function config(b = {}) {
  if (!b.extends || !b.extends.length) return b;
  if (!Array.isArray(b.extends)) throw new Error(".extends must be array");
  let a = [];
  for (let i = 0; i < b.extends.length; i++) {
    b.extends[i] = config(b.extends[i]);
  }

  const f = Object.assign({}, ...b.extends.map(e => e), { ...b });

  delete f.extends;
  return f;
}

console.log(
  config({
    extends: [{ extends: [{ bar: "zoo", extends: [{ cow: "cow" }] }] }],
    bar: "bar"
  })
);