You are given a condition that if num = 0, then return 1. Using an if-else statement for this is correct. Generally, you will want to check for the special conditions first, for better syntax and readability.
function factorialize(num) {
if (num === 0)
return 1;
else {
for (var i = num - 1; i > 0; i–-) {
num *= i;
};
}
return num;
}