Worker with import and globally defined window variable causes an error

[Yehya22]

Describe the bug

I have encountered a bug in Vite when using a worker that calls import, even if the import isn't directly called, and there is a globally defined window variable.

Expected Behavior

The application should run without any errors.

Actual Behavior

An error occurs when the worker is instantiated, even though the import function is not directly called.

Steps to Reproduce

1. Create a new Svelte app using Vite:

   pnpm create vite@latest my-app -- --template svelte
   cd my-app
   pnpm install

2. Create a worker file (worker.js) that defines a function calling import:

   const foo = () => import(url)

3. In the main application file (App.svelte), import the worker and create a new instance of it:

   <script>
   import Worker from './worker.js?worker&inline'
   new Worker
   </script>

4. In the Vite configuration file (vite.config.js), define a global window variable:

   export default defineConfig({
     plugins: [svelte()],
     define: {
       'window.__DEBUG__': true,
     },
   })

5. Run the application using pnpm run dev and observe the error.

Reproduction

You can find a minimal reproduction of the bug in the following repository:
https://github.com/mustafa0x/vite-worker-bug

You can also find a live reproduction on StackBlitz:
https://stackblitz.com/edit/vitejs-vite-zyhbeh

System Info

Vite version: 6.0.11
@sveltejs/vite-plugin-svelte version: 5.0.3
Svelte version: 5.19.3
Browser: Google Chrome (latest version)
Operating System: macOS

Used Package Manager

pnpm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

I think it's kind a expected behavior as define's description says "Entries will be defined as globals during dev" and you have window._DEBUG_ in define which will create window global variable in everywhere and causes these kinds of issues.
https://vite.dev/config/shared-options.html#define:~:text=Entries%20will%20be%20defined%20as%20globals%20during%20dev
But I'm fine with having an additional check at this condition.

vite/packages/vite/src/client/client.ts

Line 107 in e8c783f

if (typeof window !== 'undefined') {

Feel free to send a PR.

[Yehya22]

1. This is a regression I believe (but I'm not sure when it changed)
2. It only occurs if the worker contains import(url), even if it isn't called. So it's unclear why an import() would trigger a different code path. Perhaps there is an underlying bug.