Structured Error Cause Chaining with Exhaustive Narrowing

class ApplicationError<TCode extends string = string> extends Error {
  constructor(
    public readonly code: TCode,
    message: string,
    options?: { cause?: unknown }
  ) {
    super(message, options);
    this.name = 'ApplicationError';
  }
}
 
type DatabaseError = ApplicationError<'ERR_DB_OFFLINE' | 'ERR_DB_QUERY'>;
type NetworkError = ApplicationError<'ERR_NET_TIMEOUT'>;
type DomainError = DatabaseError | NetworkError;
 
function handleDomainError(err: DomainError): string {
  switch (err.code) {
    case 'ERR_DB_OFFLINE':
      return 'Database connection lost';
    case 'ERR_DB_QUERY':
      return 'Failed to execute query';
    case 'ERR_NET_TIMEOUT':
      return 'Network timed out';
    default: {
      const _exhaustiveCheck: never = err;
      throw new Error(`Unhandled error code: ${(_exhaustiveCheck as ApplicationError).code}`);
    }
  }
}