DTO and Response Class Guide

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
  }),
);