How do you implement custom validation logic for DTOs in Nest.js?

To implement custom validation logic for DTOs in Nest.js, we typically use the class-validator library, which provides decorators and functions to define these validation rules. Here are some steps and examples to implement custom validation logic:

Step 1: Install Dependencies

First, ensure your project has the class-validator and class-transformer packages installed.

bash
npm install class-validator class-transformer

Step 2: Define DTO

Use decorators in your DTO to define validation rules. For example, let's define a CreateUserDto:

typescript
import { IsNotEmpty, IsEmail, Validate } from 'class-validator';

export class CreateUserDto {
  @IsNotEmpty()
  name: string;

  @IsEmail()
  email: string;

  @IsNotEmpty()
  password: string;

  @Validate(CustomPasswordValidator)
  password: string;
}

Step 3: Create a Custom Validator

To add custom validation logic, create a custom validator class. For example, let's implement a validator for password complexity:

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

@ValidatorConstraint({ name: 'customPasswordValidator', async: false })
export class CustomPasswordValidator implements ValidatorConstraintInterface {
  validate(password: string, args: ValidationArguments) {
    // Password must be at least 8 characters long and include at least one number, one uppercase letter, and one lowercase letter
    return /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[a-zA-Z\d]{8,}$/.test(password);
  }

  defaultMessage(args: ValidationArguments) {
    // Custom message when validation fails
    return 'Password must be at least 8 characters long and contain at least one number, one uppercase letter, and one lowercase letter.';
  }
}

Step 4: Use DTO in Controller

Ensure you use this DTO in your controller and leverage Nest.js's pipes to automatically apply validation.

typescript
import { Body, Controller, Post } from '@nestjs/common';
import { CreateUserDto } from './create-user.dto';

@Controller('users')
export class UsersController {
  @Post()
  async create(@Body() createUserDto: CreateUserDto) {
    // The createUserDto object is validated before reaching this point
    // If validation fails, Nest.js automatically throws an exception
    return 'User created successfully';
  }
}

Summary

Through these steps, we demonstrate how to implement custom DTO validation logic in a Nest.js project. By using the @Validate decorator from class-validator, you can easily add complex custom validation logic to meet business requirements. This ensures the data integrity and security of your application.