peek(k, peekOptions = {}) {
        const { allowStale = this.allowStale } = peekOptions;
        const index = this.#keyMap.get(k);
        if (index === undefined ||
            (!allowStale && this.#isStale(index))) {
            return;
        }
        const v = this.#valList[index];
        // either stale and allowed, or forcing a refresh of non-stale value
        return this.#isBackgroundFetch(v) ? v.__staleWhileFetching : v;
    }
Like {@link LRUCache#get} but doesn't update recency or delete stale
items.

Returns `undefined` if the item is stale, unless
{@link LRUCache.OptionsBase.allowStale} is set.