Currying function

sum.js

const sum = (...args) => {
  let result = 0;
  if (!args.length) {
    return result;
  }
  return (...a) => {
    result = args.reduce((accumulator, item) => accumulator + item, result);
    if (!a.length) {
      return result;
    }
    return sum(...args, ...a);
  };
};

console.log(`sum(): ${sum()} // 0`);
console.log(`sum(1)(): ${sum(1)()} // 1`);
console.log(`sum(1)(1)(): ${sum(1)(1)()} // 2`);
console.log(`sum(1, 1)(): ${sum(1, 1)()} // 2`);
console.log(`sum(1)(1, 1)(1)(): ${sum(1)(1, 1)(1)()} // 4`);
console.log(`sum(1): ${sum(1)} // function`);
console.log(`sum(1, 1, ..., 1): ${sum(1, 1, 1, 1)} // function`);