Liskov Substitution Principle

If you have a function, that works for a base type, it should work for a derived type.

class TodoList {
    items: ITodo[];

    getAllItems() {
        return this.items;
    }

}

class PendingTodoList extends TodoList {
    constructor() {
        super();
    }
}

Example Solution

class PendingTodoList extends TodoList {
    getAllItems() {
        return this.items.filter(it => it.status === EStatus.pending);
    }
}