recursive-sum-count-max-with-es6-head-and-tail.js

const sum = ([x, ...xs]) => (
  xs.length === 0
    ? x
    : x + sum(xs)
);
console.log(sum([2, 4, 6])); // 12

const count = ([x, ...xs]) => (
  xs.length === 0
    ? 1
    : 1 + count(xs)
);
console.log(count([3, 6, 7, 4, 11, 39, 19])); // 7

const max = (x, y) => x > y ? x : y;
const maximum = ([x, ...xs]) => (
  xs.length === 0
    ? x
    : max(x, maximum(xs))
);

console.log(maximum([1, 339, 22, 31, 5, 7])); // 339