JSFiddle - React, Tailwind, and code Playground

by ciju

JavaScript

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

var s4 = function (h) {
    return sum(function (n) {
        return h(h)(n);
    });
};
console.assert(s4(s4)(5) === 15);