namae/src/util/hooks.test.tsx

import { render, waitFor } from '@testing-library/react';
import 'mutationobserver-shim';
import React from 'react';
import { useDeferredState } from './hooks';

const App: React.FC = () => {
  const [value, setValue] = useDeferredState(500, 0);
  React.useEffect(() => {
    setValue(1);
    setValue(2);
    setValue(3);
  }, [setValue]);
  return <div data-testid="root">{value}</div>;
};

it('provoke state flow after certain time passed', async () => {
  const { getByTestId } = render(<App />);
  expect(getByTestId('root').textContent).toBe('0');
  await waitFor(() => {
    expect(getByTestId('root').textContent).toBe('3');
  });
});