Find and count duplicate values in array

by 420

JavaScript

function findDuplicate(x) {
  const dup = x.filter((item, index) => console.log(x.indexOf(item), index) || x.indexOf(item) !== index)
  const total = x.filter(item => item === dup[0])
  return total.length
}

console.log(findDuplicate([ 3, 2, 1, 3 ]));