in typescript env, I create HttpException that derive class of Error, use it at my express error handler

Question

why it can use without new HttpException() then trown a error I saw this method elsewhere，and it is work

Here is HttpException

class HttpException extends Error{
  constructor(public status: number,public message: string){
      super(message);
      this.status=status;
      this.message=message;
  }
}
export default HttpException;

Here is error handler

const errorMiddleware = (error: HttpException, _request: Request, response: Response, next: NextFunction) => {
  const status = error.status || 500;
  const message = error.message || ' something wrong';

  response
    .status(status)
    .send({
      status,
      message
    });
    
  logger.error(error.name);
  logger.error(error.message);
  logger.error(error.stack);

  next(error);
};

user in app.ts

// ...some routers
app.use(errorMiddleware)

api error in postman

{
    "status": 500,
    "message": "User validation failed: username: Error, expected `username` to be unique. Value: `aaa111`"
}