*entries() {
        for (const i of this.#indexes()) {
            if (this.#valList[i] !== undefined &&
                this.#keyList[i] !== undefined &&
                !this.#isBackgroundFetch(this.#valList[i])) {
                yield [this.#keyList[i], this.#valList[i]];
            }
        }
    }
Return a generator yielding `[key, value]` pairs,
in order from most recently used to least recently used.