Unnamed Skill

---

name: testing-msw description: Apply when mocking API calls in tests or development: intercepting requests, simulating error states, and testing loading states. version: 1.0.0 tokens: ~600 confidence: high sources:

• https://mswjs.io/docs/getting-started
• https://mswjs.io/docs/best-practices/typescript last_validated: 2025-01-10 next_review: 2025-01-24 tags: [testing, msw, mocking, api]

---

When to Use

Apply when mocking API calls in tests or development: intercepting requests, simulating error states, and testing loading states.

Patterns

Pattern 1: Setup Handlers

// Source: https://mswjs.io/docs/getting-started
// src/mocks/handlers.ts
import { http, HttpResponse } from 'msw';

export const handlers = [
  http.get('/api/users', () => {
    return HttpResponse.json([
      { id: '1', name: 'John' },
      { id: '2', name: 'Jane' },
    ]);
  }),

  http.post('/api/users', async ({ request }) => {
    const body = await request.json();
    return HttpResponse.json({ id: '3', ...body }, { status: 201 });
  }),
];

Pattern 2: Test Setup

// Source: https://mswjs.io/docs/getting-started
// src/mocks/server.ts
import { setupServer } from 'msw/node';
import { handlers } from './handlers';

export const server = setupServer(...handlers);

// jest.setup.ts or vitest.setup.ts
beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Pattern 3: Test-Specific Overrides

// Source: https://mswjs.io/docs/best-practices/typescript
import { http, HttpResponse } from 'msw';
import { server } from './mocks/server';

test('handles server error', async () => {
  // Override for this test only
  server.use(
    http.get('/api/users', () => {
      return HttpResponse.json(
        { error: 'Server error' },
        { status: 500 }
      );
    })
  );

  render(<UserList />);
  expect(await screen.findByText(/error/i)).toBeInTheDocument();
});

test('handles empty list', async () => {
  server.use(
    http.get('/api/users', () => {
      return HttpResponse.json([]);
    })
  );

  render(<UserList />);
  expect(await screen.findByText(/no users/i)).toBeInTheDocument();
});

Pattern 4: Request Assertions

// Source: https://mswjs.io/docs/best-practices/typescript
test('sends correct data', async () => {
  let capturedBody: unknown;

  server.use(
    http.post('/api/users', async ({ request }) => {
      capturedBody = await request.json();
      return HttpResponse.json({ id: '1' }, { status: 201 });
    })
  );

  render(<CreateUserForm />);
  await userEvent.type(screen.getByLabelText(/name/i), 'John');
  await userEvent.click(screen.getByRole('button', { name: /submit/i }));

  await waitFor(() => {
    expect(capturedBody).toEqual({ name: 'John' });
  });
});

Pattern 5: Delayed Responses (Loading States)

// Source: https://mswjs.io/docs/api/delay
import { http, HttpResponse, delay } from 'msw';

server.use(
  http.get('/api/users', async () => {
    await delay(100); // Simulate network delay
    return HttpResponse.json([{ id: '1', name: 'John' }]);
  })
);

test('shows loading state', async () => {
  render(<UserList />);
  expect(screen.getByText(/loading/i)).toBeInTheDocument();
  expect(await screen.findByText('John')).toBeInTheDocument();
});

Pattern 6: Browser Setup (Development)

// Source: https://mswjs.io/docs/getting-started
// src/mocks/browser.ts
import { setupWorker } from 'msw/browser';
import { handlers } from './handlers';

export const worker = setupWorker(...handlers);

// main.tsx (development only)
if (process.env.NODE_ENV === 'development') {
  const { worker } = await import('./mocks/browser');
  await worker.start();
}

Anti-Patterns

• Not resetting handlers - Always resetHandlers in afterEach
• Global mocks in tests - Use server.use() for test-specific
• No error scenarios - Test 4xx and 5xx responses
• Mocking too much - Integration tests should hit real APIs

Verification Checklist

• Server setup in test config (beforeAll/afterAll)
• Handlers reset after each test
• Error states tested with overrides
• Loading states tested with delay()
• Request body assertions where needed