Nuxt/docs/2.guide/3.going-further/1.internals.md

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---
title: "How Nuxt Works?"
description: "Nuxt is a minimal but highly customizable framework to build web applications."
---
# How Nuxt Works?
Nuxt is a minimal but highly customizable framework to build web applications. This guide helps you better understand Nuxt internals to develop new solutions and module integrations on top of Nuxt.
## The Nuxt Interface
When you start Nuxt in development mode with `nuxi dev` or building a production application with `nuxi build`,
a common context will be created, referred to as `nuxt` internally. It holds normalized options merged with `nuxt.config` file,
some internal state, and a powerful [hooking system](/docs/api/advanced/hooks) powered by [unjs/hookable](https://github.com/unjs/hookable)
allowing different components to communicate with each other. You can think of it as **Builder Core**.
This context is globally available to be used with [nuxt/kit](/docs/api/advanced/kit) composables.
Therefore only one instance of Nuxt is allowed to run per process.
To extend the Nuxt interface and hook into different stages of the build process, we can use [Nuxt Modules](/docs/guide/going-further/modules).
For more details, check out [the source code](https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/core/nuxt.ts).
## The NuxtApp Interface
When rendering a page in the browser or on the server, a shared context will be created, referred to as `nuxtApp`.
This context keeps vue instance, runtime hooks, and internal states like ssrContext and payload for hydration.
You can think of it as **Runtime Core**.
This context can be accessed using `useNuxtApp()` composable within Nuxt plugins and `<script setup>` and vue composables.
Global usage is possible for the browser but not on the server, to avoid sharing context between users.
To extend the `nuxtApp` interface and hook into different stages or access contexts, we can use [Nuxt Plugins](/docs/guide/directory-structure/plugins).
Check [Nuxt App](/docs/api/composables/use-nuxt-app) for more information about this interface.
`nuxtApp` has the following properties:
Note: this is an internal interface, and some properties might change until stable release.
```js
const nuxtApp = {
vueApp, // the global Vue application: https://vuejs.org/api/application.html#application-api
versions, // an object containing Nuxt and Vue versions
// These let you call and add runtime NuxtApp hooks
// https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/nuxt.ts#L18
hooks,
hook,
callHook,
// Only accessible on server-side
ssrContext: {
url,
req,
res,
runtimeConfig,
noSSR,
},
// This will be stringified and passed from server to client
payload: {
serverRendered: true,
data: {},
state: {}
}
provide: (name: string, value: any) => void
}
```
:StabilityEdge{title="nuxtApp.versions"}
For more details, check out [the source code](https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/nuxt.ts).
## Runtime Context vs. Build Context
Nuxt builds and bundles project using Node.js but also has a runtime side.
While both areas can be extended, that runtime context is isolated from build-time. Therefore, they are not supposed to share state, code, or context other than runtime configuration!
`nuxt.config` and [Nuxt Modules](/docs/guide/going-further/modules) can be used to extend the build context, and [Nuxt Plugins](/docs/guide/directory-structure/plugins) can be used to extend runtime.
When building an application for production, `nuxi build` will generate a standalone build
in the `.output` directory, independent of `nuxt.config` and [Nuxt modules](/docs/guide/going-further/modules).