OptionalallowAllow LRUCache#get and LRUCache#fetch calls to return stale data, if available.
By default, if you set ttl, stale items will only be deleted from the
cache when you get(key). That is, it's not preemptively pruning items,
unless OptionsBase.ttlAutopurge is set.
If you set allowStale:true, it'll return the stale value as well as
deleting it. If you don't set this, then it'll return undefined when
you try to get a stale entry.
Note that when a stale entry is fetched, even if it is returned due to
allowStale being set, it is removed from the cache immediately. You
can suppress this behavior by setting
OptionsBase.noDeleteOnStaleGet, either in the constructor, or in
the options provided to LRUCache#get.
This may be overridden by passing an options object to cache.get().
The cache.has() method will always return false for stale items.
Only relevant if a ttl is set.
Options that may be passed to the LRUCache#peek method.