sum function which returns another function

sum(1)(2)(3)(n).js

(() => {
  console.clear();

  console.log('sum is: ' + sum(1)(2)(3)(4));

  function sum(a, b = 0) {
    const res = a + b;

    console.log({res, a, b});

    fn = (b) => sum(res, b);
    fn.toString = fn.valueOf = () => res;

    return fn;
  }
})();

sum(1,2,3)(4,5)(n,n,n).js

(() => {
  console.clear();

  console.log('sum is: ' + sum(1,2,3)(4,5));

  function sum(...nums) {
    const res = nums.reduce((a, b) => a + b);

    console.log({...nums, res});

    fn = (...nums) => sum(res, ...nums);
    fn.toString = fn.valueOf = () => res;

    return fn;
  }
})();