Custom validation decorators in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the necessary decorator for custom validation.

NestJS

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

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

AIsString

BValidatorConstraint

CValidate

DIsNotEmpty

Attempts:

3 left

💡 Hint

Common Mistakes

Using decorators meant for property validation like IsString instead of ValidatorConstraint.

The ValidatorConstraint decorator is used to define a custom validation constraint class in NestJS.

2fill in blank

medium

Complete the code to implement the ValidatorConstraintInterface.

NestJS

export class CustomTextLengthValidator implements [1] {
  validate(text: string) {
    return text.length > 5;
  }
}

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

AValidatorConstraintInterface

BValidationArguments

CValidatorConstraint

DValidationOptions

Attempts:

3 left

💡 Hint

Common Mistakes

Using the decorator name instead of the interface name for implementation.

The class must implement ValidatorConstraintInterface to define the validate method for custom validation logic.

3fill in blank

hard

Fix the error in the custom decorator function to apply the validation correctly.

NestJS

export function IsCustomText(validationOptions?: ValidationOptions) {
  return function (object: Object, propertyName: string) {
    [1](CustomTextLengthValidator, {
      message: 'Text is too short',
      ...validationOptions,
    })(object, propertyName);
  };
}

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

AIsString

BValidatorConstraint

CValidateIf

DValidate

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class decorator ValidatorConstraint instead of the property decorator Validate.

The Validate decorator applies the custom validator class to the property inside the custom decorator function.

4fill in blank

hard

Fill both blanks to complete the custom validator class and decorator usage.

NestJS

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

@ValidatorConstraint({ name: 'customText', [2]: true })
export class CustomTextLengthValidator implements ValidatorConstraintInterface {
  validate(text: string, args: ValidationArguments) {
    return typeof text === 'string' && text.length > 5;
  }
}

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

AValidatorConstraint

BValidate

Casync

DasyncValidation

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the async option name or missing the decorator.

The ValidatorConstraint decorator marks the class as a validator. The async option set to true allows asynchronous validation if needed.

5fill in blank

hard

Fill all three blanks to create a custom decorator that uses the validator and accepts validation options.

NestJS

import { Validate, ValidationOptions } from 'class-validator';

export function IsCustomText([1]?: ValidationOptions) {
  return function (object: Object, propertyName: string) {
    [2](CustomTextLengthValidator, {
      message: 'Text must be longer than 5 characters',
      ...[3],
    })(object, propertyName);
  };
}

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

AvalidationOptions

BValidate

DValidatorConstraint

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong decorator or mismatching parameter names.

The function parameter is named validationOptions. Inside, the Validate decorator is called with the validator class and options spread from validationOptions.

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