Industry standard API mocking for JavaScript.
"I found MSW and was thrilled that not only could I still see the mocked responses in my DevTools, but that the mocks didn't have to be written in a Service Worker and could instead live alongside the rest of my app. This made it silly easy to adopt. The fact that I can use it for testing as well makes MSW a huge productivity booster."
This README will give you a brief overview of the library, but there's no better place to start with Mock Service Worker than its official documentation.
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In-browser usage is what sets Mock Service Worker apart from other tools. Utilizing the Service Worker API, which can intercept requests for the purpose of caching, Mock Service Worker responds to intercepted requests with your mock definition on the network level. This way your application knows nothing about the mocking.
Take a look at this quick presentation on how Mock Service Worker functions in a browser:
fetch
, axios
, react-query
, you-name-it;// 1. Import the library.
import { http, HttpResponse } from 'msw'
import { setupWorker } from 'msw/browser'
// 2. Describe network behavior with request handlers.
const worker = setupWorker(
http.get('https://github.com/octocat', ({ request, params, cookies }) => {
return HttpResponse.json(
{
message: 'Mocked response',
},
{
status: 202,
statusText: 'Mocked status',
},
)
}),
)
// 3. Start mocking by starting the Service Worker.
await worker.start()
Performing a GET https://github.com/octocat
request in your application will result into a mocked response that you can inspect in your browser's "Network" tab:
Tip: Did you know that although Service Worker runs in a separate thread, your request handlers execute entirely on the client? This way you can use the same languages, like TypeScript, third-party libraries, and internal logic to create the mocks you need.
There's no such thing as Service Workers in Node.js. Instead, MSW implements a low-level interception algorithm that can utilize the very same request handlers you have for the browser. This blends the boundary between environments, allowing you to focus on your network behaviors.
fetch
, axios
, etc. As a result, your tests know nothing about mocking;Here's an example of using Mock Service Worker while developing your Express server:
import express from 'express'
import { http, HttpResponse } from 'msw'
import { setupServer } from 'msw/node'
const app = express()
const server = setupServer()
app.get(
'/checkout/session',
server.boundary((req, res) => {
// Describe the network for this Express route.
server.use(
http.get(
'https://api.stripe.com/v1/checkout/sessions/:id',
({ params }) => {
return HttpResponse.json({
id: params.id,
mode: 'payment',
status: 'open',
})
},
),
)
// Continue with processing the checkout session.
handleSession(req, res)
}),
)
This example showcases
server.boundary()
to scope request interception to a particular closure, which is extremely handy!
Mock Service Worker is trusted by hundreds of thousands of engineers around the globe. It's used by companies like Google, Microsoft, Spotify, Amazon, Netflix, and countless others. Despite that, it remains a hobby project maintained in a spare time and has no opportunity to financially support even a single full-time contributor.
You can change that! Consider sponsoring the effort behind one of the most innovative approaches around API mocking. Raise a topic of open source sponsorships with your boss and colleagues. Let's build sustainable open source together!
Become our golden sponsor and get featured right here, enjoying other perks like issue prioritization and a personal consulting session with us.
Learn more on our GitHub Sponsors profile.
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We've been extremely humbled to receive awards and mentions from the community for all the innovation and reach Mock Service Worker brings to the JavaScript ecosystem.
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Solution Worth PursuingTechnology Radar (2020–2021) |
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The Most Exciting Use of TechnologyOpen Source Awards (2020) |