Stack Overflow, Commented (001): Get all non-unique values in an array

Might make this a series. Might not.

Recently, I was looking for ways (via JavaScript) to get all items in an array that appeared more than once.

Not too long ago, I was confused about map(), filter() and reduce(). Sometimes, I still get confused – with reduce in particular...

So, here's a shoutout to past me, who would have appreciated this, and anybody else out there who wants to look at an attempt at a commented breakdown.

Thank you to Christian Landgren for your solution.

For those on a smaller device, this might look a tad better for you in comparison to the embed below.