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Document limitations of jsdom

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Winnie Hellmann 2019-06-26 11:39:30 +00:00 committed by Marcia Ramos
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doc/development/testing_guide/frontend_testing.md
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@ -27,7 +27,7 @@ we need to solve before being able to use Jest for all our needs.
### Differences to Karma
- Jest runs in a Node.js environment, not in a browser. Support for running Jest tests in a browser [is planned](https://gitlab.com/gitlab-org/gitlab-ce/issues/58205).
- Because Jest runs in a Node.js environment, it uses [jsdom](https://github.com/jsdom/jsdom) by default.
- Because Jest runs in a Node.js environment, it uses [jsdom](https://github.com/jsdom/jsdom) by default. See also its [limitations](#limitations-of-jsdom) below.
- Jest does not have access to Webpack loaders or aliases.
The aliases used by Jest are defined in its [own config](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/jest.config.js).
- All calls to `setTimeout` and `setInterval` are mocked away. See also [Jest Timer Mocks](https://jestjs.io/docs/en/timer-mocks).
@ -40,6 +40,17 @@ we need to solve before being able to use Jest for all our needs.
- Unhandled Promise rejections.
- Calls to `console.warn`, including warnings from libraries like Vue.
### Limitations of jsdom
As mentioned [above](#differences-to-karma), Jest uses jsdom instead of a browser for running tests.
This comes with a number of limitations, namely:
- [No scrolling support](https://github.com/jsdom/jsdom/blob/15.1.1/lib/jsdom/browser/Window.js#L623-L625)
- [No element sizes or positions](https://github.com/jsdom/jsdom/blob/15.1.1/lib/jsdom/living/nodes/Element-impl.js#L334-L371)
- [No layout engine](https://github.com/jsdom/jsdom/issues/1322) in general
See also the issue for [support running Jest tests in browsers](https://gitlab.com/gitlab-org/gitlab-ce/issues/58205).
### Debugging Jest tests
Running `yarn jest-debug` will run Jest in debug mode, allowing you to debug/inspect as described in the [Jest docs](https://jestjs.io/docs/en/troubleshooting#tests-are-failing-and-you-don-t-know-why).