Nuxt/docs/2.guide/2.directory-structure/3.app-config.md

---
title: app.config.ts
head.title: 'app.config.ts'
description: Expose reactive configuration within your application with the App Config file.
navigation.icon: i-ph-file-duotone
---

Nuxt 3 provides an `app.config` config file to expose reactive configuration within your application with the ability to update it at runtime within lifecycle or using a nuxt plugin and editing it with HMR (hot-module-replacement).

You can easily provide runtime app configuration using `app.config.ts` file. It can have either of `.ts`, `.js`, or `.mjs` extensions.

```ts twoslash [app.config.ts]
export default defineAppConfig({
  foo: 'bar'
})
```

::caution
Do not put any secret values inside `app.config` file. It is exposed to the user client bundle.
::

## Usage

To expose config and environment variables to the rest of your app, you will need to define configuration in `app.config` file.

```ts twoslash [app.config.ts]
export default defineAppConfig({
  theme: {
    primaryColor: '#ababab'
  }
})
```

When adding `theme` to the `app.config`, Nuxt uses Vite or webpack to bundle the code. We can universally access `theme` both when server-rendering the page and in the browser using [`useAppConfig`](/docs/api/composables/use-app-config) composable.

```vue [pages/index.vue]
<script setup lang="ts">
const appConfig = useAppConfig()

console.log(appConfig.theme)
</script>
```

When configuring a custom [`srcDir`](/docs/api/nuxt-config#srcdir), make sure to place the `app.config` file at the root of the new `srcDir` path.

## Typing App Config

Nuxt tries to automatically generate a TypeScript interface from provided app config so you won't have to type it yourself.

However, there are some cases where you might want to type it yourself. There are two possible things you might want to type.

### App Config Input

`AppConfigInput` might be used by module authors who are declaring what valid _input_ options are when setting app config. This will not affect the type of `useAppConfig()`.

```ts [index.d.ts]
declare module 'nuxt/schema' {
  interface AppConfigInput {
    /** Theme configuration */
    theme?: {
      /** Primary app color */
      primaryColor?: string
    }
  }
}

// It is always important to ensure you import/export something when augmenting a type
export {}
```

### App Config Output

If you want to type the result of calling [`useAppConfig()`](/docs/api/composables/use-app-config), then you will want to extend `AppConfig`.

::warning
Be careful when typing `AppConfig` as you will overwrite the types Nuxt infers from your actually defined app config.
::

```ts [index.d.ts]
declare module 'nuxt/schema' {
  interface AppConfig {
    // This will entirely replace the existing inferred `theme` property
    theme: {
      // You might want to type this value to add more specific types than Nuxt can infer,
      // such as string literal types
      primaryColor?: 'red' | 'blue'
    }
  }
}

// It is always important to ensure you import/export something when augmenting a type
export {}
```

## Merging Strategy

Nuxt uses a custom merging strategy for the `AppConfig` within [the layers](/docs/getting-started/layers) of your application.

This strategy is implemented using a [Function Merger](https://github.com/unjs/defu#function-merger), which allows defining a custom merging strategy for every key in `app.config` that has an array as value.

::note
The function merger can only be used in the extended layers and not the main `app.config` in project.
::

Here's an example of how you can use:

::code-group

```ts twoslash [layer/app.config.ts]
export default defineAppConfig({
  // Default array value
  array: ['hello'],
})
```

```ts twoslash [app.config.ts]
export default defineAppConfig({
  // Overwrite default array value by using a merger function
  array: () => ['bonjour'],
})
```

::