class-transformer usage in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this class-transformer code?

Given the following NestJS code using class-transformer, what will be the output of the console.log?

NestJS

import { plainToInstance } from 'class-transformer';

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

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

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

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

C { id: 1, name: 'Alice', password: 'secret' }

D User { id: 1 }

💡 Hint

Check if any decorators like @Expose or @Exclude are used to filter properties.

Since no @Expose or @Exclude decorators are used, excludeExtraneousValues has no effect. All properties from plainUser are assigned to the User instance.

🧠 Conceptual

intermediate

What does the @Exclude decorator do in class-transformer?

In class-transformer, what is the effect of adding the @Exclude() decorator to a class property?

A It makes the property required during transformation.

B It prevents the property from being included when transforming class instances to plain objects.

C It renames the property during transformation.

D It automatically validates the property value.

💡 Hint

Think about hiding sensitive data when sending responses.

The @Exclude decorator hides the property when converting class instances to plain objects, useful for removing sensitive fields like passwords.

❓ Hyperparameter

advanced

Which option correctly enables transformation of nested objects with class-transformer?

You have a User class with an Address property which is another class. Which option correctly transforms nested plain objects into class instances?

NestJS

import { plainToInstance } from 'class-transformer';

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

class User {
  name: string;
  address: Address;
}

const plainUser = {
  name: 'Bob',
  address: { street: '123 Main St', city: 'Townsville' }
};

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

A { strategy: 'excludeAll' }

B { excludeExtraneousValues: true }

C { enableImplicitConversion: true }

D { exposeDefaultValues: true }

💡 Hint

Look for the option that allows automatic conversion of nested objects.

enableImplicitConversion allows class-transformer to convert nested plain objects into instances of their respective classes automatically.

❓ Metrics

advanced

What is the effect of using @Transform decorator in class-transformer?

Consider this code snippet using @Transform on a property. What does the @Transform decorator do?

NestJS

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

class Product {
  @Transform(({ value }) => value.toUpperCase())
  name: string;
}

const plainProduct = { name: 'laptop' };
const productInstance = plainToInstance(Product, plainProduct);
console.log(productInstance.name);

A It converts the name property to uppercase during transformation.

B It validates the name property to be uppercase.

C It excludes the name property from the output.

D It renames the name property to 'NAME'.

💡 Hint

Check what the function inside @Transform does to the value.

The @Transform decorator applies a function to the property value during transformation, here converting it to uppercase.

🔧 Debug

expert

Why does this class-transformer code fail to exclude the password property?

Given this code, why is the password property still present in the output?

NestJS

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

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

const plainUser = { id: 1, name: 'Eve', password: '1234' };
const userInstance = plainToInstance(User, plainUser, { excludeExtraneousValues: true });
console.log(userInstance);

A Because password property is not defined in the class.

B Because @Exclude decorator is missing parentheses.

C Because plainToInstance does not support excludeExtraneousValues option.

D Because excludeExtraneousValues only works if properties have @Expose decorator, not @Exclude.

💡 Hint

Check how excludeExtraneousValues interacts with @Expose and @Exclude decorators.

excludeExtraneousValues removes properties not decorated with @Expose. @Exclude alone does not remove properties unless @Expose is used on others.