Why Custom validation decorators in NestJS? - Purpose & Use Cases

The Big Idea

What if you could write your validation rules once and have them magically work everywhere?

The Scenario

Imagine you have many forms in your app, and you need to check if user inputs follow specific rules like "must be a valid phone number" or "password must have special characters".

You write the same validation code again and again in every controller or service.

The Problem

Manually repeating validation code everywhere is tiring and easy to forget or make mistakes.

It also makes your code messy and hard to update if rules change.

The Solution

Custom validation decorators let you write your validation logic once and reuse it everywhere by just adding a simple tag on your data fields.

This keeps your code clean, consistent, and easy to maintain.

Before vs After

✗ Before

if (!isValidPhone(user.phone)) { throw new Error('Invalid phone'); }

✓ After

@IsPhoneNumber() phone: string;

What It Enables

You can create clear, reusable, and easy-to-read validation rules that automatically check user input wherever needed.

Real Life Example

In a signup form, instead of writing phone number checks in the controller, you just add @IsPhoneNumber() on the phone field in your DTO, and NestJS handles the rest.

Key Takeaways

Manual validation is repetitive and error-prone.

Custom decorators let you write validation once and reuse it.

This makes your code cleaner and easier to maintain.

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