factorial of a number

Question

I have the following code but it is not giving perfect result for factorial can u find it out plz

Answer 1

i am quite new to javascript and would be happy to know any improvements that could be made to this answer

var a = 1;
function factorial(num) {
    if (num == 0) {
        return 1;
    } else if (num < 0) {
        return undefined;
    } else {
    for(i = num; i > 0; i--){
        a *= i;
    }
    return a;
    }
}
var b = factorial(5);
console.log(b);