Return duplicates within a JS array

getDuplicates.js

const count = names => names.reduce((acc, name) => Object.assign(acc, {[name]: (acc[name] || 0) + 1}), {})

const duplicates = dict => Object.keys(dict).filter((a) => dict[a] > 1)

const getDuplicates = vals => duplicates(count(vals))

export default getDuplicates

Alternate version:

const getDupes = (arr) => {
  const data = arr.join('')
  const dupes = arr.filter(val => data.match(new RegExp(val, 'gi')).length > 1)
  const uniqueValues = new Set(dupes)
  return [...uniqueValues]
}
export default getDupes