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();

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
}

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
}

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;
  }
}

Hint

Add @UseInterceptors(ClassSerializerInterceptor) above the controller class.

Practice

1. What is the main purpose of the class-transformer library in a NestJS application?

easy

A. To convert plain JavaScript objects into class instances and vice versa

B. To handle HTTP requests and responses

C. To manage database connections

D. To style the frontend components

Solution

1. Step 1: Understand the role of class-transformer

   The library is designed to transform plain objects into instances of classes and back, which helps in managing data cleanly.

2. Step 2: Compare with other options

   Options B, C, and A relate to other parts of NestJS or frontend, not data transformation.

3. Final Answer:

   To convert plain JavaScript objects into class instances and vice versa -> Option A

4. Quick Check:

   class-transformer = object-class conversion [OK]

Hint: Remember: class-transformer changes plain objects to class instances [OK]

Common Mistakes:

• Confusing class-transformer with HTTP or database modules
• Thinking it styles frontend components
• Assuming it manages server routing

2. Which decorator from class-transformer is used to exclude a property from the transformed output?

easy

A. @Expose()

B. @Exclude()

C. @Transform()

D. @Type()

Solution

1. Step 1: Identify the decorator for exclusion

   The @Exclude() decorator marks properties to be left out during transformation.

2. Step 2: Understand other decorators

   @Expose() includes properties, @Transform() customizes transformation, and @Type() defines property types.

3. Final Answer:

   @Exclude() -> Option B

4. Quick Check:

   Exclude means leave out = @Exclude() [OK]

Hint: Exclude properties with @Exclude() decorator [OK]

Common Mistakes:

• Using @Expose() to exclude instead of include
• Confusing @Transform() with exclusion
• Not importing decorators correctly

3. Given the following code, what will be the output of console.log(userTransformed)?

import { plainToInstance, Exclude } from 'class-transformer';

class User {
  id: number;
  name: string;
  @Exclude() password: string;
}

const plainUser = { id: 1, name: 'Alice', password: 'secret' };
const userTransformed = plainToInstance(User, plainUser);
console.log(userTransformed);

medium

A. User { id: 1, name: 'Alice', password: 'secret' }

B. { id: 1, name: 'Alice' }

C. User { id: 1, name: 'Alice' }

D. Error: password property missing

Solution

1. Step 1: Understand @Exclude effect on transformation

   The @Exclude() decorator removes the password property from the transformed class instance.

2. Step 2: Analyze the output of plainToInstance

   The result is an instance of User with id and name, but no password.

3. Final Answer:

   User { id: 1, name: 'Alice' } -> Option C

4. Quick Check:

   @Exclude removes password = User without password [OK]

Hint: Properties with @Exclude() won't appear in class instances [OK]

Common Mistakes:

• Expecting password to be present after transformation
• Confusing plain object with class instance output
• Assuming error occurs due to missing password

4. Identify the error in this NestJS code using class-transformer:

import { plainToInstance } from 'class-transformer';

class Product {
  id: number;
  name: string;
}

const plainProduct = { id: 10, name: 'Book' };
const product = plainToInstance(Product, plainProduct, { excludeExtraneousValues: true });
console.log(product);

Why does product print an empty object?

medium

A. Missing @Expose() decorators on class properties

B. Wrong import of plainToInstance function

C. Plain object keys do not match class property names

D. excludeExtraneousValues option is not supported

Solution

1. Step 1: Understand excludeExtraneousValues option

   This option removes all properties not marked with @Expose() during transformation.

2. Step 2: Check class property decorators

   The Product class properties lack @Expose(), so all are excluded, resulting in an empty object.

3. Final Answer:

   Missing @Expose() decorators on class properties -> Option A

4. Quick Check:

   excludeExtraneousValues needs @Expose() [OK]

Hint: Use @Expose() with excludeExtraneousValues to keep properties [OK]

Common Mistakes:

• Assuming excludeExtraneousValues disables transformation
• Thinking plain object keys cause the issue
• Ignoring need for decorators with this option

5. You want to transform a plain object with nested properties into class instances using class-transformer. Which approach correctly handles nested objects?

import { plainToInstance, Type } from 'class-transformer';

class Address {
  street: string;
  city: string;
}

class User {
  name: string;
  @Type(() => Address)
  address: Address;
}

const plainUser = {
  name: 'Bob',
  address: { street: 'Main St', city: 'Townsville' }
};

const user = plainToInstance(User, plainUser);
console.log(user.address instanceof Address);

What will console.log output and why?

hard

A. undefined, because address property is missing

B. false, because nested objects are not transformed automatically

C. Error, because @Type decorator is used incorrectly

D. true, because @Type decorator converts nested object to Address instance

Solution

1. Step 1: Recognize the role of @Type decorator

   The @Type(() => Address) decorator tells class-transformer to convert the nested plain object into an Address class instance.

2. Step 2: Check the output of instanceof

   Since transformation happens correctly, user.address is an instance of Address, so the output is true.

3. Final Answer:

   true, because @Type decorator converts nested object to Address instance -> Option D

4. Quick Check:

   @Type enables nested class conversion = true [OK]

Hint: Use @Type(() => Class) for nested object transformation [OK]

Common Mistakes:

• Forgetting @Type causes nested objects to stay plain
• Assuming nested objects transform automatically
• Misusing @Type decorator syntax