We are going to use the waitForNextUpdate function to test debouncing in our useDebouncedUndo Hook by following these steps:

1. Create a new src/hooks/useDebouncedUndo.test.js file.
2. Import the renderHook and act functions as well as the useDebouncedUndo Hook:

import { renderHook, act } from '@testing-library/react-hooks'
import useDebouncedUndo from './useDebouncedUndo'

3. First of all, we test whether the Hook returns a proper result, including the content value, setter function, and the undoRest object:

test('should use debounced undo', () => {
    const { result } = renderHook(() => useDebouncedUndo())
    const [ content, setter, undoRest ] = result.current

    expect(content).toBe('')
    expect(typeof setter).toBe('function')
    expect(typeof undoRest.undo).toBe('function')
    expect(typeof undoRest.redo).toBe('function')
    expect(undoRest.canUndo).toBe(false)
    expect(undoRest.canRedo).toBe(false)
})

4. Next, we test whether the content value gets updated immediately:

test('should update content immediately', () => {
    const { result } = renderHook(() => useDebouncedUndo())
    const [ content, setter ] = result.current

    expect(content).toBe('')
    act(() => setter('test'))
    const [ newContent ] = result.current
    expect(newContent).toBe('test')
})

5. Finally, we use waitForNextUpdate to wait for the debounced effect to trigger. After debouncing, we should now be able to undo our change:

test('should debounce undo history update', async () => {
    const { result, waitForNextUpdate } = renderHook(() => useDebouncedUndo())
    const [ , setter ] = result.current

    act(() => setter('test'))

    const [ , , undoRest ] = result.current
    expect(undoRest.canUndo).toBe(false)

    await act(async () => await waitForNextUpdate())

    const [ , , newUndoRest ] = result.current
    expect(newUndoRest.canUndo).toBe(true)
})

As we can see, we can use async/await in combination with the waitForNextUpdate function to easily handle testing asynchronous operations in our Hooks.