sum(1, 2)(3)(4, 5) == 15

sum.js

const add = (a, b) => a + b;

const sumList = numbers => numbers.reduce(add, 0);

const sum = (...startValues) => {
  let total = sumList(startValues);

  const fn = (...values) => (total += sumList(values), fn);

  fn.valueOf = () => total;

  return fn;
};