Add nonUnique function

[aleclarson]

The opposite of unique

export function nonUnique<T>(array: readonly T[]) {
  const seen = new Set<T>()
  return array.filter(value => {
    if (seen.has(value)) {
      return true
    }
    seen.add(value)
    return false
  })
}

Todo

• Add optional “mapping function” argument

[Minhir]

We can think about more generic util for which I don't have a nice name (could be a sign that we don't need such util, just share my ideas).

const uniqueItems = countAndSelect(array, 1);
const pairedItems = countAndSelect(array, 2);
const nonUniqueItems = countAndSelect(array, count => count > 1);

But we already have counting, so countAndSelect can be too much sugar:

const countedItems = _.counting(array, v => v);
const pairedItems = array.filter(item => countedItems[item] === 2);

And the same for nonUnique:

const countedItems = _.counting(array, v => v);
const nonUniqueItems = array.filter(item => countedItems[item] > 1);

BTW, should the second argument in counting function be optional?