Recursive sum function inside an IIFE. (Not my idea!) Uses ES2015 syntax.

ES2015_sum.js

// Recursive sum function inside an IIFE. (Not my idea!)
(() => {
    main();

    function main() {
        console.write(sum(10));
    }

    function sum(n) {
        if (n === 1) {
            return 1;
        }

        return n + sum(n - 1);
    }
})();