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JavaScript

function gamma(n) {
        //some magic constants 
        var g = 7, // g represents the precision desired, p is the values of p[i] to plug into Lanczos' formula
            p = [0.99999999999980993, 676.5203681218851, -1259.1392167224028, 771.32342877765313, -176.61502916214059, 12.507343278686905, -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7];
        if (n < 0.5) {
            return Math.PI / Math.sin(n * Math.PI) / gamma(1 - n);
        } else {
            n--;
            var x = p[0];
            for (var i = 1; i < g + 2; i++) {
                x += p[i] / (n + i);
            }
            var t = n + g + 0.5;
            return Math.sqrt(2 * Math.PI) * Math.pow(t, (n + 0.5)) * Math.exp(-t) * x;
        }
    }

    function factorial(n) {
        return gamma(n + 1);
    }

    alert(factorial(3));