Here is a codepen I cooked up, if you check the browser console you can see a significant performance increase. :) http://codepen.io/anon/pen/bqoeWM?editors=0011

For what it's worth I actually think you older code is more readable, here is how I would have improved it: const followers = (nodes) => { return _ (nodes) .flatMap( node => node.followers) .filter( follower => follower.email) . map ( follower => follower.email) .uni q() } I think some would disagree with this, but I personally favour readability in code and I think your old code is easier to reason about. As a bonus, the time required to complete this also faster.