class-transformer usage in NestJS

Introduction

We use class-transformer to easily convert plain data into class objects and back. This helps keep data organized and safe in NestJS apps.

When receiving JSON data from a user and you want to turn it into a class instance.

When sending data back to the user and you want to control which properties are shown.

When you want to automatically convert nested objects into class instances.

When you want to apply simple transformations like changing date strings into Date objects.

When you want to exclude sensitive fields like passwords from API responses.

Syntax

NestJS

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

const classObject = plainToInstance(ClassName, plainObject);
const plainObject = instanceToPlain(classObject);

plainToInstance converts plain objects to class instances.

instanceToPlain converts class instances back to plain objects.

Examples

This converts a simple object into a User class instance.

NestJS

import { plainToInstance } from 'class-transformer';

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

const plainUser = { id: 1, name: 'Alice' };
const user = plainToInstance(User, plainUser);

This hides the password field when converting to JSON.

NestJS

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

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

const plainUser = { id: 1, name: 'Alice', password: 'secret' };
const user = plainToInstance(User, plainUser);
const output = JSON.stringify(instanceToPlain(user));

Sample Program

This example shows how to convert plain data into a class instance and then back to a plain object while excluding the password field.

NestJS

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

class User {
  id: number;

  @Expose()
  name: string;

  @Exclude()
  password: string;

  constructor(id: number, name: string, password: string) {
    this.id = id;
    this.name = name;
    this.password = password;
  }
}

// Plain data from a client
const plainUser = { id: 1, name: 'Alice', password: 'mypassword' };

// Convert plain object to class instance
const userInstance = plainToInstance(User, plainUser);

// Convert class instance back to plain object (password excluded)
const safeUser = instanceToPlain(userInstance);

console.log('User instance:', userInstance);
console.log('Safe user object:', safeUser);

Important Notes

Remember to use decorators like @Exclude() and @Expose() to control which fields appear in output.

Nested objects can also be transformed automatically if you use @Type(() => ClassName) decorator.

Always validate your data separately; class-transformer only converts data shapes.

Summary

class-transformer helps convert plain data to class objects and back.

Use decorators to include or exclude fields easily.

This keeps your data clean and safe in NestJS applications.

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