class-validator decorators in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a property with @IsEmail() decorator receives invalid input?

Consider a NestJS DTO class with a property decorated by @IsEmail(). What will the validation result be if the property value is "not-an-email"?

NestJS

import { IsEmail } from 'class-validator';

class UserDto {
  @IsEmail()
  email: string;
}

const user = new UserDto();
user.email = "not-an-email";
// Assume validation is run here

A Validation fails with an error indicating the email is invalid.

B Validation passes because the value is a string.

C Validation throws a runtime error due to invalid decorator usage.

D Validation ignores the property and passes.

💡 Hint

Think about what @IsEmail() checks for in the input.

The @IsEmail() decorator checks if the property value is a valid email format. If the value is not a valid email, validation fails with an error message.

📝 Syntax

intermediate

Which option correctly uses @Length decorator to enforce string length between 5 and 10?

Choose the correct way to use the @Length decorator from class-validator to ensure a string property has length between 5 and 10 characters.

NestJS

import { Length } from 'class-validator';

class ProductDto {
  @Length(?, ?)
  name: string;
}

A @Length(10, 5)

B @Length(5, 10)

C @Length(min=5, max=10)

D @Length({ min: 5, max: 10 })

💡 Hint

Check the official @Length decorator signature.

The @Length decorator takes two numeric arguments: minimum length first, then maximum length. So @Length(5, 10) is correct.

🔧 Debug

advanced

Why does validation not fail when @IsNotEmpty() is used on a number property set to 0?

Given this DTO:

class ScoreDto {
  @IsNotEmpty()
  score: number;
}

const dto = new ScoreDto();
dto.score = 0;
// Validation is run here

Why does validation pass even though 0 might be considered empty?

A Validation fails but the error is swallowed silently.

B @IsNotEmpty() only works on strings, so it ignores numbers.

C The decorator is missing a required option to check for zero.

D @IsNotEmpty() considers 0 a valid non-empty value because it checks for null or undefined, not falsy values.

💡 Hint

Think about what @IsNotEmpty() actually checks internally.

@IsNotEmpty() checks if the value is not null or undefined. The number 0 is a valid value and not considered empty by this decorator.

❓ state_output

advanced

What is the validation error message for a missing required property with @IsDefined()?

Consider this DTO:

class CreateUserDto {
  @IsDefined()
  username: string;
}

const dto = new CreateUserDto();
// username is not set
// Validation is run here

What error message will the validator produce?

A username must be defined

B username must be a string

C username should not be null or undefined

D username should not be empty

💡 Hint

Look at the default message for @IsDefined().

The default error message for @IsDefined() is ' should not be null or undefined'.

🧠 Conceptual

expert

Which decorator combination ensures a property is a non-empty array of strings with at least 2 items?

Choose the correct set of class-validator decorators to enforce that a property is:

• An array
• Contains only strings
• Has at least 2 items
• Is not empty

NestJS

class TagsDto {
  // decorators here
  tags: string[];
}

A @IsArray() @ArrayMinSize(2) @IsString({ each: true }) @IsNotEmpty()

B @IsArray() @ArrayMinSize(2) @IsString() @IsNotEmpty()

C @IsArray() @ArrayMinSize(2) @IsString({ each: true })

D @IsArray() @ArrayMinSize(2) @IsNotEmpty({ each: true }) @IsString()

💡 Hint

Remember to validate each item in the array and the array itself.

@IsArray() checks the property is an array.
@ArrayMinSize(2) ensures at least 2 items.
@IsString({ each: true }) validates each item is a string.
@IsNotEmpty() ensures the array itself is not empty.