Lately I've been writing pure functions more and more in my code, they convey meaning very well, are easy to write and test

Now that most of my logic is in pure function I tend to write them outside of components because they don't depend on the state of the component.

Example getUsername is pure and not used in the template and as such doesn't really need to be a method of the component.

@Component({selector: ..., templateUrl: ..., styleUrls: ...})
export class UserComponent implements OnInit {
    @Input() user: User;
    username: string

    ngOnInit() {
        this.username = getUsername(this.user);
    }
}

export const getUsername = (user: User): string => `${user.firstname} ${user.lastname}`;

When I have too many of these pure functions I sometimes move them to a helper file.

Everything works perfectly, except when I try to spy on them for unit testing. At this point jasmine refuses to spy on them (relevant github issue)

What do you guys think I should do ? Move my pure functions to method of components ? Change my helper files into services eventhough they don't use any state ?

I feel like I have to remove relevant information from my code (purity of the functions) for the sole reason of unit testing and I don't like it

Edit: spying on an exported function used to work but stopped working with Angular 9, cf this github issue