By default, Nuxt is configured to cover most use cases. The [`nuxt.config.ts`](/guide/directory-structure/nuxt.config) file can override or extend this default configuration.
## Nuxt Configuration
The `nuxt.config.ts` file is located at the root of a Nuxt project and can override or extend the application's behavior.
A minimal configuration file exports the `defineNuxtConfig` function containing an object with your configuration. The `defineNuxtConfig` helper is globally available without import.
```ts [nuxt.config.ts]
export default defineNuxtConfig({
// My Nuxt config
})
```
This file will often be mentioned in the documentation, for example to add custom scripts, register modules or change rendering modes.
You don't have to use TypeScript to build an application with Nuxt. However, it is strongly recommended to use the `.ts` extension for the `nuxt.config` file. This way you can benefit from hints in your IDE to avoid typos and mistakes while editing your configuration.
::
### Environment Variables and Private Tokens
The `runtimeConfig` API exposes values like environment variables to the rest of your application. By default, these keys are only available server-side. The keys within `runtimeConfig.public` are also available client-side.
Those values should be defined in `nuxt.config` and can be overridden using environment variables.
::code-group
```ts [nuxt.config.ts]
export default defineNuxtConfig({
runtimeConfig: {
// The private keys which are only available server-side
apiSecret: '123',
// Keys within public are also exposed client-side
public: {
apiBase: '/api'
}
}
})
```
```text [.env]
# This will override the value of apiSecret
NUXT_API_SECRET=api_secret_token
```
::
These variables are exposed to the rest of your application using the [`useRuntimeConfig`](/api/composables/use-runtime-config) composable.
The `app.config.ts` file, also located at the root of a Nuxt project, is used to expose public variables that can be determined at build time. Contrary to the `runtimeConfig` option, these can not be overriden using environment variables.
A minimal configuration file exports the `defineAppConfig` function containing an object with your configuration. The `defineAppConfig` helper is globally available without import.
```ts [app.config.ts]
export default defineAppConfig({
title: 'Hello Nuxt',
theme: {
dark: true,
colors: {
primary: '#ff0000'
}
}
})
```
These variables are exposed to the rest of your application using the [`useAppConfig`](/api/composables/use-app-config) composable.
As stated above, `runtimeConfig` and `app.config` are both used to expose variables to the rest of your application. To determine whether you should use one or the other, here are some guidelines:
-`runtimeConfig`: Private or public tokens that need to be specified after build using environment variables.
-`app.config` : Public tokens that are determined at build time, website configuration such as theme variant, title and any project config that are not sensitive.
