How to validate a DTO fields?

I have an endpoint with no entry params:

async myendpoint(): Promise<any> {
   const customer = await this.customerService.findOne(1);
   if (customer) {
      return await this.customerService.mapToDestination(customer);
   }...
}

Then I have my method mapToDestination where I simply assign vars:

async mapToDestination(customer: Customer): Promise<DestinationDto> {
    const destination: DestinationDto = {
          lastname: customer.lastname,
          firstname: customer.firstname,...

Finally, I have my DTO:

import {IsEmail, IsNotEmpty, IsOptional, IsNumber, IsBoolean, IsString, IsDate, MaxLength, Length, NotEquals} from 'class-validator';
import {ApiProperty} from '@nestjs/swagger';

export class DestinationDto {


  @IsString()
  @IsNotEmpty()
  @MaxLength(32)
  lastname: string;

  @IsString()
  @IsNotEmpty()
  @MaxLength(20)
  firstname: string; ...

I would like my DTO fields to be validated automatically following the decorators when I'm mapping it in my mapToDestination() method. I look through the web and the official documentation and I gave a try to Validators (ValidationPipe) but it does not seem to be my need as it validates the endpoint entry params.

Please, could you explain to me how to achieve this automatic validation? Thanks in advance.

Solution

I won't be "automatic" but you could instantiate your own instance of the validator from class validator and use it against the DTO in your service. Otherwise, it won't ever happen automatically because as you said, the ValidationPipe only works on the entry of the endpoint.

Example

Inside of mapToDestination so long as customer is an instance of DestinationDTO` you can have something like this:

@Injectable()
export class CustomerService {

  async mapToDestination(customer: DestinationDTO) {
    const errors = await validate(customer);
    if (errors) {
      throw new BadRequestException('Some Error Message');
    }
    ...
  }
  
  ...
}