Nested DTO validation in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the validation decorator for nested objects in NestJS.

NestJS

import { [1] } from 'class-validator';

Drag options to blanks, or click blank then click option'

AValidateNested

BIsString

CIsNumber

DIsNotEmpty

Attempts:

3 left

💡 Hint

Common Mistakes

Using IsString or IsNumber which validate primitive types, not nested objects.

The ValidateNested decorator is used to validate nested objects inside DTOs in NestJS.

2fill in blank

medium

Complete the code to apply nested validation on the 'address' property inside a DTO.

NestJS

export class UserDto {
  @[1]()
  address: AddressDto;
}

Drag options to blanks, or click blank then click option'

AIsNotEmpty

BIsString

CValidateNested

DIsArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using IsString which only validates strings, not objects.

Forgetting to use ValidateNested for nested DTOs.

The ValidateNested decorator tells NestJS to validate the nested AddressDto object inside UserDto.

3fill in blank

hard

Fix the error in the code to ensure nested DTO validation works correctly with class-transformer.

NestJS

export class UserDto {
  @ValidateNested()
  @[1](() => AddressDto)
  address: AddressDto;
}

Drag options to blanks, or click blank then click option'

AType

BTransform

CExpose

DExclude

Attempts:

3 left

💡 Hint

Common Mistakes

Using Transform which is for custom transformations, not type hinting.

Omitting the Type decorator causes nested validation to fail.

The Type decorator from class-transformer is required to tell NestJS how to transform and validate nested objects.

4fill in blank

hard

Fill both blanks to define a nested DTO with validation decorators for its properties.

NestJS

export class AddressDto {
  @[1]()
  street: string;

  @[2]()
  city: string;
}

Drag options to blanks, or click blank then click option'

AIsString

BIsNumber

CIsNotEmpty

DIsBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using IsNumber or IsBoolean for string fields.

Forgetting to check that city is not empty.

IsString ensures the property is a string, and IsNotEmpty ensures it is not empty. Both are common validations for address fields.

5fill in blank

hard

Fill all three blanks to create a nested DTO with validation and type transformation.

NestJS

import { [1] } from 'class-transformer';
import { [2], ValidateNested } from 'class-validator';

export class ProfileDto {
  @ValidateNested()
  @[3](() => UserDto)
  user: UserDto;
}

Drag options to blanks, or click blank then click option'

AType

BIsString

CValidateNested

DIsNotEmpty

Attempts:

3 left

💡 Hint

Common Mistakes

Not importing Type causes nested validation to fail.

Confusing IsString with Type decorator.

The Type decorator from class-transformer is imported and used to specify the nested type. IsString is imported but not used here, showing common imports. ValidateNested is imported from class-validator but already used in code.

Practice

1. What is the main purpose of using @ValidateNested() in a NestJS DTO?

easy

A. To define default values for nested properties

B. To convert nested objects to plain JavaScript objects

C. To skip validation of nested objects

D. To validate properties of nested objects inside the DTO automatically

Solution

1. Step 1: Understand the role of @ValidateNested()

   This decorator tells NestJS to validate nested objects inside a DTO, not just the top-level properties.

2. Step 2: Compare with other options

   @Type() handles transformation, not validation. Skipping validation or setting defaults is unrelated to @ValidateNested().

3. Final Answer:

   To validate properties of nested objects inside the DTO automatically -> Option D

4. Quick Check:

   @ValidateNested() = validate nested objects [OK]

Hint: Remember: @ValidateNested() triggers nested object validation [OK]

Common Mistakes:

• Confusing @ValidateNested() with @Type()
• Thinking it skips validation
• Assuming it sets default values

2. Which of the following is the correct way to apply nested validation in a DTO property?

easy

A. Use both @ValidateNested() and @Type(() => NestedDto)

B. Use @ValidateNested() only

C. Use @Type(() => NestedDto) only

D. Use @IsString() on the nested property

Solution

1. Step 1: Identify decorators needed for nested validation

   @ValidateNested() triggers validation of nested objects, but @Type() is needed to transform plain objects into class instances.

2. Step 2: Understand why both are required

   Without @Type(), validation won't work properly because the nested object won't be recognized as the correct class type.

