MSW Patterns

Quick Guide: MSW intercepts requests at the network layer, so application code never learns it is mocked. One handler set serves both environments — setupWorker from msw/browser in development, setupServer from msw/node in tests — and the two are not interchangeable. Keep response bodies in their own module so each variant (default, empty, error) is reusable and typed against your API's generated types.

Detailed Resources:

• examples/core.md — mock data modules, variant handlers, server setup and test lifecycle, per-test overrides, runtime variant switching, simulated latency
• examples/browser.md — browser worker setup and app integration for client-rendered and server-rendered entry points

---

Which path applies

Handlers are shared; only the setup module and the lifecycle differ.

• Browser, during development — setupWorker from msw/browser, awaited before the app renders, with runtime variant switching to walk the UI through its states. Follow examples/browser.md.
• Node, in tests — setupServer from msw/node, listening for the suite and reset between tests, with server.use() for one-test overrides. Follow examples/core.md.

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<critical_requirements>

Before writing MSW code

Match the setup function to the environment. setupWorker needs service worker APIs and setupServer patches Node's request layer, so each fails in the other with an error that names a missing global rather than the swap.

Reset handlers in afterEach with server.resetHandlers(), so a server.use() override cannot decide the outcome of the next test.

Await worker.start() before rendering. Requests fired before the worker is ready reach the real network, which makes the first render of a suite intermittently different from the rest.

Keep response bodies in their own module, typed against your API's generated types — one fixture then serves several handlers, and a schema change fails at compile time instead of inside an assertion.

</critical_requirements>

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Auto-detection: msw, setupWorker, setupServer, msw/browser, msw/node, http.get, http.all, HttpResponse.json, server.use, resetHandlers, onUnhandledRequest, mockServiceWorker.js, delay()

Applies to:

• Mocking HTTP responses in development before the backend exists
• Exercising empty, error and slow responses without changing application code
• Sharing one handler set between a dev server and a test suite
• Overriding a single endpoint for a single test

Handled elsewhere:

• Test runner configuration and lifecycle hooks — this skill says what goes inside beforeAll and afterEach, not which runner provides them
• Rendering components and querying the result
• Integration against a real backend, where the server's own behaviour is the thing under test

---

<philosophy>

MSW mocks the network, not the code that calls it. Nothing in the application is injected, wrapped or swapped, so what runs in a test is what runs in production — and the same handlers can drive a dev server, a test suite and a demo build.

That only holds while the handler set stays a description of the API rather than of one test's needs. Data lives in fixtures, handlers pick a fixture, and anything a single test needs differently arrives through an override that is thrown away afterwards.

</philosophy>

---

<decision_framework>

Which mechanism for changing a response

You want	Use
A scenario several tests share	A named variant handler exported beside the default
One test to see something different	server.use(variant()) — discarded by resetHandlers()
To flip states by hand while developing	A variant map the default handler reads at request time
A response the code under test waits on	delay(ms) with an explicit duration

Runtime variant switching belongs to development only. In a test it is shared mutable state that survives the test that set it, where server.use() is scoped and self-cleaning.

</decision_framework>

---

<patterns>

Core patterns

Pattern 1: Response Bodies in Their Own Module

Fixtures live apart from handlers, typed against the API's generated types, so one body serves several handlers and a schema change surfaces as a type error.

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

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

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

Data genuinely specific to one test stays inline in that test.

Full code: examples/core.md

---

Pattern 2: Variant Handlers

Export the default handler and each alternative scenario from one module, so tests and the dev server pick a variant by name.

import { http, HttpResponse } from "msw";

export const getFeaturesHandlers = {
  defaultHandler: () =>
    http.get(ENDPOINT, () => HttpResponse.json(defaultFeatures)),
  emptyHandler: () =>
    http.get(ENDPOINT, () => HttpResponse.json(emptyFeatures)),
  errorHandler: () =>
    http.get(ENDPOINT, () => new HttpResponse("Server error", { status: 500 })),
};

HttpResponse.json sets the JSON content type and answers 200 unless init.status says otherwise, so only the error variant states a code. A non-JSON body — plain text, an empty error — goes through new HttpResponse(body, init).

Full code, including a default handler that reads a variant map at request time: examples/core.md

---

Pattern 3: One Handler Set, Two Setups

The handler array is shared; the module that consumes it is chosen by environment.

// 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);

Full code, with app integration: examples/browser.md

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Pattern 4: Test Lifecycle and Per-Test Overrides

Listen once, reset between tests, close at the end. server.use() prepends a handler that resetHandlers() then removes.

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

it("renders the empty state", async () => {
  server.use(getFeaturesHandlers.emptyHandler());
  renderApp();
});

Full code: examples/core.md

</patterns>

---

<red_flags>

Red flags

Breaks at runtime:

• setupServer in a browser bundle, or setupWorker in Node — the error names a missing global, so it reads as an environment problem rather than a swapped import
• worker.start() without await — the first requests race the worker and reach the real network, which shows up as an intermittent failure in whichever spec runs first
• A top-level import of the browser worker in a server-rendered entry point — service worker code lands in the server bundle and the build fails; import it dynamically behind a typeof window check
• No resetHandlers() in afterEach — an override outlives its test, so a suite passes alone and fails in a different order
• A worker.start() that is not behind an environment guard — the service worker registers in the production bundle and real users are served fixtures

Surprising behaviour:

• delay() with no argument is a random 100–400ms wait in the browser and is negated in Node, so a test that needs a pause has to pass an explicit duration
• server.use() overrides persist until resetHandlers(); they do not expire when the test that added them ends
• http.all() matches every method on a path, so a request sent with the wrong verb still gets a successful response
• once: true makes a handler match a single request and then stop, which is what sequential responses need and what makes a later request fall through unnoticed
• Without onUnhandledRequest on start/listen, a request nobody wrote a handler for passes through quietly, so a missing mock looks like a backend fault — it takes "bypass", "warn" or "error", which is the difference between silent, noisy and fatal

</red_flags>