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.)

Usage nonunique(Array collection [, Boolean returnBool=false, Boolean shortcircuit=false, Function uniqBy])

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:

// 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
}