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

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---
navigation.icon: IconFile
title: app.config.ts
head.title: 'app.config.ts'
description: Nuxt 3 provides the app.config file to expose reactive configuration within your application.
---
# App Config File
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 [app.config.ts]
export default defineAppConfig({
foo: 'bar'
})
```
::alert{type=warning}
Do not put any secret values inside `app.config` file. It is exposed to the user client bundle.
::
## Defining App Config
To expose config and environment variables to the rest of your app, you will need to define configuration in `app.config` file.
**Example:**
```ts [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.
```js
const appConfig = useAppConfig()
console.log(appConfig.theme)
```
### Manually Typing App Config
Nuxt tries to automatically generate a TypeScript interface from provided app config.
It is also possible to type app config manually. There are two possible things you might want to type.
#### Typing 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 {}
```
#### Typing App Config Output
If you want to type the result of calling `useAppConfig()`, then you will want to extend `AppConfig`.
::alert{type=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 {}
```