API Mock Builder

API mocking means simulating API endpoints so your frontend or tests can work without a real backend. Use it when: the backend isn't built yet (frontend-first development), you want tests to run without network calls, you need to test error states (401, 429, 500) that are hard to trigger in development, or you want to prototype UI before the API design is finalized.

MSW (Mock Service Worker) intercepts real network requests at the service worker level in browsers, and at the Node.js level in tests. Unlike simple fetch mocks, MSW intercepts requests transparently — no changes to application code. The handlers this tool generates are compatible with MSW v2. Install with: npm install msw --save-dev, then run npx msw init public/.

Use the MSW handlers from this tool with setupServer() from msw/node: import { setupServer } from 'msw/node'; import { handlers } from './handlers'; const server = setupServer(...handlers); beforeAll(() => server.listen()); afterEach(() => server.resetHandlers()); afterAll(() => server.close());. This gives you real network-level mocking with no axios/fetch monkey-patching.

Test the happy path (200/201) and all error cases: 400 Bad Request (validation errors), 401 Unauthorized (expired token), 403 Forbidden (wrong role), 404 Not Found (deleted resource), 409 Conflict (duplicate), 422 Unprocessable Entity (semantic validation), 429 Too Many Requests (rate limiting), and 500 Internal Server Error (unexpected failures). Good test coverage mocks each of these.

Yes — switch to the 'cURL' export tab for a curl command for the selected endpoint. You can paste it directly into your terminal to test the response format, or share it with teammates to document your API expectations. Combine with tools like jq for pretty-printing: curl -s ... | jq .

API mocking replaces the real API with a fake that returns predefined responses — used during development and unit/integration testing. API testing sends real requests to a running API and validates the actual responses — used for end-to-end and contract testing. Both are complementary: mock for speed and isolation during dev, test against real endpoints in CI/CD.