dp factorial as map reduce in javascript.

factorial

function factorial(num) {
    if (num <= 2) return num;
    return Array.apply(null, Array(num))
                .map(function (_, i) { return i + 1; })
                .reduce(function(p, c, i, a) {
                    a[i] = (i < 2) ? i+1 : (i + 1) * a[i-1];
                    return a[i];
                });
}