Implement a defaut while miss cache provider

src/defaultWhileMiss.ts

@@ -0,0 +1,40 @@
+import {CacheLoader, CacheProvider} from './cacheProvider';
+
+export type Configuration<K, V> = {
+    provider: CacheProvider<K, V>,
+    defaultValue: V,
+    /**
+     * Handler for background revalidation errors
+     */
+    errorHandler?: (error: Error) => void,
+};
+
+export class DefaultWhileMissCache<K, V> implements CacheProvider<K, V> {
+    private readonly provider: Configuration<K, V>['provider'];
+
+    private readonly defaultValue: V;
+
+    private readonly errorHandler: (error: Error) => void;
+
+    public constructor(config: Configuration<K, V>) {
+        this.provider = config.provider;
+        this.defaultValue = config.defaultValue;
+        this.errorHandler = config.errorHandler ?? ((): void => { /* noop */ });
+    }
+
+    public get(key: K, loader: CacheLoader<K, V>): Promise<V> {
+        return this.provider.get(key, innerKey => {
+            loader(innerKey).catch(this.errorHandler);
+
+            return Promise.resolve(this.defaultValue);
+        });
+    }
+
+    public set(key: K, value: V): Promise<void> {
+        return this.provider.set(key, value);
+    }
+
+    public delete(key: K): Promise<void> {
+        return this.provider.delete(key);
+    }
+}

src/index.ts

@@ -9,4 +9,5 @@ export * from './staleWhileRevalidate';
 export * from './timestampedCacheEntry';
 export * from './errorResilient';
 export * from './sharedInFlight';
-export {NoopCache} from './noop';
+export * from './noop';
+export * from './defaultWhileMiss';

test/defaultWhileMiss.test.ts

@@ -0,0 +1,79 @@
+import {DefaultWhileMissCache, NoopCache} from '../src';
+
+describe('A cache provider that returns a default value while loading in the background', () => {
+    it('should return a default value calling the loader the background', async () => {
+        const provider = new NoopCache();
+
+        jest.spyOn(provider, 'get');
+
+        const loader = jest.fn().mockResolvedValue('loaderValue');
+
+        const cache = new DefaultWhileMissCache({
+            provider: provider,
+            defaultValue: 'defaultValue',
+        });
+
+        await expect(cache.get('key', loader)).resolves.toBe('defaultValue');
+
+        expect(provider.get).toHaveBeenCalledWith('key', expect.any(Function));
+
+        expect(loader).toHaveBeenCalledWith('key');
+    });
+
+    it('should return a default value handling a loading error in background', async () => {
+        const provider = new NoopCache();
+
+        jest.spyOn(provider, 'get');
+
+        const error = new Error('Some error.');
+
+        const loader = jest.fn().mockRejectedValue(error);
+
+        const errorHandler = jest.fn();
+
+        const cache = new DefaultWhileMissCache({
+            provider: provider,
+            defaultValue: 'defaultValue',
+            errorHandler: errorHandler,
+        });
+
+        await expect(cache.get('key', loader)).resolves.toBe('defaultValue');
+
+        expect(provider.get).toHaveBeenCalledWith('key', expect.any(Function));
+
+        expect(loader).toHaveBeenCalledWith('key');
+
+        expect(errorHandler).toHaveBeenCalledTimes(1);
+        expect(errorHandler).toHaveBeenCalledWith(error);
+    });
+
+    it('should delegate setting a value to the underlying provider', async () => {
+        const provider = new NoopCache();
+
+        jest.spyOn(provider, 'set');
+
+        const cache = new DefaultWhileMissCache({
+            provider: provider,
+            defaultValue: 'defaultValue',
+        });
+
+        await cache.set('key', 'value');
+
+        expect(provider.set).toHaveBeenCalledWith('key', 'value');
+    });
+
+    it('should delegate deleting a value to the underlying provider', async () => {
+        const provider = new NoopCache();
+
+        jest.spyOn(provider, 'delete');
+
+        const cache = new DefaultWhileMissCache({
+            provider: provider,
+            defaultValue: 'defaultValue',
+        });
+
+        await cache.delete('key');
+
+        expect(provider.delete).toHaveBeenCalledWith('key');
+    });
+});