Mocking Requests

Because Vitest runs in Node, mocking network requests is tricky; web APIs are not available, so we need something that will mimic network behavior for us. We recommend Mock Service Worker to accomplish this. It allows you to mock http, WebSocket and GraphQL network requests, and is framework agnostic.

Mock Service Worker (MSW) works by intercepting the requests your tests make, allowing you to use it without changing any of your application code. In-browser, this uses the Service Worker API. In Node.js, and for Vitest, it uses the @mswjs/interceptors library. To learn more about MSW, read their introduction

Configuration

You can use it like below in your setup file

HTTP Setup

import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { http, HttpResponse } from 'msw'

const posts = [
  {
    userId: 1,
    id: 1,
    title: 'first post title',
    body: 'first post body',
  },
  // ...
]

export const restHandlers = [
  http.get('https://rest-endpoint.example/path/to/posts', () => {
    return HttpResponse.json(posts)
  }),
]

const server = setupServer(...restHandlers)

// Start server before all tests
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))

// Close server after all tests
afterAll(() => server.close())

// Reset handlers after each test for test isolation
afterEach(() => server.resetHandlers())

GraphQL Setup

import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { graphql, HttpResponse } from 'msw'

const posts = [
  {
    userId: 1,
    id: 1,
    title: 'first post title',
    body: 'first post body',
  },
  // ...
]

const graphqlHandlers = [
  graphql.query('ListPosts', () => {
    return HttpResponse.json({
      data: { posts },
    })
  }),
]

const server = setupServer(...graphqlHandlers)

// Start server before all tests
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))

// Close server after all tests
afterAll(() => server.close())

// Reset handlers after each test for test isolation
afterEach(() => server.resetHandlers())

WebSocket Setup

import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { ws } from 'msw'

const chat = ws.link('wss://chat.example.com')

const wsHandlers = [
  chat.addEventListener('connection', ({ client }) => {
    client.addEventListener('message', (event) => {
      console.log('Received message from client:', event.data)
      // Echo the received message back to the client
      client.send(`Server received: ${event.data}`)
    })
  }),
]

const server = setupServer(...wsHandlers)

// Start server before all tests
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))

// Close server after all tests
afterAll(() => server.close())

// Reset handlers after each test for test isolation
afterEach(() => server.resetHandlers())

Configuring the server with onUnhandledRequest: 'error' ensures that an error is thrown whenever there is a request that does not have a corresponding request handler.

More

There is much more to MSW. You can access cookies and query parameters, define mock error responses, and much more! To see all you can do with MSW, read their documentation.