sentry/tests/js/spec/utils/usePrevious.spec.tsx

import {reactHooks} from 'sentry-test/reactTestingLibrary';

import usePrevious from 'sentry/utils/usePrevious';

describe('usePrevious', () => {
  it('stores initial value', () => {
    const {result} = reactHooks.renderHook(() => usePrevious('Initial Value'));
    expect(result.current).toBe('Initial Value');
  });

  it('provides initial value', () => {
    const {result} = reactHooks.renderHook(value => usePrevious(value), {
      initialProps: 'Initial Value',
    });

    expect(result.current).toBe('Initial Value');
  });

  it('provides previous value', () => {
    const {result, rerender} = reactHooks.renderHook(
      (value: string | undefined) => usePrevious(value),
      {
        initialProps: undefined,
      }
    );

    rerender('New Value');
    // We did not pass anything under initialProps
    expect(result.current).toBe(undefined);
    rerender('New New Value');
    // Result should point to old value
    expect(result.current).toBe('New Value');
  });
});