How would you create an object?

Question

Angular

I receive data from the server - an array of categories: interface Category { id: number, name: string, items: []}

I want to create a group based on each category, because the category has items: [], so that the group works as a dropdown element - opens and hides on click.

Imagine that the group is defined as follows: interface Group { id: number, name: elements: []}.

Suppose that in the parent component I get the categories:

public categories$: Observable<Category[]>;

And in the template of this component I display the categories in a group:

<app-category-group *ngFor="let cat of categories$  | async"></app-category-group>

How not to mix types Categoryand Grouphow to make it so that no matter what the content is in Group. Where to do it - in the parent after receiving the categories - to convert to another type? At the same time, I do not want to mutate the original data