Taming the Testing Beast: Resolving RTK Query Errors with MSW and Vitest
The Problem: You're building a React application with RTK Query for data fetching, and you want to write robust tests using MSW (Mock Service Worker) and Vitest. But your tests are failing, throwing cryptic errors, and you're left wondering why your mocked API responses aren't reaching your RTK Query setup.
Reframing the Problem: Imagine you're trying to test your car's engine. You've built a mock engine for testing, but the real car isn't receiving the signals from your mock engine. You need to ensure the communication channel between your real car (your application) and your mock engine (MSW) is working correctly.
The Scenario:
Let's say you have a simple API call to fetch a list of users:
// usersSlice.js
import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query/react';
export const usersApi = createApi({
reducerPath: 'usersApi',
baseQuery: fetchBaseQuery({ baseUrl: 'https://api.example.com' }),
endpoints: (builder) => ({
getUsers: builder.query({
query: () => '/users',
}),
}),
});
export const { useGetUsersQuery } = usersApi;
And you're trying to mock this API call with MSW:
// tests/users.test.ts
import { setupServer } from 'msw/node';
import { rest } from 'msw';
import { vi, expect, describe, it } from 'vitest';
const server = setupServer(
rest.get('https://api.example.com/users', (req, res, ctx) => {
return res(ctx.status(200), ctx.json({ data: [{ id: 1, name: 'John' }] }));
})
);
describe('Users API', () => {
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());
it('should fetch users successfully', async () => {
const { result, error } = await useGetUsersQuery();
expect(result.data).toEqual({ data: [{ id: 1, name: 'John' }] });
expect(error).toBeUndefined();
});
});
The Insight: The issue lies in the way MSW interacts with RTK Query within the context of your testing environment (Vitest). When you use RTK Query, it internally sets up a fetch implementation that might not be compatible with MSW's default mocking behavior.
The Solution:
To solve this, you need to manually tell Vitest to use MSW's mock environment:
// tests/users.test.ts
import { setupServer } from 'msw/node';
import { rest } from 'msw';
import { vi, expect, describe, it } from 'vitest';
import { usersApi } from '../src/features/users/usersSlice'; // Import your API
// Start the MSW server outside the test suite
const server = setupServer(
rest.get('https://api.example.com/users', (req, res, ctx) => {
return res(ctx.status(200), ctx.json({ data: [{ id: 1, name: 'John' }] }));
})
);
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());
describe('Users API', () => {
it('should fetch users successfully', async () => {
// Create a mock environment for Vitest
const environment = await usersApi.injectEndpoints({
// Provide the MSW's mock environment
fetchFn: server.fetch.bind(server)
});
// Run your test as usual
const { result, error } = await environment.endpoints.getUsers.query();
expect(result.data).toEqual({ data: [{ id: 1, name: 'John' }] });
expect(error).toBeUndefined();
});
});
Explanation:
- MSW Setup: The MSW server is set up outside the test suite to ensure it's active throughout your tests.
- Vitest Environment: The
injectEndpointsmethod of RTK Query is used to manually create a testing environment. This environment is then provided with MSW'sfetchmethod, allowing RTK Query to use the mocked responses. - Testing Logic: The test code then runs within the configured environment, allowing you to assert against the mocked responses.
Additional Value:
- Debugging Tips: If you're still encountering errors, check the console logs for any network requests or warnings related to your API calls. Verify that the mocked responses are being correctly set up and accessed by your RTK Query endpoints.
- Customization: The provided example showcases a simple scenario. Adapt the mock responses and test logic to fit your specific use cases and API endpoints.
References and Resources:
- MSW Documentation: https://mswjs.io/docs
- RTK Query Documentation: https://redux-toolkit.js.org/rtk-query
- Vitest Documentation: https://vitest.dev/
By following these steps and understanding the underlying principles, you can confidently test your RTK Query interactions using MSW and Vitest, ensuring the reliability of your applications.
