Typescript mixins

// Generic constructor signature
type ClassConstructor<T> = new (...args: any[]) => T;

// The contraint we might want to apply to our constructor signature
export interface IHelperRepository {
  getEntity<T extends EntityDTO>(entityQuery: EntityQuery): Promise<Maybe<T>>;
  getItems<T extends SummaryDTO>(itemsQuery: ItemsQuery): Promise<T[]>;
}
type HelperRepoConstructor = ClassConstructor<IHelperRepository>;

// This is the mixin, a function that takes a class constructor and returns a class constructor
function withPostProcessing<C extends HelperRepoConstructor>(Class: C) {
  return class extends Class implements IHelperRepository {
    public getEntity<T extends EntityDTO>(
      entityQuery: EntityQuery
    ): Promise<Maybe<T>> {
      return super.getEntity<T>(entityQuery);
      //Post process: filtering, logging, transforming,...
    }

    getItems<T extends SummaryDTO>(itemsQuery: ItemsQuery): Promise<T[]> {
      return super.getItems<T>(itemsQuery);
      //Post process: filtering, logging, transforming,...
    }
  };
}

// The class with the mixin applied
const MyRepoClassWithMixin = withPostProcessing(class implements IHelperRepository{
  getEntity<T extends EntityDTO>(entityQuery: EntityQuery): Promise<Maybe<T>> {
    throw Error()
  }

  getItems<T extends SummaryDTO>(itemsQuery: ItemsQuery): Promise<T[]> {
    throw Error()
  }
});

// Instance of the class with the mixin applied
const repoWithMixin = new MyRepoClassWithMixin();

repoWithMixin.getItems(...);