NestJs: Why do we need DTOs and interfaces both in NestJS

Question

The NestJS documentation showcases how to add DTOs to use in Controllers to validate request objects by using class-validator package. DTOs described there are TypeScript classes. Now, while controllers deal with DTOs(TS Classes), NestJS providers (or services), on the other hand, makes use of TypeScript interfaces. These DTOs and interfaces are pretty much of the same shape.

Now, I am seeing duplication of shape definition here. And wondering if the interfaces are needed at all?

Can we not make DTOs source of truth for the shape and validations? One of the approaches we were considering (to make the DTO source of truth) was, to have an openapi generator take the DTOs as input and generate openapi definition and from there another codegen can generate a set of typescript interfaces to be consumed by NestJS itself and which can be shared with another set of consumer applications like Angular too.

Have any people come across a similar problem? What do you think of the above? Feedback appreciated.

Answer 1

I think it's important to know what DTO is.

DTO(Data Transfer Object) is a concept of Java(J2EE) design.

It looks like a normal Java Bean object, which was created for transferring data object through multiple layers (such as controller, service, database) in our backend, especially in Distributed Systems.

Without DTO Model

We send lots of requests to query some data we want, which may be duplicated.

Application -> WebService -> Database

1. It consumes large amount of bandwidth because of some duplicated requests.
2. Unsafely, it returns the whole object from database, which contains some attributes should not be exposed. BTW we should manually add some additional code to limit it, which sucks.

With DTO Model

It helps us to handle our data object.

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In the guide of NestJS, DTO acts like a HTTP Request body.

In my opinion, DTO contains:

• some attributes we would use but not in Database.

and DTO masks:

• some attributes we don't want to expose.

To use with class-validator, DTO can also help us validate data in an elegant way.

Sometimes it looks same with object interface.

I think DTO matters when our database collection is huge and complex.