class-transformer usage in NestJS - Mini Project: Build & Apply

class-transformer usage

📖 Scenario: You are building a NestJS API that returns user data. You need to ensure sensitive fields like passwords are never sent in responses, and that incoming data is properly transformed into class instances with type coercion.

🎯 Goal: Use class-transformer decorators to control serialization of response data and enable automatic transformation of incoming request bodies in NestJS.

📋 What You'll Learn

Install class-transformer and enable transform in ValidationPipe

Create a DTO with @Expose and @Exclude decorators

Use @Transform for custom type conversion

Apply ClassSerializerInterceptor to strip excluded fields from responses

💡 Why This Matters

🌍 Real World

class-transformer is essential in NestJS for securing API responses by hiding sensitive data and ensuring incoming data is properly typed and validated.

💼 Career

Understanding serialization and transformation is a core NestJS skill for building secure, type-safe backend APIs.

1

Enable transformation in ValidationPipe

In main.ts, configure the global ValidationPipe with transform: true and whitelist: true so that incoming request bodies are automatically converted into DTO class instances.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Add useGlobalPipes with transform and whitelist enabled
  // Your code here
  await app.listen(3000);
}
bootstrap();

Need a hint?

Use app.useGlobalPipes(new ValidationPipe({ transform: true, whitelist: true })).

2

Create DTO with @Exclude decorator

Create a UserResponseDto class with properties id (number), name (string), email (string), and password (string). Use @Exclude() on the password property so it is never included in API responses.

NestJS

import { Exclude } from 'class-transformer';

export class UserResponseDto {
  // Add id, name, email properties
  // Add password with @Exclude() decorator
  // Your code here
}

Need a hint?

Place @Exclude() directly above the password property declaration.

3

Add @Transform for type conversion

Create a CreateEventDto class with a title (string) and a startDate property. Use @Transform() to automatically convert the incoming string date value into a JavaScript Date object.

NestJS

import { Transform } from 'class-transformer';

export class CreateEventDto {
  // Add title property
  // Add startDate with @Transform to convert string to Date
  // Your code here
}

Need a hint?

Use @Transform(({ value }) => new Date(value)) above the startDate property.

4

Apply ClassSerializerInterceptor

In your controller, use @UseInterceptors(ClassSerializerInterceptor) on the class so that all response objects are serialized through class-transformer, automatically stripping @Exclude() fields.

NestJS

import { Controller, Get, UseInterceptors, ClassSerializerInterceptor } from '@nestjs/common';
import { UserResponseDto } from './user-response.dto';

// Add @UseInterceptors with ClassSerializerInterceptor
// Your code here
export class UsersController {
  @Get()
  findOne(): UserResponseDto {
    return { id: 1, name: 'Alice', email: '[e-mail]', password: 'secret' } as UserResponseDto;
  }
}

Need a hint?

Add @UseInterceptors(ClassSerializerInterceptor) above the controller class.