Mock Service Worker usage example with Jest and JSDOM.
Using MSW with JSDOM requires additional configuration. Unfortunately, JSDOM (jest-environment-jsdom, to be precise) has a number of issues that prevent you from running valid JavaScript code in tests.
- JSDOM uses browser module resolution while running in Node.js. This doesn't work with the packages that provide conditional exports, like MSW does.
- JSDOM strips off all Node.js globals, like
fetch. This means you cannot use APIs that have been available in the browser and Node.js for years until they've been explicitly added to JSDOM.
Please see the setup steps below to properly configure Jest when using in combination with JSDOM.
Opt-out from the browser module resolution in JSDOM by setting the testEnvironmentOptions.customExportConditions option in jest.config.ts. This will force JSDOM to use Node.js module resolution, correctly resolving export conditions of third-party packages.
Despite JSDOM predenting to be a browser environment, your code still runs in Node.js. Using the browser module resolution can cause all sorts of import issues with third-party packages that depend on the standard Node.js API.
Create a jest.polyfills.ts file in your project (you can copy it) and include it in the setupFiles option in jest.config.ts. This will re-add some of the Node.js globals (and browser) missing in JSDOM, like fetch, Request, Response, etc.