When building scalable and maintainable backend applications, data validation and type safety are essential. NestJS promotes these best practices through DTOs (Data Transfer Objects) and Validation Pipes, enabling developers to enforce consistent data structures and validate user input effortlessly.
In this article, we’ll explore how to use DTOs and Validation Pipes in NestJS, why they matter, and how they can dramatically improve your development workflow.
What Are DTOs in NestJS?
DTOs, or Data Transfer Objects, are TypeScript classes that define the shape of data being sent over the network. In NestJS, DTOs are used to describe the expected structure of request bodies, query parameters, or route parameters.
They are especially useful when working with TypeScript because they provide static typing, autocomplete, and error detection at compile time.
Example: Creating a DTO
This DTO defines validation rules for creating a user. You’ll notice decorators from the class-validator package. These rules ensure that user input matches the desired format.
Introducing Validation Pipes
Validation Pipes are a powerful feature in NestJS that enable you to apply transformation and validation logic to incoming requests. By combining Validation Pipes with DTOs, you can automatically check and transform data before it reaches your controller methods.
Setting Up ValidationPipe Globally
To activate validation across your application, use the ValidationPipe globally in the main bootstrap file:
The { whitelist: true } option strips out any properties that are not in your DTO, which is a good practice for security and cleanliness.
Using DTOs and Pipes in Controllers
With DTOs and ValidationPipe in place, you can apply them in your route handlers:
Here, NestJS automatically validates the incoming request body against the rules defined in CreateUserDto. If the validation fails, a 400 error is returned with helpful error messages.
Transformation with ValidationPipe
ValidationPipe also allows you to transform incoming data types. For example, converting strings to numbers:
And then use @Type() from class-transformer in your DTO:
This ensures the limit query parameter is treated as a number instead of a string.
Best Practices with DTOs and Validation
- Keep DTOs simple and focused on one purpose.
- Use whitelisting to strip unwanted data.
- Always return clear error messages for validation failures.
- Reuse DTOs where applicable to maintain consistency.
- Use
PartialType()from@nestjs/mapped-typesfor update DTOs:
This automatically makes all fields optional for update operations.
Error Handling with ValidationPipe
When validation fails, NestJS throws a BadRequestException with a standard structure:
You can customise the error format globally by passing an exceptionFactory to the pipe.
Final Thoughts
Using DTOs and Validation Pipes in NestJS is one of the cleanest ways to manage data validation in modern APIs. With a few lines of code, you ensure that your application accepts only valid, expected data, reducing bugs and enhancing security.
If you’re building production-grade backends with NestJS, mastering DTOs and validation is essential.
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