class-transformer usage in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the class-transformer library in NestJS?

It helps convert plain JavaScript objects into class instances and vice versa, making it easier to work with typed data and apply transformations or validations.

beginner

How do you use @Expose() decorator in class-transformer?

The <code>@Expose()</code> decorator marks a class property to be included when transforming an object to plain or class instance, allowing control over which properties are visible.

beginner

What does the plainToInstance() function do?

It converts a plain JavaScript object into an instance of a specified class, applying any decorators like @Type() or @Expose() during the transformation.

intermediate

Why would you use @Type(() => SomeClass) in a DTO class?

To tell class-transformer how to transform nested objects or arrays into instances of the specified class, ensuring proper typing and transformation of nested data.

intermediate

What is the effect of using excludeExtraneousValues: true option in transformation?

It removes any properties from the plain object that do not have @Expose() decorator in the class, helping to keep only intended data and improve security.

Which function converts a plain object to a class instance in class-transformer?

A plainToInstance()

B classToPlain()

C transformObject()

D toClass()

The plainToInstance() function converts plain objects to class instances.

What does the @Expose() decorator do?

A Validates property values

B Hides properties from transformation

C Marks properties to be included during transformation

D Converts property types automatically

@Expose() marks properties to be included when transforming objects.

How do you specify the type of a nested object for transformation?

A @ExposeNested(ClassName)

B @NestedType(ClassName)

C @TransformType(ClassName)

D @Type(() => ClassName)

Use @Type(() => ClassName) to specify nested object types.

What happens if you set excludeExtraneousValues: true during transformation?

A Only properties with @Expose() are included

B All properties are included

C Transformation fails if extra properties exist

D Properties are renamed automatically

Only properties decorated with @Expose() are included when excludeExtraneousValues is true.

Which decorator is NOT part of class-transformer?

A @Expose()

B @Validate()

C @Type()

D @Exclude()

@Validate() is from class-validator, not class-transformer.

Explain how you would use class-transformer to convert a plain JSON object into a typed class instance with nested objects in NestJS.

Think about how to prepare your classes and call the right function.

Describe the benefits of using excludeExtraneousValues: true option in data transformation.

Consider what happens to properties without @Expose() when this option is enabled.

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