Function callback

test/MultiItemCache.unittest.js:29–35

(
			/** @type {Error | null | undefined} */ err,
			/** @type {unknown} */ res
		)

		const itemCaches = generateItemCaches(10);
		const multiItemCache = new MultiItemCache(itemCaches);
		const callback = (
			/** @type {Error | null | undefined} */ err,
			/** @type {unknown} */ res
		) => {
			expect(err).toBeNull();
			expect(res).toBeInstanceOf(Object);
		};
		for (let i = 0; i < 10; ++i) {
			multiItemCache.get(callback);
		}