fn with acc

fn.js

function fibonacci (n) {
  const fn = (n, a, b) => n === 0 ? a : fn(n - 1, a + b, a);
  return fn(n, 0, 1);
};

function factorial (n) {
  const fn = (n, acc) => n === 0 ? acc : fn(n - 1, n * acc);
  return fn(n, 1);
}