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


factorialRec(n) {
    if (n == 0) return 1;
    return n * factorialRec(n - 1);
}
 
factorialIter(n) {
    for (i in range(1, n))
        n *= i;
    return n;
}
 
factorialFold(n) {
    return reduce(multiply, 1, range(1, n + 1));
}