Currying sum hack

currying-sum.js

// The problem is to write sum() such that it produces the following output -
// console.log(sum(2,3)); // Produces 5
// console.log(sum(2)(3)); // Produces 5

function sum() {
    var s = Array.prototype.reduce.call(arguments, function (x, y) { return x + y; }, 0);
    var f = function () {
        var a = Array.prototype.slice.call(arguments);
        a.push(s);
        return sum.apply(null, a);
    };
    f.valueOf = function () { return s; };
    return f;
}