AddonVitest: Improve perf & fix loading incorrect .env file

[ndelangen]

Closes #33101

What I did

I am doing 2 things in this PR:

1: I'm improving the performance of the addon-vitest package, by only evaluating what config to add/change when the code is being called in a vitest environment.
2: Fix a bug where the incorrect .env file is loaded

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

I discussed with @JReinhold about how to approach testing this, and though we discussed options like, extending a sandbox in some way or writing a unit-test for the function, both would be rather hard to do.

So I'm going to do a manual test using a canary release.
Here's my plan:

• I'm going to take the repro: https://stackblitz.com/edit/github-szu539pq?file=vite.config.ts
• Upgrade/switch it to my canary
• Ensure the .env files exist
• Run npm run build (executes vite build --mode=uat)
• Check if the built JS (in /dist/client/assets/index-*.js) contains the wrong environment variable (VITE_ENV_EXAMPLE=PRODUCTION instead of VITE_ENV_EXAMPLE=UAT)

🦋 Canary release

This pull request has been released as version 0.0.0-pr-33469-sha-4021d8de. Try it out in a new sandbox by running npx storybook@0.0.0-pr-33469-sha-4021d8de sandbox or in an existing project with npx storybook@0.0.0-pr-33469-sha-4021d8de upgrade.

More information

Published version	0.0.0-pr-33469-sha-4021d8de
Triggered by	@ndelangen
Repository	storybookjs/storybook
Branch	norbert/improve-vitest-addon-plugin
Commit	4021d8de
Datetime	Tue Jan 6 10:32:21 UTC 2026 (1767695541)
Workflow run	20745609634

To request a new release of this pull request, mention the @storybookjs/core team.

core team members can create a new canary release here or locally with gh workflow run --repo storybookjs/storybook publish.yml --field pr=33469

Summary by CodeRabbit

• Improvements
  • Enhanced Vitest plugin initialization to better handle environment configuration and build mode setup.
  • Streamlined configuration resolution for improved Vitest integration.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Vitest plugin initialization now returns an empty plugin array when the VITEST env flag is falsy. Env resolution is deferred into the plugin config hook via presets.apply('env', {}); BUILD_TARGET is set from the plugin mode to align with Vitest mode-based env loading. The transform runtime guard was removed. (49 words)

Changes

Cohort / File(s)	Summary
Vitest plugin changes code/addons/vitest/src/vitest-plugin/index.ts	Plugin now returns an empty array early if VITEST is falsy. Removed initial storybookEnv handling and env preset resolution; env is now populated inside the config hook via presets.apply('env', {}). Propagates BUILD_TARGET from plugin mode. Removed runtime VITEST guard in transform. Adjusted destructuring / Promise.all ordering accordingly.

Sequence Diagram(s)

sequenceDiagram
    %% Styling for clarity
    participant CLI as CLI/Env
    participant Presets as Presets
    participant VitestPlugin as Vitest Plugin (index.ts)
    participant VitestCfg as Vitest Config
    participant Transformer as Transform

    rect rgb(240,248,255)
    CLI->>VitestPlugin: load plugin (with env flags, mode)
    alt VITEST falsy
        VitestPlugin-->>CLI: return [] (no plugin)
    else VITEST truthy
        VitestPlugin->>VitestPlugin: enter config hook
        VitestPlugin->>Presets: presets.apply('env', {})
        Presets-->>VitestPlugin: env object
        VitestPlugin->>VitestCfg: set BUILD_TARGET = mode (if present)
        VitestPlugin->>VitestCfg: merge env into config
        VitestCfg-->>VitestPlugin: finalized config
        VitestPlugin->>Transformer: register transform (no VITEST runtime guard)
    end
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• Build: Upgrade to Vitest 4 #33086 — Related changes touching the same addons/vitest/src/vitest-plugin/index.ts area (env loading, BUILD_TARGET propagation, transform behavior).

✨ Finishing touches

• 📝 Generate docstrings

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📥 Commits

Reviewing files that changed from the base of the PR and between 4021d8d and 75ea041.

📒 Files selected for processing (1)

• code/addons/vitest/src/vitest-plugin/index.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• code/addons/vitest/src/vitest-plugin/index.ts

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• GitHub Check: normal
• GitHub Check: Core Unit Tests, windows-latest
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☁️ Nx Cloud last updated this comment at 2026-01-06 12:20:42 UTC

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