I believe the concern here is that you're mapping and filtering over potentially huge data sets, which can become inefficient.

Ah ha, thank you sir!

How does one get the values out of that method? I'm pretty naive to this particular usage, but console.log(followers(nodes)); isn't giving me what I anticipated.

I haven't used _() would you be able to put that in a jsbin/jsfiddle example? I can't get it to run correctly. Keeps returning a list of wrapped functions.

You got me thinking this morning, so I was looking for potential optimizations. I'm far from a guru on optimizations but I thought I'd take your idea and extend it into the followers node. forEach is notably a slow function, so I thought why not use a reducer? https://jsperf.com/multi-reducer-comparison Looks like a clear win, and we increase readability, separate the concerns a bit, and made each reducer a little more testable. Cheers!

getSetOfFollowerEmails doesn't return anything. Good article. Array#reduce is underused. If I see one more var isTrue; and a for loop, I'm going to lose it.