function deepCopy(obj, hash = new WeakMap()) {
  if (obj === null) return null;
  if (typeof obj !== 'object') return obj;
  if (obj instanceof Date) return new Date(obj);
  if (obj instanceof RegExp) return new RegExp(obj.source, obj.flags);
  if (hash.has(obj)) return hash.get(obj);

  const result = Array.isArray(obj) ? [] : {};
  hash.set(obj, result);

  Object.keys(obj).forEach(key => {
    if (typeof obj[key] === 'object' && obj[key] !== null) {
      result[key] = deepCopy(obj[key], hash);
    } else {
      result[key] = obj[key];
    }
  });

  return result;
}