Codestantin C++dorov

const arrayOfDuplicateElements = [ '-.........', '.....', '-....-', '.....' ]

const getNumberOfDuplicateItems = arr => {
  const set = new Set()
  arr.forEach(el => set.add(el))

  const initialArrayLength = arr.length
  const uniqueArrayLength = [...set].length

  return initialArrayLength - uniqueArrayLength
}

console.log(getNumberOfDuplicateItems(arrayOfDuplicateElements))

const arrayOfDuplicateElements = [ '-.........', '.....', '-....-', '.....' ]

const getNumberOfDuplicateItems = arr => arr.length - [...new Set(arr)].length

console.log(getNumberOfDuplicateItems(arrayOfDuplicateElements))