function product(array) {
if(array instanceof Array) {
var len = array.length;
var result = len > 0 ? 1 : null;
for(var i = 0; i < len; i++) {
result *= array[i];
}
return result;
}
return null;
}
function factorial(num) {
var result = num > 0 ? 1 : 0;
while(num > 0) {
result *= num;
num--;
}
return result;
}
var result1 = product([1,2,3,4,5]);
var result2 = factorial(5);