This is a great solution to the thundering herd problem, but isn't OP explicitly trying to cache the results for later? In that case, the promise map is a clever way to make sure the cachable value is fetched at most once, and you want the keys to build up in the map, so GC'ing them is counterproductive.
Ya it obviously depends on the intended goal, but caching items in a map indefinitely better be done on a set of values that is known to be limited to a certain size over the lifetime of the server or it'll lead to a memory leak.
