find(fn, getOptions = {}) {
        for (const i of this.#indexes()) {
            const v = this.#valList[i];
            const value = this.#isBackgroundFetch(v)
                ? v.__staleWhileFetching
                : v;
            if (value === undefined)
                continue;
            if (fn(value, this.#keyList[i], this)) {
                return this.get(this.#keyList[i], getOptions);
            }
        }
    }
Find a value for which the supplied fn method returns a truthy value,
similar to Array.find().  fn is called as fn(value, key, cache).