Add vitest browser test framework

[codingjoe]

First step towards a robust test suite to enable JavaScript changes with confidence.
I chose the cookie functions as PoC.

@django/django-website was this what you had regarding
"We need to test the changed javascript for regressions"?

I didn't add Edge as a test browser since Edge and Chromium are engine-wise almost identical.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
djangoproject/static/js/mod/switch-dark-mode.js	88.23%	2 Missing ⚠️
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@@           Coverage Diff           @@
##             main    #2537   +/-   ##
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  Coverage        ?   85.49%           
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  Files           ?      185           
  Lines           ?     6727           
  Branches        ?      395           
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  Hits            ?     5751           
  Misses          ?      894           
  Partials        ?       82           

Flag	Coverage Δ
javascript	52.72% <88.23%> (?)
python	85.76% <ø> (?)

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[Copilot]

Pull request overview

This PR introduces vitest as a browser test framework for JavaScript code, using the cookie functions in switch-dark-mode.js as a proof of concept. It adds vitest configuration, package.json, initial tests, and CI integration, while also refactoring switch-dark-mode.js to use const declarations, strict equality, arrow functions, and exports for testability.

Changes:

• Sets up vitest browser testing framework with WebdriverIO provider, coverage via istanbul, and CI integration in GitHub Actions.
• Adds browser tests for getCookie and setCookie functions from switch-dark-mode.js.
• Refactors switch-dark-mode.js to export functions, use const/let, strict equality, globalThis, and arrow functions.

Reviewed changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
vitest.config.js	New vitest configuration with browser mode (WebdriverIO, Chrome + Firefox) and istanbul coverage
package.json	New package metadata with devDependencies for vitest and test runner
djangoproject/static/js/mod/tests/switch-dark-mode.test.js	New browser tests for getCookie and setCookie
djangoproject/static/js/mod/switch-dark-mode.js	Refactored to use exports, const/let, globalThis, strict equality, arrow functions
.nvmrc	New file pinning Node.js to the LTS version
.gitignore	Added node_modules/ and coverage/ ignore rules
.github/workflows/tests.yml	Renamed tests job to python, added vitest CI job, and added Codecov reporting to both

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[codingjoe]

Ref #2325 @ulgens I added codecov here, mainly to test it and to make sure I add tags. I can drop the changes, but someone needs to remember to tag both languages.

[sarahboyce]

We recently added Playwright tests. I just opened a pr to show how we could add some tests here: #2623
@codingjoe Do you still feel this is needed?

[codingjoe]

We recently added Playwright tests. I just opened a pr to show how we could add some tests here: #2623 @codingjoe Do you still feel this is needed?

Yes, I do believe it's needed. I want to add unit tests, not integration tests. The latter is done via Playwright but comes at a performance and reliability cost. Besides, writing unit tests is a more rewarding experience in most cases.

We recently added the same framework to django-debug-toolbar too, and it serves us well.

[adamzap]

There's not much JavaScript in this project currently, and there should be less as time goes on. My preference would be to invest in Playwright tests now and finish the in-progress JavaScript refactor before evaluating the need for JavaScript unit tests.

[codingjoe]

There's not much JavaScript in this project currently, and there should be less as time goes on. My preference would be to invest in Playwright tests now and finish the in-progress JavaScript refactor before evaluating the need for JavaScript unit tests.

While I don't believe there will be less JavaScript in the future, I only added this to provide tests for the fix of the setCookie utility, link in the pr description.

If you say I don't have to, I won't. However, a refactor may benefit from unit test. I would use Playwright with the same caution, as Selenium. I love Python, but not to test JavaScript.

[marksweb]

I'm not against more things to help us test. And I'm sure there will always be some JS that could be tested.