function modify(obj) {
  obj.a.push(4);
  obj.b = 'changed';
  return obj;
}

const original = { a: [1, 2, 3], b: 'original', c: { deep: true } };
const copy1 = { ...original };
const copy2 = JSON.parse(JSON.stringify(original));

modify(copy1);
console.log(original.a, original.b, copy2.c === original.c);