Factorial algo using recursion

factorial.js

function factorial(num, product = 1) {
  // base case: num === 1; return product
  // 1! = 1 || 0! = 1
  if (num <= 1) return product
  // recursive case: factorial (num -1, num * product)
  return factorial(num - 1, num * product);
}

console.log(factorial(4)); // -> 24
console.log(factorial(6)); // -> 720
console.log(factorial(0)); // -> 1

// reference: https://www.khanacademy.org/computing/computer-science/algorithms/recursive-algorithms/a/recursive-factorial