[SuperZen]

const fn = (a, b) => a + b;
const a = [1, 2, 3];
const b = [4, 5, 6];
Array.prototype.sum = function (...args) {
  return this.reduce(...args);
};
console.log([a, b].map(v => v.sum(fn)).sum(fn, 100));

тоже поучаствую )