For tests that should exercise your application’s normal request code, use Mock Service Worker (MSW) to intercept requests where your test runtime supports it. For a small unit test that only needs to check how a function calls or responds to fetch, a direct mock is also a reasonable choice. The key is to test the failure you mean: an HTTP 404 response is not the same as a rejected network request.
Choose between MSW and a direct fetch mock
| Approach | What it replaces | Best fit | Trade-off |
|---|---|---|---|
| MSW request interception | Intercepts the outgoing request while leaving the application’s ordinary request path in place. | Tests that should model realistic request and response behavior, including headers, status codes, and network failures. | Requires reusable request handlers and test-runner lifecycle setup. |
| Direct function mock | Replaces the fetch function with a mock that resolves or rejects as specified. | A narrowly isolated unit test checking a function’s fetch-call contract or its handling of a returned value. | You must supply the response methods your code calls; the test models the function boundary rather than intercepting a request. |
Vitest’s request-mocking guide recommends MSW for network request mocking and describes Node interception through @mswjs/interceptors. MSW’s Node.js integration guide names both Jest and Vitest. Choose based on what the test needs to preserve, not on a universal rule.
Set up MSW for Vitest
Keep handlers reusable and separate from runner setup. The following TypeScript adaptation uses the documented MSW Node setup pattern and Vitest lifecycle hooks. Add its setup file to Vitest’s setupFiles configuration; check the imports and APIs against the versions installed in your project.
// test/server.ts
import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { http, HttpResponse } from 'msw'
export const server = setupServer(
http.get('https://api.example.test/items', () =>
HttpResponse.json([{ id: 'item-1' }]),
),
)
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))
afterEach(() => server.resetHandlers())
afterAll(() => server.close())Starting the server before tests, resetting per-test overrides afterward, and closing the server when the suite finishes keeps one test’s handler from leaking into another. With onUnhandledRequest: 'error', an unhandled request fails rather than silently escaping as an accidental real network call; see the Vitest lifecycle guidance. MSW’s quick start shows handlers built with http.get and HttpResponse.json, then passed to setupServer for Node.
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Test a successful response
Call the production function and assert its observable result. For example, suppose getItems fetches the endpoint and returns the parsed JSON:
export async function getItems(): Promise<{ id: string }[]> {
const response = await fetch('https://api.example.test/items')
if (!response.ok) {
throw new Error(`Request failed: ${response.status}`)
}
return response.json()
}A test can exercise that ordinary function without replacing fetch:
import { expect, it } from 'vitest'
import { getItems } from '../src/items'
it('returns items from the API', async () => {
await expect(getItems()).resolves.toEqual([{ id: 'item-1' }])
})The registered handler supplies the JSON response. This follows the MSW quick-start pattern, which demonstrates fetching a registered endpoint and asserting on the parsed response.
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Test an HTTP error status such as 404
A 404 or 500 is still an HTTP response. Fetch does not turn a non-OK status into a rejected promise by itself, so a function that should reject on such a status needs an explicit response.ok or status check. The example above throws its own error for that reason.
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import { expect, it } from 'vitest'
import { http, HttpResponse } from 'msw'
import { server } from '../test/server'
import { getItems } from '../src/items'
it('throws when the API returns 404', async () => {
server.use(
http.get('https://api.example.test/items', () =>
new HttpResponse(null, { status: 404 }),
),
)
await expect(getItems()).rejects.toThrow('Request failed: 404')
})Because the setup calls server.resetHandlers() after each test, this override is temporary.
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Test a rejected request or network failure
A network failure is a different input: the request itself rejects instead of resolving to an HTTP response. MSW documents HttpResponse.error() for simulating this case. The client receives the generic TypeError: Failed to fetch; the API does not let you customize that message, so avoid asserting DNS- or timeout-specific wording. See MSW’s network errors guide.
import { expect, it } from 'vitest'
import { http, HttpResponse } from 'msw'
import { server } from '../test/server'
import { getItems } from '../src/items'
it('rejects when the request fails at the network level', async () => {
server.use(
http.get('https://api.example.test/items', () => HttpResponse.error()),
)
await expect(getItems()).rejects.toThrow()
})If you are testing an error state rather than a rejected promise directly, assert the state your application exposes—for example, that a caller receives an error result or that a UI renders its failure state. Keep that assertion about your function’s behavior, not a particular network stack’s wording.
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try {
await getItems()
} catch (error: unknown) {
const message = error instanceof Error ? error.message : 'Unexpected failure'
// Report or return message
}This is a TypeScript safety practice; it does not change whether the underlying failure was an HTTP response or a rejected request.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use a direct Jest mock when the function boundary is the point
For a focused unit test, mock the fetch dependency with a promise that resolves to the response shape the production code actually uses. If that code calls response.json(), the mock needs a json method; if it calls response.text(), it needs text. A simple mocked response might look like this:
const fetchMock = jest.fn().mockResolvedValue({
ok: true,
json: async () => [{ id: 'item-1' }],
})Wire the mock through the module or dependency boundary used by your function, then assert the result and, if relevant, the request arguments. A mock of fetch is not automatically a complete Fetch Response.
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Jest’s “Bypassing module mocks” guide documents a specific node-fetch pitfall: mocking node-fetch can also mock its Response, leaving methods such as response.text() unavailable. Its example retrieves the real response with jest.requireActual. That concern is specific to the module-mocking setup described there; do not assume a node-fetch response and a runtime’s global Fetch API are interchangeable.
Keep aborts distinct from network errors
Cancellation is another case, separate from both an HTTP error status and a generic network rejection. The node-fetch documentation describes abort rejections as AbortError and other operational errors as FetchError (node-fetch error handling). That taxonomy applies to node-fetch; it should not be generalized to browser Fetch or every runtime. If your function supports cancellation, give it a separate test using the cancellation mechanism and runtime your application actually uses.
Check the test runtime before choosing globals
Available web APIs depend on the test environment. Vitest’s request-mocking guide notes the effect of its Node environment and presents MSW as a way to mimic network behavior. Confirm which fetch and response implementation the project uses before writing direct mocks—especially when a project imports node-fetch instead of relying on a global. The examples here illustrate patterns; they are not a claim that a specific package-version combination was executed.
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