Conquer useSuspenseQuery Testing: A Practical Guide

Struggling to test useSuspenseQuery in React Query v5? This guide breaks down the common pitfalls and provides actionable solutions to write robust unit tests, covering loading, success, and error states. Let's boost your test coverage!

Understanding the Challenges

useSuspenseQuery throws a promise when data is loading or an error occurs. This behavior differs from useQuery, requiring a shift in your testing approach. Directly accessing result.current during the initial render (while suspending) will likely give you null because the component hasn't resolved yet.

Scenario Setup: Mocking Made Easy

We use MSW (Mock Service Worker) to simulate API responses. The key is to create separate mock configurations for success, error, and delayed success scenarios:

// Example MSW setup (server.js)
import { rest } from 'msw'

const handlers = [
  rest.get('/api/data', (req, res, ctx) => {
    // Success case
    if (req.url.searchParams.get('delay')) {
      // Delayed success
      return res(ctx.delay(Number(req.url.searchParams.get('delay'))), ctx.json([{ id: 1, name: 'Delayed Data' }]))
    }
    return res(ctx.json([{ id: 1, name: 'Success Data' }]))
  }),
  rest.get('/api/error', (req, res, ctx) => {
    // Error case
    return res(ctx.status(500), ctx.json({ message: 'Internal Server Error' }))
  }),
]

export const server = setupServer(...handlers)

The QueryWrapperWithSuspense: Your Testing Ally

This component handles the QueryClient and the crucial Suspense boundary. The fallback prop is essential for testing loading states:

// mockWrappers.tsx
import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
import { Suspense, PropsWithChildren } from 'react'

export const QueryWrapperWithSuspense = ({ children }: PropsWithChildren) => {
  const queryClient = new QueryClient()
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <QueryClientProvider client={queryClient}>
        {children}
      </QueryClientProvider>
    </Suspense>
  )
}

Testing Success: The Happy Path

Here's how to confidently test that data is correctly fetched:

it('Should have data on success', async () => {
  server.use(
    rest.get('/api/data', (req, res, ctx) => {
      return res(ctx.json([{ id: 1, name: 'Success Data' }]))
    })
  )
  const { result } = renderHook(() => useGetData(), { wrapper: QueryWrapperWithSuspense })

  // Wait for the Suspense boundary to resolve
  await waitFor(() => expect(result.current).toBeDefined(), { timeout: 3000 });// Adjust timeout as needed.

  expect(result.current.data).toHaveLength(1);
  expect(result.current.isSuccess).toBe(true);
});

Key Points:

• server.use(): Overrides the global MSW setup for this specific test. This is great when your tests use different API responses.
• waitFor & .toBeDefined(): Wait for the result.current to resolve within the suspense boundary, meaning that the Promise thrown by useSuspenseQuery has completed.
• Timeout: Adjust the timeout in waitFor appropriately if your API responses are slow.

Testing Loading States with useSuspenseQuery

The magic is in the fallback of Suspense. Check that the fallback is rendered while the hook suspends.

import { renderHook, waitFor } from '@testing-library/react'
import { QueryWrapperWithSuspense } from '../../../../internal/test/mocks/mockWrappers'
import { useGetData } from '../hooks/useGetData'
import { server } from '../../../../internal/test/server/server'
import { screen } from '@testing-library/dom'
import { rest } from 'msw'

it('Should show loading state', async () => {
    server.use(
        rest.get('/api/data', (req, res, ctx) => {
            return res(ctx.delay(200), ctx.json([{id: 1, name: 'Delayed Data'}]))
        })
    )

    const { result } = renderHook(() => useGetData(), { wrapper: QueryWrapperWithSuspense })

    //check the loading state, we need to use screen
    expect(screen.getByText('Loading...')).toBeInTheDocument()

    await waitFor(() => expect(result.current).toBeDefined(), { timeout: 3000 })

    expect(screen.queryByText('Loading...')).not.toBeInTheDocument()
    expect(result.current.isSuccess).toBe(true)
})

Explanation:

1. screen.getByText: Checks if "Loading..." text (set in Suspense's fallback) is present in the DOM before the useSuspenseQuery hook resolves.
2. server.use and ctx.delay(): Configure MSW to return the mocked data after a delay, simulating a loading state.
3. waitFor: Waits for the component to resolve and result.current to be defined.
4. screen.queryByText: After the data is loaded, we assert that the "Loading..." text is no longer in the DOM, meaning the Suspense boundary has resolved and the data is displayed.

Testing Errors: Catching the Rejections

When useSuspenseQuery encounters an error, it throws a promise that rejects. Here's the correct way to test error handling:

import { renderHook, waitFor } from '@testing-library/react'
import { QueryWrapperWithSuspense } from '../../../../internal/test/mocks/mockWrappers'
import { useGetData } from '../hooks/useGetData'
import { server } from '../../../../internal/test/server/server'
import { rest } from 'msw'

it('Should handle error state', async () => {
    server.use(
        rest.get('/api/data', (req, res, ctx) => {
            return res(ctx.status(500), ctx.json({ message: 'Request failed' }))
        })
    )

    const { result } = renderHook(() => useGetData(), { wrapper: QueryWrapperWithSuspense })

    // Wait for the hook to settle in error state via Suspense
    await waitFor(() => {
        expect(result.error).toBeInstanceOf(Error);
    }, { timeout: 3000 }); // Adjust timeout as needed

});

Important Points:

• await assertThrowsAsync: Because useSuspenseQuery throws the rejection, we need to await a function that calls the renderHook statement. React Testing Library's built-in result.error reflects the state of the error with the useSuspenseQuery hook.
• result.error Assertion Verify if the result.error (or the error provided by your API) is what's expected. A good implementation can narrow down the type of error and display customized error messages.

Key Takeaways for useSuspenseQuery

• Embrace Suspense: Testing useSuspenseQuery fundamentally relies on understanding and utilizing the Suspense component.
• waitFor is your friend: Use waitFor to ensure your assertions run after the promise resolves (or rejects).
• MSW for Control: MSW gives you precise control over your mock API responses, allowing you to simulate various scenarios.
• Check Loading states with screen: Use screen.getByText and screen.queryByText from @testing-library/dom to make assertions on the Loading states when using Suspense.

By following these guidelines, you will develop more reliable and effective tests for components using useSuspenseQuery, leading to more robust React applications and better user experiences.