Definitions

   The factorial of   0   (zero)   is defined as being   1   (unity).
   The   Factorial Function   of a positive integer,    n,    is defined as the product of the sequence:
                 n,   n-1,   n-2,   ...   1 



Task

Write a function to return the factorial of a number.

Solutions can be iterative or recursive.

Support for trapping negative    n    errors is optional.



Related task

   Primorial numbers


function factorial(n) {
  //check our edge case
  if (n < 0) { throw "Number must be non-negative"; }
 
  var result = 1;
  //we skip zero and one since both are 1 and are identity
  while (n > 1) {
    result *= n;
    n--;
  }
  return result;
}