gistfile1.txt

function sum() {
  let result = 0;

  function _sum() {
    result += [].slice.call(arguments).reduce((r, i) => r + i, 0);
    return _sum;
  }

  _sum.toString = function() {
    return result;
  }

  return _sum.apply(null, arguments);
}

const result = sum(1)(2,3)(4,5,6);