✅ Add tests for vitest setupFiles duplication fix

[Robdel12]

Summary

• Adds regression tests validating that the Vitest plugin doesn't duplicate user-provided setupFiles
• Tests both array and string setupFiles configurations

Context

These tests validate the fix from #82. Vitest automatically merges plugin config with user config, so the plugin should only return its own setup file—not spread the user's files into its return value.

Test plan

• Merge fix: don't duplicate setupFiles #82 first
• Rebase this branch onto main
• Tests should pass

[claude]

Code Review ✅

Overall Assessment: This PR adds excellent regression tests for the setupFiles duplication fix from #82. The tests are well-written, clearly documented, and validate the correct behavior.

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✅ Strengths

1. Clear Test Intent: The test names and inline comments clearly explain why the plugin shouldn't duplicate setupFiles—because Vitest handles the merging automatically. This makes the tests self-documenting.

2. Edge Case Coverage: Testing both array and string setupFiles configurations is thorough. Users can configure setupFiles either way, and both should work correctly.

3. Focused Assertions: The tests verify:

   • Plugin only returns its own setup file (length check)
   • Plugin's setup file is present (toContain('setup.js'))
   • User's setup files are NOT included in plugin's return value

   This validates the exact contract between plugin and Vitest's config merging.

4. Consistent Style: Follows the existing test patterns in vitest-plugin.spec.js (structure, variable naming, assertion style).

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💡 Suggestions

1. Consider Testing the Merged Result

The current tests verify what the plugin returns, but don't verify what Vitest actually sees after merging. Consider adding a test that validates the final merged config to prove setupFiles aren't duplicated:

it('should result in correct merged config after vitest merging', async () => {
  const { vizzlyPlugin } = await import('../src/index.js');
  
  const userConfig = {
    test: {
      setupFiles: ['./user-setup.js'],
    },
  };
  
  const plugin = vizzlyPlugin();
  const pluginConfig = plugin.config(userConfig, { mode: 'test' });
  
  // Simulate Vitest's merging behavior
  const mergedSetupFiles = [
    ...(userConfig.test.setupFiles || []),
    ...(pluginConfig.test.setupFiles || []),
  ];
  
  // Should have exactly 2 files (user's + plugin's), not 3
  expect(mergedSetupFiles).toHaveLength(2);
  expect(mergedSetupFiles).toContain('./user-setup.js');
  expect(mergedSetupFiles.some(f => f.includes('setup.js'))).toBe(true);
});

This would make the regression test even more explicit about the bug that was fixed.

2. Minor: Variable Naming Consistency

In the first test you use userConfig and pluginConfig, but in the second test you reuse userConfig. Consider using consistent names across both tests for readability:

// First test
let userConfig = { test: { setupFiles: ['./user-setup.js'] } };
let pluginConfig = plugin.config(userConfig, { mode: 'test' });

// Second test - could also use pluginConfig for consistency
let userConfig = { test: { setupFiles: './user-setup.js' } };
let pluginConfig = plugin.config(userConfig, { mode: 'test' });

3. Documentation Enhancement

Consider adding a brief comment at the top of these tests explaining the Vitest plugin config merging behavior for future maintainers:

// Vitest's plugin system automatically merges plugin config with user config.
// When a plugin returns { test: { setupFiles: [...] } }, Vitest merges it with
// the user's test.setupFiles array. If the plugin spreads the user's files into
// its return value, those files would run twice.
// See: https://vitest.dev/guide/using-plugins.html

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🔒 Security & Performance

• ✅ No security concerns
• ✅ No performance impact (tests run in milliseconds)

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🧪 Test Coverage

• ✅ Tests validate the exact fix from fix: don't duplicate setupFiles #82
• ✅ Edge cases covered (array and string setupFiles)
• ℹ️ Could be enhanced with merged config validation (see suggestion ✨ Add npmignore file, reduce release size #1)

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📝 Final Recommendation

Approve with optional improvements. The tests are solid as-is and ready to merge. The suggestions above would make them even more robust but aren't blockers.

Great work validating the fix! 🎉