factorial.ex

defmodule Factorial do
  def factorial(n) when n>1 do
    n * factorial(n-1)
  end
  def factorial(n) when n <=1 do
    1 
  end
end

factorial.js

function factorial(num){
  if (num == 0) {
    return 1;
  }
  var acc = num;
  while (num-- > 2) {
    acc *= num;
  }
  return acc;
}

factorial.rb

def factorial(num)
  unless num.nonzero?
    1
  else
    num * factorial(num)
  end
end