[cache] Fix defaultSupplier called on every get invocation (#518)

Reported on the community:

https://community.openhab.org/t/cache-private-get-defaultsupplier-is-always-called/168578

Signed-off-by: Florian Hotze <dev@florianhotze.com>

Author: florian-h05

src/cache.js

@@ -32,12 +32,13 @@ class JSCache {
    * @returns {*|null} the current object for the supplied key, the value returned by defaultSupplier (if provided), or `null`
    */
   get (key, defaultSupplier) {
-    if (typeof defaultSupplier === 'function') {
-      if (defaultSupplier() === null) return null;
-      return this.#valueCache.get(key, defaultSupplier);
-    } else {
-      return this.#valueCache.get(key);
-    }
+    if (this.exists(key)) return this.#valueCache.get(key);
+    // key doesn't exist in cache: invoke supplier if provided
+    if (typeof defaultSupplier !== 'function') return null;
+    const supplied = defaultSupplier();
+    if (supplied === null) return null; // do not store null values in cache
+    this.#valueCache.put(key, supplied);
+    return supplied;
   }
 
   /**

test/cache.spec.js

@@ -42,19 +42,31 @@ describe('cache.js', () => {
         expect(console.warn).not.toHaveBeenCalled();
       });
 
-      it('passes supplier, if present.', () => {
-        const supplier = () => 'value';
-        cache.get(key, supplier);
-        expect(mockGet).toHaveBeenCalledWith(key, supplier);
-        expect(console.warn).not.toHaveBeenCalled();
-      });
-
       it('returns value from cache instance.', () => {
         const returnValue = 'value';
         mockGet.mockImplementation(() => returnValue);
         expect(cache.get(key)).toBe(returnValue);
         expect(console.warn).not.toHaveBeenCalled();
       });
+
+      it('returns null, if key does not exist in cache instance and no supplier is provided.', () => {
+        mockGet.mockImplementation(() => null);
+        expect(cache.get(key)).toBe(null);
+      });
+
+      it('returns value from supplier, if key does not exist in cache instance and supplier is provided.', () => {
+        const supplier = jest.fn(() => 'value');
+        mockGet.mockImplementation(() => null);
+        expect(cache.get(key, supplier)).toBe('value');
+        expect(supplier).toHaveBeenCalledTimes(1);
+      });
+
+      it('puts value from supplier into cache instance, if key does not exist in cache instance and supplier is provided.', () => {
+        const supplier = () => 'value';
+        mockGet.mockImplementation(() => null);
+        cache.get(key, supplier);
+        expect(mockPut).toHaveBeenCalledWith(key, 'value');
+      });
     });
 
     describe('put', () => {