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docs(config): add TS callback literal widening note #21702

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2 changes: 2 additions & 0 deletions docs/config/index.md
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Expand Up @@ -101,6 +101,8 @@ export default defineConfig(async ({ command, mode }) => {
})
```

When using TypeScript with callback-based config (`defineConfig(() => ...)` or `defineConfig(async () => ...)`), some nested string literals may be widened to `string` by TypeScript. If you see an overload error for a literal option (for example `build.minify`), add `as const` to that value or annotate the callback return type.

## Using Environment Variables in Config

Environment variables available while the config itself is being evaluated are only those that already exist in the current process environment (`process.env`). Vite deliberately defers loading any `.env*` files until _after_ the user config has been resolved because the set of files to load depends on config options like [`root`](/guide/#index-html-and-project-root) and [`envDir`](/config/shared-options.md#envdir), and also on the final `mode`.
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