class-validator decorators in NestJS - Mini Project: Build & Apply

Validating User Input with class-validator Decorators in NestJS

📖 Scenario: You are building a simple user registration form in a NestJS backend. You want to ensure that the data sent by users is valid before processing it.

🎯 Goal: Create a User Data Transfer Object (DTO) class using class-validator decorators to validate user input fields like username, email, and age.

📋 What You'll Learn

Create a class called UserDto with properties username, email, and age.

Add validation decorators @IsString() and @Length(3, 20) to username.

Add validation decorators @IsEmail() to email.

Add validation decorators @IsInt(), @Min(18), and @Max(99) to age.

💡 Why This Matters

🌍 Real World

Validating user input is essential in web applications to prevent bad data and security issues. Using class-validator decorators in NestJS makes this easy and clean.

💼 Career

Backend developers working with NestJS often create DTOs with validation decorators to ensure data integrity and improve application reliability.

1

Create the UserDto class with properties

Create a class called UserDto with three properties: username of type string, email of type string, and age of type number.

NestJS

export class UserDto {
  // Define username, email, and age properties here
}

Hint

Define a class with three properties and their types exactly as specified.

2

Add validation decorators for username

Import IsString and Length from class-validator. Add the decorators @IsString() and @Length(3, 20) above the username property.

NestJS

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

export class UserDto {
  // Add @IsString() and @Length(3, 20) decorators to username
  username: string;
  email: string;
  age: number;
}

Hint

Remember to import the decorators before using them.

3

Add validation decorator for email

Import IsEmail from class-validator. Add the decorator @IsEmail() above the email property.

NestJS

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

export class UserDto {
  @IsString()
  @Length(3, 20)
  username: string;
  // Add @IsEmail() decorator to email
  email: string;
  age: number;
}

Hint

Use @IsEmail() to check if the email is valid.

4

Add validation decorators for age

Import IsInt, Min, and Max from class-validator. Add the decorators @IsInt(), @Min(18), and @Max(99) above the age property.

NestJS

import { IsString, Length, IsEmail, IsInt, Min, Max } from 'class-validator';

export class UserDto {
  @IsString()
  @Length(3, 20)
  username: string;

  @IsEmail()
  email: string;

  // Add @IsInt(), @Min(18), and @Max(99) decorators to age
  age: number;
}

Hint

Use these decorators to ensure age is an integer between 18 and 99.

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