class-validator decorators in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to validate that the 'email' property is a valid email address.

NestJS

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

class UserDto {
  @[1]()
  email: string;
}

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

AIsEmail

BIsString

CIsNotEmpty

DIsNumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using @IsString instead of @IsEmail

Forgetting to import the decorator

Using @IsNotEmpty which only checks presence, not format

The @IsEmail() decorator checks if the property is a valid email address.

2fill in blank

medium

Complete the code to ensure the 'age' property is a number and at least 18.

NestJS

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

class UserDto {
  @IsNumber()
  @[1](18)
  age: number;
}

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

AMax

BIsPositive

CMin

DIsInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Max instead of @Min

Using @IsInt which only checks integer, not minimum value

Not combining with @IsNumber

The @Min(18) decorator ensures the number is at least 18.

3fill in blank

hard

Fix the error in the code to validate that 'username' is a non-empty string.

NestJS

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

class UserDto {
  @IsString()
  @[1]()
  username: string;
}

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

AIsNotEmpty

BIsEmail

CIsNumber

DMinLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using @MinLength without specifying length

Using @IsEmail which is for emails

Using @IsNumber on a string

The @IsNotEmpty() decorator ensures the string is not empty.

4fill in blank

hard

Fill both blanks to validate that 'password' is a string with minimum length 8.

NestJS

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

class UserDto {
  @[1]()
  @[2](8)
  password: string;
}

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

AIsString

BMinLength

CIsNotEmpty

DMaxLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using @MaxLength instead of @MinLength

Forgetting to check type with @IsString

Using @IsNotEmpty which does not check length

@IsString() ensures the value is a string, and @MinLength(8) ensures it has at least 8 characters.

5fill in blank

hard

Fill all three blanks to create a validation that 'tags' is an optional array of strings with each string not empty.

NestJS

import { [1], [2], [3] } from 'class-validator';

class UserDto {
  @[1]()
  @[2]({ each: true })
  @[3]({ each: true })
  tags?: string[];
}

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

AIsOptional

BIsString

CIsNotEmpty

DIsArray

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting @IsOptional and causing validation errors when property is missing

Not using { each: true } and validating the whole array instead of items

Using @IsArray but missing item validations

@IsOptional() allows the property to be missing, @IsString({ each: true }) checks each item is a string, and @IsNotEmpty({ each: true }) ensures no empty strings in the array.

Practice

1. What is the main purpose of @IsEmail() decorator in class-validator when used in a NestJS DTO class?

easy

A. To convert the property value to lowercase

B. To check if the property value is a valid email address format

C. To make the property optional

D. To trim whitespace from the property value

Solution

1. Step 1: Understand the decorator's role

   The @IsEmail() decorator validates that the decorated property contains a valid email format string.

2. Step 2: Compare options with decorator behavior

   Only To check if the property value is a valid email address format matches the validation purpose of @IsEmail(). Other options describe unrelated behaviors.

3. Final Answer:

   To check if the property value is a valid email address format -> Option B

4. Quick Check:

   @IsEmail() validates email format [OK]

Hint: Remember: @IsEmail() checks email format only [OK]

Common Mistakes:

• Thinking @IsEmail() modifies the value instead of validating
• Confusing @IsEmail() with @IsOptional() or @Transform decorators

2. Which of the following is the correct way to use @Length() decorator to require a string property to be between 5 and 10 characters in NestJS?

easy

A. @Length(5, 10)

B. @Length(10, 5)

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

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

Solution

1. Step 1: Recall the syntax of @Length decorator

   The @Length() decorator takes two numeric arguments: minimum length first, then maximum length.

2. Step 2: Match options with correct syntax

   @Length(5, 10) correctly uses @Length(5, 10). Options A, B, and D use incorrect argument order or syntax.

3. Final Answer:

   @Length(5, 10) -> Option A

4. Quick Check:

   @Length(min, max) = @Length(5, 10) [OK]

Hint: @Length(min, max) uses two numbers, min first [OK]

Common Mistakes:

• Swapping min and max values
• Using object syntax instead of positional arguments
• Using named parameters which are invalid

3. Given this DTO class, what will be the validation result for username = 'abc' and age = 17?

class UserDto {
  @Length(5, 15)
  username: string;

  @Min(18)
  age: number;
}

medium

A. Validation passes with no errors

B. Validation fails because username is too short

C. Validation fails because age is less than 18

D. Validation fails because both username and age are invalid

Solution

1. Step 1: Check username length

   The username 'abc' has length 3, which is less than the minimum 5 required by @Length(5, 15). So username is invalid.

2. Step 2: Check age minimum

   The age is 17, which is less than the minimum 18 required by @Min(18). So age is invalid.

3. Final Answer:

   Validation fails because both username and age are invalid -> Option D

4. Quick Check:

   username too short + age too low = both invalid [OK]

Hint: Check each decorator rule separately for all properties [OK]

Common Mistakes:

• Ignoring one invalid field and assuming validation passes
• Confusing @Min with @Max for age
• Assuming length includes spaces or trimming automatically

4. Identify the error in this DTO class using class-validator decorators:

class ProductDto {
  @IsInt()
  @Min(1)
  price: string;
}

medium

A. price should be a number type, not string

B. Missing @IsString() decorator for price

C. @Min(1) cannot be used with @IsInt()

D. No error, code is correct

Solution

1. Step 1: Analyze property type and decorators

   The property price is declared as string, but decorators @IsInt() and @Min(1) expect a number type.

2. Step 2: Identify mismatch causing validation error

   Since price is string, validation will fail because @IsInt() requires a number. The type should be number.

3. Final Answer:

   price should be a number type, not string -> Option A

4. Quick Check:

   Type mismatch between property and decorators = error [OK]

Hint: Match property type with decorator expectations [OK]

Common Mistakes:

• Using string type with number validators
• Assuming decorators convert types automatically
• Ignoring TypeScript type declarations

5. You want to validate a DTO property tags which is an array of strings, each string must be at least 3 characters. Which combination of class-validator decorators correctly enforces this?

hard

A. @IsString()\n@Length(3, 50)\n@IsArray()

B. @IsArray()\n@IsString()\n@Length(3, 50)

C. @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true })

D. @IsArray()\n@ArrayMaxSize(1)\n@IsString({ each: false })

Solution

1. Step 1: Validate the property is an array

   Use @IsArray() to ensure tags is an array type.

2. Step 2: Validate each element is a string with length constraints

   Use @IsString({ each: true }) and @Length(3, 50, { each: true }) to apply string and length validation to every item in the array.

3. Step 3: Ensure array has at least one element

   @ArrayMinSize(1) ensures the array is not empty.

4. Final Answer:

   @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true }) -> Option C

5. Quick Check:

   Array + each:true for element validation = @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true }) [OK]

Hint: Use { each: true } to validate every array element [OK]

Common Mistakes:

• Forgetting { each: true } for array element validation
• Using @IsString() without array context
• Setting wrong array size constraints