usePrevious.test.ts

import { renderHook } from '@testing-library/react-hooks';
import usePrevious from '../src/usePrevious';

const setUp = () => renderHook(({ state }) => usePrevious(state), { initialProps: { state: 0 } });

it('should return undefined on initial render', () => {
  const { result } = setUp();

  expect(result.current).toBeUndefined();
});

it('should always return previous state after each update', () => {
  const { result, rerender } = setUp();

  rerender({ state: 2 });
  expect(result.current).toBe(0);

  rerender({ state: 4 });
  expect(result.current).toBe(2);

  rerender({ state: 6 });
  expect(result.current).toBe(4);
});