Very nice! Do you think it would be worthwhile to use a Set here? const emails = nodes.reduce( (set, node) => { return node.followers.reduce( (set, follower) => { follower.email && set.add(follower.email); return set; }, set); }, new Set()); console .log([...emails]); https://jsfiddle.net/auzwang/cod8y1hm/1/

#2's reducer functions needs to return acc because acc.push returns the pushed item: const squaresOfNumbers = numArray => numbers.reduce( (acc, item) => { acc.push(item * item); return acc; }, [] );