DTO class creation in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a NestJS DTO class is used for validation?

Consider this DTO class in NestJS using class-validator decorators. What will happen if the input object misses the email property?

import { IsString, IsEmail } from 'class-validator';

export class UserDto {
  @IsString()
  name: string;

  @IsEmail()
  email: string;
}

A Validation passes because only the name property is required.

B Validation fails because the email property is missing.

C Validation fails because name is missing, not email.

D Validation passes and email is set to null automatically.

💡 Hint

Think about what happens when a required decorated property is missing in the input.

The @IsEmail() decorator requires the email property to be present and valid. Missing it causes validation to fail.

📝 Syntax

intermediate

Which DTO class syntax is correct for optional properties in NestJS?

You want to create a DTO class where the age property is optional. Which code snippet correctly defines this?

A

export class PersonDto {
  @IsOptional()
  @IsNumber()
  age?: number;
}

B

export class PersonDto {
  @IsNumber()
  age?: number;
}

C

export class PersonDto {
  @IsOptional()
  age: number;
}

D

export class PersonDto {
  age: number | undefined;
}

💡 Hint

Optional properties need a special decorator to skip validation if missing.

The @IsOptional() decorator tells the validator to ignore the property if it is not present. The property must also be marked optional with ?.

🔧 Debug

advanced

Why does this DTO class cause a runtime error in NestJS?

Look at this DTO class code. Why does it cause a runtime error when used in a controller?

export class ProductDto {
  name: string;
  price: number;

  constructor() {
    this.name = '';
  }
}

A NestJS DTO classes should not have constructors; this causes runtime errors.

B The constructor does not initialize the price property, causing an error.

C Missing decorators on properties cause runtime errors.

D The constructor overwrites the name property, causing validation to fail.

💡 Hint

Think about how NestJS creates instances of DTO classes and how constructors affect this.

NestJS uses plain objects and class-transformer to create DTO instances. Having a constructor can interfere with this process and cause runtime errors.

❓ state_output

advanced

What is the value of the isActive property after validation?

Given this DTO class and input object, what will be the value of isActive after validation?

import { IsBoolean, IsOptional } from 'class-validator';

export class StatusDto {
  @IsBoolean()
  @IsOptional()
  isActive?: boolean = true;
}

const input = {};

A true, because the default value is used when property is missing.

B false, because missing boolean defaults to false.

C Validation fails because isActive is missing.

D undefined, because the input does not provide isActive.

💡 Hint

Default values in DTO classes do not apply automatically during validation.

class-validator does not assign default values during validation. The property remains undefined if missing in input.

🧠 Conceptual

expert

Which statement best describes the role of DTO classes in NestJS?

Choose the most accurate description of what DTO classes do in NestJS applications.

A DTO classes are used to store data permanently in the database.

B DTO classes automatically generate API documentation without extra tools.

C DTO classes define the shape of data and enable validation and transformation before it reaches business logic.

D DTO classes replace entity classes and contain all business logic.

💡 Hint

Think about how data flows from outside requests into your app.

DTO classes specify how input data should look and help validate and transform it before use. They do not store data or contain business logic.