Answers

1. The problem is that the file extension is tests.tsx rather than test.tsx.
2. The toBe matcher should only be used for checking primitive values such as numbers and strings—this is an object. The toStrictEqual matcher should be used to check objects because it checks the values of all its properties instead of the object reference:

   expect({ name: 'Bob' }).toStrictEqual({ name: 'Bob' });

3. The not and toBeNull matchers can be combined to check a variable isn’t null:

   expect(something).not.toBeNull();

4. The toBeDisabled matcher can be used from jest-dom:

   expect(screen.getByText('Save')).toBeDisabled();

5. Here’s a test:

   test('should remove id when already in checked ids', () => {

     const result = getNewCheckedIds([1, 2, 3], 2);

     expect(result).toStrictEqual([1, 3]);

   });

6. The findBy query type requires awaiting because it is asynchronous:

   expect(await screen.findByText('Save')).toBeInTheDocument...