_store.js

@@ -110,6 +110,10 @@ function redisStore(args) {
         });
     };
 
+    self.isCacheableValue = function(value) {
+        return value !== null && value !== undefined;
+    };
+
     return self;
 }
 

lib/caching.js

@@ -34,6 +34,8 @@ var caching = function(args) {
 
     if (typeof args.isCacheableValue === 'function') {
         self._isCacheableValue = args.isCacheableValue;
+    } else if (typeof self.store.isCacheableValue === 'function') {
+        self._isCacheableValue = self.store.isCacheableValue;
     } else {
         self._isCacheableValue = function(value) {
             return value !== undefined;

test/caching.unit.js

@@ -707,4 +707,25 @@ describe("caching", function() {
             });
         });
     });
+
+    describe("overloading with custom store", function() {
+        it("allows us to override isCacheableValue", function(done) {
+            var store = memoryStore.create({ttl: defaultTtl});
+            var onlyOne = true;
+            store.isCacheableValue = function(result) {
+                if (onlyOne) {
+                    onlyOne = false;
+                    done();
+                }
+                return result !== undefined;
+            };
+            cache = caching({store: store});
+            cache.wrap(key, function(cb) {
+                methods.getWidget(name, cb);
+            }, function(err, widget) {
+                checkErr(err);
+                assert.deepEqual(widget, {name: name});
+            });
+        });
+    });
 });