An example of Recursion. A factorial function is defined as: factorial of n is n times the factorial of n - 1, except for factorial of 0 which is 1. Therefore, we can write this as:

factorialFunction.js

function factorial(number) {
  if (number === 0) { return 1; }
  return number * factorial(number - 1);
}

// factorial(0) -> 1
// factorial(1) -> 1
// factorial(2) -> 2
// factorial(3) -> 6
// factorial(4) -> 24
// factorial(5) -> 120