3. Final Answer:

   Use both @ValidateNested() and @Type(() => NestedDto) -> Option A

4. Quick Check:

   Nested validation = @ValidateNested() + @Type() [OK]

Hint: Always pair @ValidateNested() with @Type() for nested DTOs [OK]

Common Mistakes:

• Using only one decorator and expecting full validation
• Applying @IsString() on an object property
• Forgetting to import Type from class-transformer

3. Given the following DTOs, what will happen if address.zipCode is missing when validating UserDto?

class AddressDto {
  @IsString()
  street: string;

  @IsString()
  zipCode: string;
}

class UserDto {
  @IsString()
  name: string;

  @ValidateNested()
  @Type(() => AddressDto)
  address: AddressDto;
}

medium

A. Validation will fail because zipCode is missing

B. Validation will pass because zipCode is optional

C. Validation will fail because name is missing

D. Validation will pass but address will be undefined

Solution

1. Step 1: Check required properties in AddressDto

   Both street and zipCode are decorated with @IsString(), so they are required and must be strings.

2. Step 2: Understand nested validation in UserDto

   @ValidateNested() with @Type() ensures address is validated as AddressDto, so missing zipCode causes validation to fail.

3. Final Answer:

   Validation will fail because zipCode is missing -> Option A

4. Quick Check:

   Missing required nested property = validation fails [OK]

Hint: Missing required nested fields cause validation failure [OK]

Common Mistakes:

• Assuming missing nested fields are allowed by default
• Ignoring the effect of @ValidateNested()
• Confusing missing nested fields with missing top-level fields

4. Consider this DTO code snippet:

class ProfileDto {
  @IsString()
  bio: string;
}

class UserDto {
  @IsString()
  username: string;

  @ValidateNested()
  profile: ProfileDto;
}

Why will validation NOT work correctly for the nested profile property?

medium

A. Because @ValidateNested() should be on ProfileDto class

B. Because @IsString() is missing on profile

C. Because @Type(() => ProfileDto) decorator is missing on profile

D. Because bio property is missing @ValidateNested()

Solution

1. Step 1: Identify missing decorator for nested validation

   For nested validation to work, @Type(() => ProfileDto) must be added to the nested property to transform plain objects into class instances.

2. Step 2: Understand why other options are incorrect

   @IsString() is for string properties, not objects. @ValidateNested() belongs on the nested property, not the nested class. bio is a string, no nested validation needed.

3. Final Answer:

   Because @Type(() => ProfileDto) decorator is missing on profile -> Option C

4. Quick Check:

   Missing @Type() breaks nested validation [OK]

Hint: Always add @Type() with @ValidateNested() for nested DTOs [OK]

Common Mistakes:

• Forgetting @Type() on nested properties
• Misplacing @ValidateNested() on nested class
• Confusing property decorators with class decorators

5. You have a CompanyDto with a nested array of EmployeeDto objects. Which is the correct way to validate this nested array properly?

class EmployeeDto {
  @IsString()
  name: string;

  @IsInt()
  age: number;
}

class CompanyDto {
  @IsString()
  companyName: string;

  @ValidateNested({ each: true })
  @Type(() => EmployeeDto)
  employees: EmployeeDto[];
}

hard

A. You must remove { each: true } from @ValidateNested() for arrays

B. This setup correctly validates each employee in the employees array

C. You should use @IsArray() instead of @ValidateNested() for employees

D. You need to add @IsOptional() to employees to validate nested arrays

Solution

1. Step 1: Understand nested array validation

   For arrays of nested objects, @ValidateNested({ each: true }) tells the validator to check each item in the array.

2. Step 2: Confirm use of @Type() for transformation

   @Type(() => EmployeeDto) converts plain objects to EmployeeDto instances, enabling validation on each employee.

3. Final Answer:

   This setup correctly validates each employee in the employees array -> Option B

4. Quick Check:

   Nested arrays need @ValidateNested({ each: true }) + @Type() [OK]

Hint: Use { each: true } with @ValidateNested() for nested arrays [OK]

Common Mistakes:

• Omitting { each: true } for arrays
• Using @IsArray() alone without nested validation
• Forgetting @Type() for nested array items