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
}

Need a 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
}

Need a 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;
}

Need a 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
}

Need a hint?

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