一、正向求阶乘

    function factorial(n) {
        var product = 1, i = 2;
        while (i <= n) {
            product *= i;
        }
        return product;
    }
    alert(factorial(4));

二、逆向求阶乘

    function factorial(n) {
        var product = 1;
        while (n > 1) {
            product *= n;
            n--;
        }
        return product;
    }
    alert(factorial(4));

三、递归求阶乘

    function factorial(n) {
        if (n <= 1) {
            return 1;
        } else {
            return n * factorial(n - 1);
        }
    }
    alert(factorial(4));