Api mocking

API Mocking (pointer)

MSW handler API, browser/node setup, install commands and request-interception internals are canonical at vendor docs and drift on each release:

• MSW — mswjs.io (recommended default; browser worker + Node server, single handler set)
• nock — github.com/nock/nock (Node-only HTTP interception)
• json-server — github.com/typicode/json-server (full REST fake server from a JSON file)
• Mirage JS — miragejs.com (browser-only, models + factories)

Tool selection (version-agnostic)

Need to mock HTTP for tests/dev?
├── Browser AND Node from one handler set → MSW
├── Node-only unit tests, low ceremony → nock
├── Need a stateful REST fake server (CRUD) → json-server
└── Browser-only with relational fixtures (factories) → Mirage JS

Foundation discipline (keep across releases)

• Type-safe mocks: derive mock payloads from the same TypeScript types as the real API (e.g. shared types/api.ts). A mock that compiles against stale types is the #1 source of mock-prod divergence.
• Reset between tests: always resetHandlers() (MSW) or cleanAll() (nock) in afterEach — leaked state across tests is a debugging tax.
• Realistic failure modes: simulate 5xx/timeouts/auth errors, not just happy paths. The mock is only useful if it exercises the same edge cases as prod.
• Unhandled = error: in tests use onUnhandledRequest: 'error' (MSW) so a typo'd URL fails loudly instead of silently passing.

See also

• dev-tdd — mocks live in test setup; this skill activates from TDD work
• qa-e2e — Playwright/Cypress tests often layer MSW for deterministic stubs
• dev-tdd (test-infrastructure setup) — wires the global setupServer into vitest/jest config