nonunique

A quick helper function to find the non-unique items in a collection.

Inspired by lodash's _.uniq, but I wanted a _.nonunique. This turned out a little different from that.

Requires ES6. (Specifically, Sets.)

$3

collection - The group of objects where you want to pick out the non-unique members. A collection should be able to be any iterable, but it is only tested with arrays for now.

returnBool - optional - If you just want to know WHETHER the collection's members are unique, and you're not interested in which values are duplicates. Automatically short-circuits for speed. Returns a boolean. (See "Return Values" below.)

shortcircuit - optional - If you want to return the FIRST non-uniqe member of the collection only. (speeds up runtime on large collections.)

uniqBy - optional - if passed, every value in the interator will be run through this function before being compared to the others.

#### Return Values

nonunique returns an array unless you pass returnBool as true. REMEMBER that an empty array coerces to true in javascript, leading to a potentially tricky gotcha:

``javascript
// BAD - don't do this.
if(nonunique(myCollection)){
// this will always be true because [] coerces to true.
}

// GOOD - do this
if(nonunique(myCollection, true)){
// second arg turns on boolean mode, so it will return true/false
}

``