import { repository } from './repositories';

export function usecase() {
  const result = repository();
  return result * 10;
}

export function repository() {
  const result = db;
  return result;
}

const db = 42;


import { repository } from './repositories';
import { usecase } from './usecases';

jest.mock('./repositories.ts');

describe('', () => {
  test('', () => {
    (repository as jest.Mock).mockReturnValue(10);
    const sut = usecase();

    expect(sut).toEqual(100);
  });
});

に置くなどすると、他のテストでも再利用できる