Why DTOs enforce data contracts in NestJS - See It in Action

Why DTOs Enforce Data Contracts in NestJS

📖 Scenario: You are building a simple NestJS application to manage user profiles. You want to ensure that the data sent to your API follows a strict format to avoid errors and keep your app reliable.

🎯 Goal: Build a DTO (Data Transfer Object) in NestJS that enforces a data contract for creating a user profile. This will help your API accept only the correct data shape and types.

📋 What You'll Learn

Create a DTO class with exact properties and types

Add validation decorators to enforce data rules

Use the DTO in a controller method parameter

Ensure the controller method accepts only valid data matching the DTO

💡 Why This Matters

🌍 Real World

APIs need to accept only correct and safe data to avoid bugs and security issues. DTOs help by defining clear data contracts.

💼 Career

Understanding DTOs and validation is essential for backend developers working with NestJS or similar frameworks to build reliable and maintainable APIs.

1

Create the UserProfileDto class

Create a class called UserProfileDto with these exact properties: username as a string, age as a number, and email as a string.

NestJS

# Create the UserProfileDto class with username, age, and email properties
# Your code here

Need a hint?

Think of this class as a form that defines exactly what information a user profile must have.

2

Add validation decorators to UserProfileDto

Import IsString and IsInt from class-validator. Add @IsString() to username and email, and @IsInt() to age in the UserProfileDto class.

NestJS

import { IsString, IsInt } from 'class-validator';

export class UserProfileDto {
  // Add validation decorators here
  username: string;
  age: number;
  email: string;
}

Need a hint?

These decorators check that the data matches the expected type before your app uses it.

3

Use UserProfileDto in a controller method

Create a controller class called UserController with a method createUser that takes a parameter userData of type UserProfileDto. Use the @Body() decorator on userData.

NestJS

import { Controller, Post, Body } from '@nestjs/common';

export class UserProfileDto {
  @IsString()
  username: string;

  @IsInt()
  age: number;

  @IsString()
  email: string;
}

@Controller('users')
export class UserController {
  // Add createUser method here
}

Need a hint?

This method will only accept data that matches the UserProfileDto structure.

4

Enable validation pipe in main app

In your main application bootstrap function, import ValidationPipe from @nestjs/common and use app.useGlobalPipes(new ValidationPipe()) to enable automatic validation of DTOs.

NestJS

import { ValidationPipe } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Enable validation pipe here
  await app.listen(3000);
}
bootstrap();

Need a hint?

This step makes sure your app checks the data against the DTO rules automatically.