NestJS pipes

Pipes transform or validate input before it reaches the handler. `ValidationPipe` with class-validator DTOs is the standard boundary for HTTP bodies and query params.

					export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsInt()
  @Min(18)
  age: number;
}

@Post()
create(@Body(new ValidationPipe({ whitelist: true })) dto: CreateUserDto) {
  return this.users.create(dto);
}
				

Built-in pipes: `ParseIntPipe`, `ParseUUIDPipe`, `DefaultValuePipe`. Global `ValidationPipe` in `main.ts` enforces `whitelist` and `forbidNonWhitelisted` app-wide. Pipes throw `BadRequestException` on failure — filters shape the response.

On interviews: pipes versus manual validation, `transform: true` for coercion, and why DTO classes beat plain interfaces at runtime.

Common pitfalls: interfaces instead of classes (no runtime metadata), missing whitelist allowing mass assignment, and validation only on body but not params.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Use ValidationPipe with class-validator DTOs.
• Enable whitelist and forbid unknown properties.
• Apply Parse* pipes to route and query params.
• Register a global ValidationPipe for consistency.