Custom validation decorators in NestJS - Mini Project: Build & Apply

Custom Validation Decorators in NestJS

📖 Scenario: You are building a NestJS backend for a user registration system. You want to ensure that the username field follows a custom rule: it must start with the letter 'A'. To do this, you will create a custom validation decorator.

🎯 Goal: Create a custom validation decorator called StartsWithA that checks if a string starts with the letter 'A'. Use this decorator in a DTO class to validate the username property.

📋 What You'll Learn

Create a custom validation decorator named StartsWithA using class-validator.

Create a DTO class called CreateUserDto with a username property.

Apply the StartsWithA decorator to the username property.

Use the IsString decorator from class-validator on username.

💡 Why This Matters

🌍 Real World

Custom validation decorators help enforce business rules on data inputs in backend APIs, improving data quality and user experience.

💼 Career

Backend developers often create custom validators to handle unique validation needs beyond built-in rules, making this skill valuable for NestJS and Node.js jobs.

1

Create the DTO class with username property

Create a class called CreateUserDto with a public property username of type string.

NestJS

class CreateUserDto {
  // Define username property here
  // Your code here
}

Hint

Define a class with a property named username of type string.

2

Add IsString decorator to username

Import IsString from class-validator and add the @IsString() decorator above the username property in CreateUserDto.

NestJS

import { IsString } from 'class-validator';

class CreateUserDto {
  // Add @IsString() decorator here
  username: string;
  // Your code here
}

Hint

Use @IsString() decorator from class-validator on the username property.

3

Create the custom StartsWithA decorator

Import registerDecorator, ValidationOptions, and ValidationArguments from class-validator. Create a function called StartsWithA that takes an optional ValidationOptions parameter. Inside, use registerDecorator to define a validator that checks if the value starts with the letter 'A'.

NestJS

import { registerDecorator, ValidationOptions, ValidationArguments } from 'class-validator';

function StartsWithA(validationOptions?: ValidationOptions) {
  // Use registerDecorator here
  // Your code here
}

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

Hint

Create a function that uses registerDecorator to check if the string starts with 'A'.

4

Apply the StartsWithA decorator to username

Add the @StartsWithA() decorator above the username property in CreateUserDto to enforce the custom validation rule.

NestJS

import { registerDecorator, ValidationOptions, ValidationArguments, IsString } from 'class-validator';

function StartsWithA(validationOptions?: ValidationOptions) {
  return function (object: Object, propertyName: string) {
    registerDecorator({
      name: 'startsWithA',
      target: object.constructor,
      propertyName: propertyName,
      options: validationOptions,
      validator: {
        validate(value: any, args: ValidationArguments) {
          return typeof value === 'string' && value.startsWith('A');
        },
        defaultMessage(args: ValidationArguments) {
          return `${args.property} must start with the letter A`;
        }
      },
    });
  };
}

class CreateUserDto {
  @IsString()
  // Add @StartsWithA() decorator here
  username: string;
  // Your code here
}

Hint

Use the custom decorator @StartsWithA() on the username property.

Practice

1. What is the main purpose of creating a custom validation decorator in NestJS?

easy

A. To create new routes in the application

B. To automatically generate database schemas

C. To handle HTTP requests and responses

D. To define your own rules for validating data inputs

Solution

1. Step 1: Understand validation decorators

   Validation decorators in NestJS are used to check if data meets certain rules before processing.

2. Step 2: Purpose of custom validation decorators

   Custom validation decorators let you create your own specific rules that are not covered by built-in validators.

3. Final Answer:

   To define your own rules for validating data inputs -> Option D

4. Quick Check:

   Custom validation decorators = custom data rules [OK]

Hint: Custom decorators create your own data validation rules [OK]

Common Mistakes:

• Confusing validation decorators with routing or database features
• Thinking custom decorators generate schemas automatically
• Assuming they handle HTTP requests directly

2. Which of the following is the correct way to start defining a custom validator class in NestJS?

easy

A. Use @Injectable decorator on the class

B. Use @Controller decorator on the class

C. Use @ValidatorConstraint decorator on the class

D. Use @Module decorator on the class

Solution

1. Step 1: Identify the decorator for custom validators

   In NestJS, custom validation classes must be marked with @ValidatorConstraint to tell the system they are validators.

2. Step 2: Differentiate from other decorators

   @Controller is for routing, @Injectable for services, and @Module for modules, not for validation.

