Follow-up for #12, to be tackled in a next major version. As written there:

Adds a lot of new code, with unfortunately some duplicate code because I wanted to avoid mixins and other forms of complexity, which means key and value iterators use classes that are separate from preexisting iterators. For example, a method like _seek() must now be implemented on three classes: AbstractIterator, AbstractKeyIterator and AbstractValueIterator. This (small?) problem extends to implementations and their subclasses, if they choose to override key and value iterators to improve performance.

To come up with a more DRY approach, it may help to first reduce the differences between the 3 iterators. Mainly: change the callback signature of AbstractIterator#next() from (err, key, value) to (err, entry). Perhaps (dare I say) remove callbacks altogether.

If we can then merge the 3 classes into one, or at least have a shared and reusable base class, then unit tests can probably be simplified too, not having to repeat like so:

Lastly (unrelated but I postponed it because of the next() callback signature and to avoid more breaking changes) perhaps rename iterator() to entries().