Atlas Whoff

Posted on Apr 7 Edited on Apr 9

Mock Service Worker: Test Your UI Without Mocking fetch

#javascript #react #testing #tutorial

The Problem With Mocking fetch

// Traditional approach — fragile
jest.spyOn(global, 'fetch').mockResolvedValue({
  ok: true,
  json: async () => ({ id: '1', name: 'Alice' }),
} as Response);

This breaks when you change the API response shape. It doesn't test your actual request headers, body, or URL patterns. It ties your tests to implementation details.

MSW (Mock Service Worker) intercepts requests at the network level—your code calls fetch normally, MSW intercepts it.

Setup

npm install msw --save-dev
npx msw init public/ --save

Define Handlers

// src/mocks/handlers.ts
import { http, HttpResponse } from 'msw';

export const handlers = [
  // GET /api/users
  http.get('/api/users', () => {
    return HttpResponse.json([
      { id: '1', name: 'Alice', email: 'alice@example.com' },
      { id: '2', name: 'Bob', email: 'bob@example.com' },
    ]);
  }),

  // GET /api/users/:id
  http.get('/api/users/:id', ({ params }) => {
    const { id } = params;
    if (id === 'not-found') {
      return new HttpResponse(null, { status: 404 });
    }
    return HttpResponse.json({ id, name: 'Alice', email: 'alice@example.com' });
  }),

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

Browser Setup (for Storybook or dev)

// src/mocks/browser.ts
import { setupWorker } from 'msw/browser';
import { handlers } from './handlers';

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

// src/main.tsx
async function enableMocking() {
  if (process.env.NODE_ENV !== 'development') return;
  const { worker } = await import('./mocks/browser');
  return worker.start({ onUnhandledRequest: 'bypass' });
}

enableMocking().then(() => {
  ReactDOM.createRoot(document.getElementById('root')!).render(<App />);
});

Node.js Setup (for Jest/Vitest)

// src/mocks/server.ts
import { setupServer } from 'msw/node';
import { handlers } from './handlers';

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

// src/setupTests.ts
import { server } from './mocks/server';

beforeAll(() => server.listen({ onUnhandledRequest: 'error' }));
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Writing Tests

import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { server } from '../mocks/server';
import { http, HttpResponse } from 'msw';
import { UserList } from './UserList';

test('displays list of users', async () => {
  render(<UserList />);

  // Waits for real async fetch to complete
  await waitFor(() => {
    expect(screen.getByText('Alice')).toBeInTheDocument();
    expect(screen.getByText('Bob')).toBeInTheDocument();
  });
});

test('shows error state when API fails', async () => {
  // Override handler for this test only
  server.use(
    http.get('/api/users', () => {
      return new HttpResponse(null, { status: 500 });
    })
  );

  render(<UserList />);

  await waitFor(() => {
    expect(screen.getByText('Failed to load users')).toBeInTheDocument();
  });
});

test('creates a user on form submit', async () => {
  const user = userEvent.setup();

  render(<CreateUserForm />);

  await user.type(screen.getByLabelText('Name'), 'Charlie');
  await user.type(screen.getByLabelText('Email'), 'charlie@example.com');
  await user.click(screen.getByRole('button', { name: 'Create' }));

  await waitFor(() => {
    expect(screen.getByText('User created successfully')).toBeInTheDocument();
  });
});

Request Assertions

test('sends correct payload', async () => {
  let capturedBody: unknown;

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

  // ... interact with form

  expect(capturedBody).toEqual({
    name: 'Charlie',
    email: 'charlie@example.com',
  });
});

Why MSW Beats the Alternatives

Approach	Tests real URL?	Tests headers?	Refactor-safe?
Mock fetch	No	No	No
Mock module	No	No	No
MSW	Yes	Yes	Yes
Real server	Yes	Yes	Yes (but slow)

MSW gives you most of the confidence of a real server with the speed of unit tests.

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