useRuntimeConfig

```ts [server/api/foo.ts]
export default defineEventHandler((event) => {
  const config = useRuntimeConfig()
})

// Public keys that are exposed to the client
public: {
  apiBase: process.env.NUXT_PUBLIC_API_BASE || '/api',
},

::note
Variables that need to be accessible on the server are added directly inside `runtimeConfig`. Variables that need to be accessible on both the client and the server are defined in `runtimeConfig.public`.
::

:read-more{to="/docs/4.x/guide/going-further/runtime-config"}

## Access Runtime Config

To access runtime config, we can use `useRuntimeConfig()` composable:

```ts [server/api/test.ts]
export default defineEventHandler(async (event) => {
  const config = useRuntimeConfig()

  // Access public variables
  const result = await $fetch(`/test`, {
    baseURL: config.public.apiBase,
    headers: {
      // Access a private variable (only available on the server)
      Authorization: `Bearer ${config.apiSecret}`,
    },
  })
  return result
})

::note
Any environment variables set within `.env` file are accessed using `process.env` in the Nuxt app during **development** and **build/generate**.
::

::warning
In **production runtime**, you should use platform environment variables and `.env` is not used.
::

:read-more{to="/docs/4.x/directory-structure/env"}

## `app` namespace

Nuxt uses `app` namespace in runtime-config with keys including `baseURL` and `cdnURL`. You can customize their values at runtime by setting environment variables.

::note
This is a reserved namespace. You should not introduce additional keys inside `app`.
::

### `app.baseURL`

By default, the `baseURL` is set to `'/'`.

However, the `baseURL` can be updated at runtime by setting the `NUXT_APP_BASE_URL` as an environment variable.

Then, you can access this new base URL using `config.app.baseURL`:

```ts [/plugins/my-plugin.ts]
export default defineNuxtPlugin((NuxtApp) => {
  const config = useRuntimeConfig()

  // Access baseURL universally
  const baseURL = config.app.baseURL
})