response to the client based on certain condition in Loopbackjs?

Question

I am new to loopback4 and I am in learning phase. And I have issue with auto-generated api which always return response that defined on decorator or something failed they return 500 (Internal Server Error). I want to return for-example 400(badRequest) in the else block. How to ? I already tried many things but I did'nt. If anyone know how to handle it. Please let me know. Thanks

@del('/dctypes/{id}')
@response(204, {
  description: 'Dctypes DELETE success',
})
async deleteById(@param.path.number('id') id: number): Promise<void> {
    console.log(await this.dctypesRepository.alreadyInUser(id));
    if ((await this.dctypesRepository.alreadyInUser(id)) === true) {
      await this.dctypesRepository.deleteById(id);

      // here I want to send 400(badRequest on this condition.
    } else response(400, {description: 'This is custom made error.'});
}