graphql-composer-decorators

# Generic types

It will certainly be very useful in your application to use generic types. Let's take the example of a list response to a request in which you send all items via a list of the type of the item (items) and the size of the list (count).
In the form of class:

class Response<Type> {
  items: Type;
  count: Number;
}

To do this with decorators we have to use an unusual method: a function returning a class!

# Process of creating a generic type

# Base class declaration

Only the number field is decorated, because the items do not have a type at this time.

The class is hidden because it serves as a template for the types that will be created from it.

ObjectType({ hidden: true })
abstract class GenericResponse<Type> {
  @Field()
  count: Number;
  items: Type[];
}

# Type creator declaration

You will need to use the value of the class passed as a parameter to the @Field decorator and the type passed as a generic parameter to the function to declare the type of the class field.

import { ClassType } from "graphql-composer";

function createResponse<ItemsType extends ClassType>(itemsType: ItemsType) {
  @ObjectType()
  class Response extends GenericResponse<ItemsType> {
    @Field(type => [itemsType])
    items: ItemsType[]
  }

  return Response as ClassType;
}

Finally, that how to use our generic type:

@ObjectType()
class User {
  @Field()
  username: string;

  @Field()
  email: string;
}

const userResponse = createResponse(User);

@Resolver()
class R {
  @Query(type => userResponse)
  query(): GenericResponse<User> {
    // ...
  }
}

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