Permutations and combinations

index.js

function factorial(n) {
  return n < 2 ? 1 : factorial(n - 1) * n;
}

function permutations(n, k) {
  return factorial(n) / factorial(n - k)
}

function combinations(n, k) {
  return factorial(n) / (factorial(k) * factorial(n - k))
}