Custom validation decorators in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom validation decorator in NestJS?

A custom validation decorator in NestJS is a user-defined function that adds specific validation rules to class properties, allowing you to check data beyond built-in validators.

beginner

Which package is commonly used to create custom validation decorators in NestJS?

The 'class-validator' package is commonly used to create custom validation decorators in NestJS, enabling easy integration with validation pipes.

intermediate

What are the two main parts needed to create a custom validation decorator in NestJS?

You need a validator constraint class implementing the validation logic and a decorator function that applies this constraint to a property.

intermediate

How do you ensure your custom validator works with NestJS validation pipe?

By using the 'ValidatorConstraint' and 'Validate' decorators from 'class-validator' and applying the custom decorator on DTO properties, the validation pipe automatically runs your custom logic.

beginner

Why might you create a custom validation decorator instead of using built-in ones?

You create custom validation decorators to handle specific rules unique to your app, like checking a username format or verifying a business rule that built-in validators don't cover.

Which decorator marks a class as a custom validator constraint in NestJS?

A @IsString

B @ValidatorConstraint

C @Validate

D @IsNotEmpty

The @ValidatorConstraint decorator marks a class as a custom validator constraint.

What does the @Validate decorator do in a custom validation setup?

A Marks a property as optional

B Defines a new validation rule

C Runs validation on the whole class

D Applies a custom validator to a property

The @Validate decorator applies a custom validator class to a specific property.

Which package provides the tools to create custom validation decorators in NestJS?

A class-validator

B express-validator

C joi

D nestjs-validation

The class-validator package is used in NestJS for validation decorators.

What must a custom validator class implement to work properly?

A ValidatorConstraintInterface

B ValidationPipe

C PipeTransform

D OnModuleInit

The custom validator class must implement ValidatorConstraintInterface.

Why use custom validation decorators in NestJS?

A To replace the validation pipe

B To speed up the app

C To validate data with rules not covered by built-in validators

D To style the UI

Custom validation decorators allow validation of specific rules not covered by built-in validators.

Explain how to create a custom validation decorator in NestJS step-by-step.

Think about the class-validator decorators and how they connect.

Describe why and when you would use a custom validation decorator in a NestJS project.

Consider unique app requirements and validation needs.

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