NestJS — Enterprise Node.js

DTOs & Validation

16 min Lesson 10 of 30

DTOs & Validation

Never trust incoming data. A DTO (Data Transfer Object) defines the exact shape of a request payload, and NestJS — with class-validator and class-transformer — validates and transforms it automatically before it ever reaches your controller. This is one of the highest-value features in the framework.

What is a DTO?

A DTO is a simple class describing the data a route accepts. Using a class (not an interface) is deliberate: classes exist at runtime, so NestJS can read their validation decorators.

// create-user.dto.ts
import { IsString, IsEmail, MinLength, IsInt, Min } from 'class-validator';

export class CreateUserDto {
  @IsString()
  @MinLength(2)
  name: string;

  @IsEmail()
  email: string;

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

Wiring up validation

Install the libraries, then enable the global ValidationPipe once in main.ts:

// terminal
npm install class-validator class-transformer

// main.ts
import { ValidationPipe } from '@nestjs/common';

app.useGlobalPipes(new ValidationPipe());

Now use the DTO as the @Body() type. Any request that violates a rule is rejected with a 400 Bad Request and a clear message — before your handler runs:

@Post()
create(@Body() dto: CreateUserDto) {
  // dto is guaranteed valid here
  return this.usersService.create(dto);
}

Common validation decorators

@IsString(), @IsInt(), @IsBoolean(), @IsEmail(), @IsUrl()
@MinLength(), @MaxLength(), @Min(), @Max()
@IsOptional() — skip validation when the field is absent
@IsEnum(), @IsArray(), @IsDateString()

Hardening the ValidationPipe

Three options make validation far safer:

app.useGlobalPipes(new ValidationPipe({
  whitelist: true,            // strip properties with no validation decorator
  forbidNonWhitelisted: true, // throw if unknown properties are sent
  transform: true,            // auto-convert payloads to DTO instances + correct types
}));

whitelist + transform are the power combo. whitelist silently removes unexpected fields (e.g. an attacker trying to set isAdmin), and transform converts the raw JSON into a real DTO instance with correctly-typed values.

Nested objects

To validate nested DTOs, mark the property with @ValidateNested() and @Type() so the transformer knows the class to build:

import { ValidateNested } from 'class-validator';
import { Type } from 'class-transformer';

export class CreateOrderDto {
  @ValidateNested()
  @Type(() => AddressDto)
  address: AddressDto;
}

Validation only works on classes, never interfaces. TypeScript interfaces are erased at compile time, so their decorators do not exist at runtime. Always use a class for a DTO.

Summary

DTOs are classes that describe and validate request data using class-validator decorators. Enable the global ValidationPipe with whitelist, forbidNonWhitelisted, and transform for safe, automatic validation and type conversion. Invalid requests are rejected with a 400 before your code runs. Next we look at pipes — the mechanism that powers this.