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


/* Factorial, in Jsish */
 
/* recursive */
function fact(n) { return ((n < 2) ? 1 : n * fact(n - 1)); }
 
/* iterative */
function factorial(n:number) {
    if (n < 0) throw format("factorial undefined for negative values: %d", n);
 
    var fac = 1;
    while (n > 1) fac *= n--;
    return fac;
}
 
if (Interp.conf('unitTest') > 0) {
;fact(18);
;fact(1);
 
;factorial(18);
;factorial(42);
try { factorial(-1); } catch (err) { puts(err); }
}