Edit in JSFiddle

var sum = function (f) {
    return function (n) {
        return n === 1 ? 1 : n + f(n - 1);
    };
};

var s5 = (function (g) {
    return g(g);
})(function (h) {
    return sum(function (n) {
        return h(h)(n);
    });
});
console.assert(s5(5) === 15);