How to open/close a list on click without adding isOpen: true to the object?

Question

Angular

Guys tell me how to open / close on click the list without adding isOpen: true to the object? So that the object was in the end without type modification , but remained . Tell me how to do it? { id: 1, title: 'Title', isOpen: true }{ id: 1, title: 'Title' }