How to Map Validation Errors to Use Exposed Property Names in NestJS DTOs?

Are you tired of receiving cryptic error messages that leave your users scratching their heads? Do you want to take your error handling game to the next level in your NestJS application? Well, you’re in luck! In this article, we’ll dive into the world of validation errors and explore how to map them to use exposed property names in NestJS DTOs. Buckle up, folks, because we’re about to take a wild ride!

What are DTOs and Why Do We Need Them?

DTOs, or Data Transfer Objects, are lightweight data containers used to transfer data between layers of your application. In NestJS, they play a crucial role in defining the structure of your data and ensuring that it’s validated properly. But, what happens when something goes wrong, and your application encounters an error? That’s where validation errors come into play.

Validation Errors 101

Validation errors occur when the data sent to your application doesn’t meet the expectations set by your DTOs. This can happen due to various reasons, such as invalid or missing data, incorrect data types, or even malicious attacks. Whatever the reason, it’s essential to handle these errors in a way that’s both informative and user-friendly.

By default, NestJS provides a built-in validation mechanism using the @nestjs/common module. When a validation error occurs, it returns an error response with a cryptic message that looks something like this:

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": ["property path is required"]
}

While this error message is helpful for developers, it’s not exactly user-friendly, is it? That’s where mapping validation errors to use exposed property names comes in.

What are Exposed Property Names?

Exposed property names are human-readable names that correspond to the properties defined in your DTOs. These names can be easily understood by users, making it easier for them to identify and correct errors. For example, instead of receiving an error message that says “property path is required,” users would see a more descriptive message like “Username is required.”

How to Map Validation Errors to Use Exposed Property Names

Now that we’ve covered the basics, let’s dive into the juicy stuff! To map validation errors to use exposed property names, we’ll need to follow these steps:

1. Create a custom DTO with exposed property names

2. Use the @nestjs/validator module to define validation rules

3. Create a custom validation pipe to map errors

4. Register the custom validation pipe in your NestJS application

Step 1: Create a Custom DTO with Exposed Property Names

Let’s create a simple DTO for a user registration form:

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

export class UserRegistrationDTO {
  @IsString()
  @IsNotEmpty()
  readonly username: string;

  @IsEmail()
  @IsNotEmpty()
  readonly email: string;

  @IsString()
  @IsNotEmpty()
  readonly password: string;
}

In this example, we’ve defined a UserRegistrationDTO with three properties: username, email, and password. We’ve also added validation rules using the @IsString(), @IsEmail(), and @IsNotEmpty() decorators.

Step 2: Define Validation Rules

We’ve already defined validation rules using the @nestjs/validator module in our DTO. But, we need to take it a step further by defining exposed property names. We can do this by adding a constraints object to each property:

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

export class UserRegistrationDTO {
  @IsString()
  @IsNotEmpty()
  @Constraints({ message: "Username is required" })
  readonly username: string;

  @IsEmail()
  @IsNotEmpty()
  @Constraints({ message: "Valid email is required" })
  readonly email: string;

  @IsString()
  @IsNotEmpty()
  @Constraints({ message: "Password is required" })
  readonly password: string;
}

In this updated example, we’ve added a constraints object to each property with a custom error message. These messages will be used later to create a more user-friendly error response.

Step 3: Create a Custom Validation Pipe

Now that we have our DTO with exposed property names, it’s time to create a custom validation pipe to map errors:

import { PipeTransform, Injectable } from '@nestjs/common';
import { validate } from 'class-validator';
import { ValidationException } from './validation.exception';

@Injectable()
export class ValidationPipe implements PipeTransform {
  async transform(value: any): Promise {
    const errors = await validate(value);

    if (errors.length > 0) {
      throw new ValidationException(errors);
    }

    return value;
  }
}

In this example, we’ve created a ValidationPipe that uses the validate() function from the class-validator module to validate the incoming data. If any errors are found, it throws a ValidationException with the error messages.

Step 4: Register the Custom Validation Pipe

Finally, we need to register our custom validation pipe in our NestJS application:

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { ValidationPipe } from './validation.pipe';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  app.useGlobalPipes(new ValidationPipe());

  await app.listen(3000);
}

bootstrap();

In this example, we’ve registered the ValidationPipe as a global pipe using the app.useGlobalPipes() method.

Putting it All Together

Let’s create a simple NestJS controller to handle user registration:

import { Controller, Post, Body } from '@nestjs/common';
import { UserRegistrationDTO } from './user-registration.dto';

@Controller('users')
export class UserController {
  @Post()
  async register(@Body() userRegistrationDTO: UserRegistrationDTO) {
    // Handle user registration logic here
  }
}

In this example, we’ve created a UserController with a single POST method to handle user registration. We’ve also injected the UserRegistrationDTO as a parameter using the @Body() decorator.

When we send a request to the /users endpoint with invalid data, our custom validation pipe will kick in and return a user-friendly error response:

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": [
    {
      "property": "username",
      "constraints": {
        "message": "Username is required"
      }
    },
    {
      "property": "email",
      "constraints": {
        "message": "Valid email is required"
      }
    }
  ]
}

Voilà! We’ve successfully mapped validation errors to use exposed property names in our NestJS DTO. This approach not only improves the user experience but also provides more informative error messages for developers.

Conclusion

In this article, we’ve explored the world of validation errors and learned how to map them to use exposed property names in NestJS DTOs. By following these simple steps, you can create more user-friendly error responses that provide valuable insights into what went wrong.

Remember, error handling is an essential part of building a robust and scalable application. By taking the extra step to map validation errors to use exposed property names, you can create a better user experience and reduce the number of support requests.

So, what are you waiting for? Start mapping those validation errors today and take your NestJS application to the next level!

DTO Property	Exposed Property Name
username	Username
email	Email Address
password	Password

1. DTO Property: The actual property defined in your DTO.

2. Exposed Property Name: The human-readable name associated with the DTO property.

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