API Mocking with MSW

Quick Guide: Handlers with variant switching (default, empty, error). Shared between browser (dev) and Node (tests). Separate mock data from handlers for reusability. Type-safe using your API's generated types. Use setupWorker (browser) and setupServer (Node) -- never swap them.

Detailed Resources:

• examples/core.md - Mock data, variant handlers, server worker, per-test overrides, runtime switching, network simulation
• examples/browser.md - Browser worker setup, SPA/SSR integration
• reference.md - Decision frameworks, red flags, anti-patterns

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<critical_requirements>

CRITICAL: Before Using This Skill

(You MUST separate mock data from handlers - handlers in handlers/, data in mocks/)

(You MUST use setupWorker for browser/development and setupServer for Node/tests - NEVER swap them)

(You MUST reset handlers after each test with server.resetHandlers() in afterEach)

(You MUST use named constants for HTTP status codes and delays - NO magic numbers)

</critical_requirements>

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Auto-detection: MSW, msw, mock handlers, mock data, API mocking, setupWorker, setupServer, http.get, HttpResponse

When to use:

• Mocking API responses during development before backend is ready
• Testing different API scenarios (success, empty, error states)
• Sharing the same mock definitions between browser dev and Node test environments
• Simulating network conditions (latency, timeouts)
• Per-test handler overrides for isolated test scenarios

When NOT to use:

• Integration tests needing real backend validation (use a test database)
• Production builds (MSW should never ship to production)
• Pure function unit tests with no network calls
• Testing actual network failure modes (use test containers)

Key patterns covered:

• Handler/data separation for reusability and type safety
• Variant-based handlers (default, empty, error scenarios)
• Browser worker for development, server worker for tests
• Per-test handler overrides with server.use()
• Runtime variant switching for UI development

---

Philosophy

MSW intercepts network requests at the service worker (browser) or class extension (Node) level, providing realistic API mocking without changing application code. Keep mock data separate from handlers for reusability, type handlers against your generated API types, and organize handlers by domain/feature.

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Core Patterns

Pattern 1: Separate Mock Data from Handlers

Define mock data as typed constants separate from MSW handlers. This enables type safety from your generated API types and reusability across handlers.

// mocks/features.ts
import type { GetFeaturesResponse } from "./api-types";

export const defaultFeatures: GetFeaturesResponse = {
  features: [
    { id: "1", name: "Dark mode", status: "done" },
    { id: "2", name: "Auth", status: "in progress" },
  ],
};

export const emptyFeatures: GetFeaturesResponse = { features: [] };

For full variant handler examples, see examples/core.md.

When not to use: When mock data is truly one-off and specific to a single test case (use inline data in the test instead).

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Pattern 2: Handlers with Variant Switching

Create handlers that support multiple response scenarios (default, empty, error) with runtime switching for development and explicit overrides for testing.

import { http, HttpResponse } from "msw";

const API_ENDPOINT = "api/v1/features";
const HTTP_STATUS_OK = 200;
const HTTP_STATUS_INTERNAL_SERVER_ERROR = 500;

export const getFeaturesHandlers = {
  defaultHandler: () =>
    http.get(API_ENDPOINT, () =>
      HttpResponse.json(defaultFeatures, { status: HTTP_STATUS_OK }),
    ),
  emptyHandler: () =>
    http.get(API_ENDPOINT, () =>
      HttpResponse.json(emptyFeatures, { status: HTTP_STATUS_OK }),
    ),
  errorHandler: () =>
    http.get(
      API_ENDPOINT,
      () =>
        new HttpResponse("Server error", {
          status: HTTP_STATUS_INTERNAL_SERVER_ERROR,
        }),
    ),
};

For full implementation with runtime switching, see examples/core.md.

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Pattern 3: Browser Worker (Development) vs Server Worker (Tests)

• Use setupWorker from msw/browser for browser/development
• Use setupServer from msw/node for Node/tests
• Never swap them -- setupWorker needs service worker APIs, setupServer needs Node APIs

// browser-worker.ts
import { setupWorker } from "msw/browser";
export const browserWorker = setupWorker(...handlers);

// server-worker.ts
import { setupServer } from "msw/node";
export const server = setupServer(...handlers);

For browser app integration (SPA and SSR), see examples/browser.md.

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Pattern 4: Test Lifecycle

Always follow this lifecycle to prevent test pollution:

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Use server.use() for per-test overrides -- they are automatically cleaned up by resetHandlers().

For per-test override examples, see examples/core.md.

---

<red_flags>

RED FLAGS

• ❌ Using setupWorker in Node tests or setupServer in browser -- wrong API for environment causes cryptic failures
• ❌ Not resetting handlers between tests (afterEach(() => server.resetHandlers())) -- causes test pollution
• ❌ Mixing handlers and mock data in same file -- reduces reusability and type safety
• ❌ Missing await when starting browser worker before render -- race conditions cause intermittent failures
• ⚠️ Only testing happy path (no empty/error variants) -- incomplete coverage
• ⚠️ No onUnhandledRequest configuration -- unclear which requests are mocked vs real

Gotchas & Edge Cases:

• delay() with no arguments is automatically negated in Node.js -- use explicit duration if you need delay in tests
• Handler overrides via server.use() persist until resetHandlers() -- they do NOT auto-reset between tests
• http.all() matches any HTTP method on a path -- convenient but can mask bugs if overused
• Dynamic imports are required for browser worker in SSR frameworks to avoid server bundling issues

See reference.md for detailed anti-pattern examples.

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

(You MUST separate mock data from handlers - handlers in handlers/, data in mocks/)

(You MUST use setupWorker for browser/development and setupServer for Node/tests - NEVER swap them)

(You MUST reset handlers after each test with server.resetHandlers() in afterEach)

(You MUST use named constants for HTTP status codes and delays - NO magic numbers)

Failure to follow these rules will cause test pollution, environment-specific failures, and hard-to-debug race conditions.

</critical_reminders>