loadEnv return type mismatch

[michumk]

Describe the bug

The loadEnv method currently declares its return type as Record<string, string>.
However, in practice, it returns undefined if we try to get env variable with key that doesn't exist.

import { loadEnv } from 'vite';

const env = loadEnv('', './');
const foo = env['XYZ']; // ts type inference marks `foo` as 'string'
console.log('foo value:', foo); // the value is actually 'undefined'

This assumption that all keys are always string might lead to potential runtime errors. Those errors could simply be avoided by forcing the if (value !== undefined) check with proper return type in loadEnv.

Proposed Fix:

Update the return type of loadEnv to Record<string, string | undefined> or Partial<Record<string, string>>.

I'm willing to submit a PR for this issue if solution is agreed on.

---

There is a similar issue, but for some reason the original author decided to close it after being asked to present an example and didn't describe the issue properly.
#18643

Reproduction

https://stackblitz.com/edit/vitejs-vite-qjnysz6r?file=vite.config.ts

Steps to reproduce

1. Run npm install && npm run dev
2. Check vite.config.ts and look at console output from dev command

Validations

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• 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.
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• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

This is how TypeScript works. You can enable that behavior by setting "noUncheckedIndexedAccess": true. I think Vite should keep the types as-is and let the user decide how they want to deal with it.