3. Final Answer:

   Use @ValidatorConstraint decorator on the class -> Option C

4. Quick Check:

   @ValidatorConstraint marks custom validator classes [OK]

Hint: Custom validators use @ValidatorConstraint decorator [OK]

Common Mistakes:

• Using @Controller or @Module instead of @ValidatorConstraint
• Forgetting to decorate the validator class
• Confusing service decorators with validation decorators

3. Given this custom validator code snippet, what will be the output when validating the value "abc123"?

import { ValidatorConstraint, ValidatorConstraintInterface } from 'class-validator';

@ValidatorConstraint({ name: 'isAlphaOnly', async: false })
export class IsAlphaOnly implements ValidatorConstraintInterface {
  validate(text: string) {
    return /^[A-Za-z]+$/.test(text);
  }
  defaultMessage() {
    return 'Text must contain only letters';
  }
}

medium

A. Validation fails because 'abc123' contains numbers

B. Validation passes because 'abc123' contains letters

C. Validation passes because regex allows numbers

D. Validation throws an error due to syntax

Solution

1. Step 1: Analyze the regex in validate method

   The regex /^[A-Za-z]+$/ matches only strings with letters from start to end, no numbers allowed.

2. Step 2: Check the test string 'abc123'

   This string contains numbers '123', so it does not match the regex and validate returns false.

3. Final Answer:

   Validation fails because 'abc123' contains numbers -> Option A

4. Quick Check:

   Regex /^[A-Za-z]+$/ rejects numbers [OK]

Hint: Check regex carefully to see what characters it allows [OK]

Common Mistakes:

• Assuming partial matches pass validation
• Ignoring numbers in the test string
• Thinking regex allows digits when it does not

4. Identify the error in this custom validation decorator code:

import { registerDecorator, ValidationOptions, ValidatorConstraint, ValidatorConstraintInterface } from 'class-validator';

@ValidatorConstraint({ async: true })
export class IsEvenNumber implements ValidatorConstraintInterface {
  validate(value: number) {
    return value % 2 === 0;
  }
}

export function IsEven(validationOptions?: ValidationOptions) {
  return function (object: Object, propertyName: string) {
    registerDecorator({
      name: 'isEven',
      target: object.constructor,
      propertyName: propertyName,
      options: validationOptions,
      validator: IsEvenNumber,
    });
  };
}

medium

A. The validate method should return a Promise because async is true

B. The registerDecorator call is missing the validate method

C. The decorator function should not accept validationOptions

D. The class should not have @ValidatorConstraint decorator

Solution

1. Step 1: Check async flag and validate method

   The @ValidatorConstraint decorator has async: true, so validate must return a Promise<boolean>.

2. Step 2: Validate method returns boolean, not Promise

   Currently, validate returns a boolean directly, which conflicts with async: true.

3. Final Answer:

   The validate method should return a Promise because async is true -> Option A

4. Quick Check:

   async: true requires Promise return [OK]

Hint: If async: true, validate must return Promise<boolean> [OK]

Common Mistakes:

• Returning boolean instead of Promise when async is true
• Omitting @ValidatorConstraint decorator
• Misusing registerDecorator parameters

5. You want to create a custom validation decorator that checks if a string property contains at least one uppercase letter and one digit. Which approach correctly combines the concepts to implement this in NestJS?

hard

A. Use built-in decorators @IsUppercase and @IsNumber together on the property

B. Create a class with @ValidatorConstraint, implement validate() using regex, then use registerDecorator in a function to apply it

C. Create a service with @Injectable and call it inside the DTO without decorators

D. Use only registerDecorator with a simple function without a class or @ValidatorConstraint

Solution

1. Step 1: Understand custom validation structure

   Custom validators require a class decorated with @ValidatorConstraint implementing validate() method.

2. Step 2: Use registerDecorator to link the class to a decorator function

   This function applies the validator to the property in the DTO.

3. Step 3: Implement validate() with regex checking uppercase and digit presence

   This ensures the string meets both conditions in one check.

4. Final Answer:

   Create a class with @ValidatorConstraint, implement validate() using regex, then use registerDecorator in a function to apply it -> Option B

5. Quick Check:

   Custom validator class + registerDecorator function = correct approach [OK]

Hint: Combine @ValidatorConstraint class with registerDecorator function [OK]

Common Mistakes:

• Skipping the validator class and using only registerDecorator
• Trying to use services instead of decorators for validation
• Assuming built-in decorators cover combined conditions