Closure and recursion

function sum(acc) {
  function input(n) {
    if (n === undefined) return acc;
    return sum(acc + n);
  }
  return acc === undefined ? 0 : input;
}
console.log(sum(1)(2)(3)()); // 6
console.log(sum(5)(10)(15)(20)()); // 50
console.log(sum()); // 0