function factorial(num)
{
    // If the number is less than 0, reject it.

    if (num < 0) {
        console.log('use a number above 1');
        return -1;

    }
    // If the number is 0, its factorial is 1.
    else if (num == 0) {
        return 1;
    }
    let tmp = num;
    while (num-- > 2) {
        tmp *= num;
    }
    return tmp;
}

let result = factorial(18);
console.log(result);

// Output: 6402373705728000