function sum(...args) {
  if (!args.length) {
    return 0;
  }
  const f = (...rest) => {
    if (!rest.length) {
      return args.reduce((a, b) => a + b);
    } else {
      return sum(...args, ...rest);
    }
  };
  f.valueOf = () => args.reduce((a, b) => a + b);
  return f;
}
 
console.log(sum()); // 0
console.log(sum(1)(2)(3)(4)(5)()); // 15
console.log(sum(1, 2, 3)(2)(3).valueOf()); // 11
console.log(sum(1, 2, 3)(2)(3) + sum(1, 2, 3)(2)); // 19