DTO and Response Class Guide

Use this skill when creating or modifying DTO files in the

dto/

folder.

Input DTO (Request)

import { IsDateString, IsEnum, IsNotEmpty, IsOptional, IsString, Length } from 'class-validator';
import { IsE164 } from 'src/app.utils';

import { EntityEnum } from '../entity.enums';

export class CreateEntityDto {
  @IsNotEmpty()
  @IsString()
  requiredField!: string;

  @IsOptional()
  @IsString()
  @Length(1, 30)
  optionalField?: string;

  @IsOptional()
  @IsDateString()
  dateField?: string;

  @IsOptional()
  @IsEnum(EntityEnum)
  enumField?: EntityEnum;
}

Input DTO Rules

1. Use class-validator decorators

   • @IsNotEmpty()

     : Required fields

   • @IsOptional()

     : Optional fields

   • @IsString()

     ,

     @IsNumber()

     ,

     @IsBoolean()

     : Type validation

   • @IsEnum()

     : Enum value validation

   • @Length()

     : String length limits

   • @IsDateString()

     : ISO 8601 date strings

2. Field Declaration

   • Required fields:

     !

     (definite assignment assertion)
   • Optional fields:

     ?

     (optional)

3. Custom Decorators

   • Phone numbers:

     @IsE164()

     (E.164 format validation)
   • Define additional decorators in

     app.utils.ts

     as needed

4. Naming Conventions

   • Find:

     Find{Entity}Dto

   • Create:

     Create{Entity}Dto

   • Update:

     Update{Entity}Dto

   • Other:

     {Action}{Entity}Dto

Output Response Class

import { Expose } from 'class-transformer';

import { EntityEnum } from '../entity.enums';

export class FindEntityRes {
  @Expose()
  id!: string;

  @Expose()
  name!: string;

  @Expose()
  enumField!: EntityEnum;

  @Expose()
  createdAt!: Date;
}

Output Response Rules

1. @Expose() decorator is required

   • Use

     @Expose()

     on all fields to expose
   • Works with

     excludeExtraneousValues: true

     option

2. Naming Conventions

   • Find:

     Find{Entity}Res

   • Create:

     Create{Entity}Res

   • Update:

     Update{Entity}Res

   • Other:

     {Action}{Entity}Res

3. Entity → Response Transformation

import { mapTo } from '../app.utils';

// Usage in controller
return mapTo(FindEntityRes, entity);

DTO/Response Same File Pattern

Define in same file when DTO and Response are closely related (e.g., update API):

// update-entity.dto.ts
import { Expose } from 'class-transformer';
import { IsOptional, IsString, Length } from 'class-validator';

export class UpdateEntityDto {
  @IsOptional()
  @IsString()
  @Length(0, 30)
  name?: string;
}

export class UpdateEntityRes {
  @Expose()
  id!: string;

  @Expose()
  name!: string;
}

mapTo Utility

Transformation function defined in

app.utils.ts

:

import { type ClassConstructor, plainToInstance } from 'class-transformer';

export function mapTo<T, I>(type: ClassConstructor<T>, instance: I): T {
  return plainToInstance(type, instance, {
    excludeExtraneousValues: true,
  });
}

Custom Validator Pattern

import { applyDecorators } from '@nestjs/common';
import { Matches } from 'class-validator';

export function IsE164(): PropertyDecorator {
  return applyDecorators(
    Matches(/^\+[1-9]\d{1,14}$/, {
      message: '전화번호는 E.164을 준수해야 합니다.',
    }),
  );
}

File Structure

src/{module}/dto/
├── create-{entity}.dto.ts
├── update-{entity}.dto.ts
├── find-{entity}.dto.ts
└── index.ts  # barrel export

index.ts (Barrel Export)

export * from './create-entity.dto';
export * from './update-entity.dto';
export * from './find-entity.dto';

ValidationPipe Configuration

Global pipe setup in

main.ts

:

app.useGlobalPipes(
  new ValidationPipe({
    whitelist: true,           // Remove undefined fields
    forbidNonWhitelisted: true, // Error on undefined fields
    transform: true,           // Auto type transformation
  }),
);