next.js/docs/01-app/03-api-reference/05-config/01-next-config-js/output.mdx

---
title: output
description: Next.js automatically traces which files are needed by each page to allow for easy deployment of your application. Learn how it works here.
---

{/* The content of this doc is shared between the app and pages router. You can use the `<PagesOnly>Content</PagesOnly>` component to add content that is specific to the Pages Router. Any shared content should not be wrapped in a component. */}

During a build, Next.js will automatically trace each page and its dependencies to determine all of the files that are needed for deploying a production version of your application.

This feature helps reduce the size of deployments drastically. Previously, when deploying with Docker you would need to have all files from your package's `dependencies` installed to run `next start`. Starting with Next.js 12, you can leverage Output File Tracing in the `.next/` directory to only include the necessary files.

Furthermore, this removes the need for the deprecated `serverless` target which can cause various issues and also creates unnecessary duplication.

## How it Works

During `next build`, Next.js will use [`@vercel/nft`](https://github.com/vercel/nft) to statically analyze `import`, `require`, and `fs` usage to determine all files that a page might load.

Next.js' production server is also traced for its needed files and output at `.next/next-server.js.nft.json` which can be leveraged in production.

To leverage the `.nft.json` files emitted to the `.next` output directory, you can read the list of files in each trace that are relative to the `.nft.json` file and then copy them to your deployment location.

## Automatically Copying Traced Files

Next.js can automatically create a `standalone` folder that copies only the necessary files for a production deployment including select files in `node_modules`.

To leverage this automatic copying you can enable it in your `next.config.js`:

```js filename="next.config.js"
module.exports = {
  output: 'standalone',
}
```

This will create a folder at `.next/standalone` which can then be deployed on its own without installing `node_modules`.

Additionally, a minimal `server.js` file is also output which can be used instead of `next start`. This minimal server does not copy the `public` or `.next/static` folders by default as these should ideally be handled by a CDN instead, although these folders can be copied to the `standalone/public` and `standalone/.next/static` folders manually, after which `server.js` file will serve these automatically.

To copy these manually, you can use the `cp` command-line tool after you `next build`:

```bash filename="Terminal"
cp -r public .next/standalone/ && cp -r .next/static .next/standalone/.next/
```

To start your minimal `server.js` file locally, run the following command:

```bash filename="Terminal"
node .next/standalone/server.js
```

<AppOnly>

> **Good to know**:
>
> - If your project needs to listen to a specific port or hostname, you can define `PORT` or `HOSTNAME` environment variables before running `server.js`. For example, run `PORT=8080 HOSTNAME=0.0.0.0 node server.js` to start the server on `http://0.0.0.0:8080`.

</AppOnly>

<PagesOnly>

> **Good to know**:
>
> - `next.config.js` is read during `next build` and serialized into the `server.js` output file. If the legacy [`serverRuntimeConfig` or `publicRuntimeConfig` options](/docs/pages/api-reference/config/next-config-js/runtime-configuration) are being used, the values will be specific to values at build time.
> - If your project needs to listen to a specific port or hostname, you can define `PORT` or `HOSTNAME` environment variables before running `server.js`. For example, run `PORT=8080 HOSTNAME=0.0.0.0 node server.js` to start the server on `http://0.0.0.0:8080`.

</PagesOnly>

## Caveats

- While tracing in monorepo setups, the project directory is used for tracing by default. For `next build packages/web-app`, `packages/web-app` would be the tracing root and any files outside of that folder will not be included. To include files outside of this folder you can set `outputFileTracingRoot` in your `next.config.js`.

```js filename="packages/web-app/next.config.js"
module.exports = {
  // this includes files from the monorepo base two directories up
  outputFileTracingRoot: path.join(__dirname, '../../'),
}
```

- There are some cases in which Next.js might fail to include required files, or might incorrectly include unused files. In those cases, you can leverage `outputFileTracingExcludes` and `outputFileTracingIncludes` respectively in `next.config.js`. Each config accepts an object with [minimatch globs](https://www.npmjs.com/package/minimatch) for the key to match specific pages and a value of an array with globs relative to the project's root to either include or exclude in the trace.

```js filename="next.config.js"
module.exports = {
  outputFileTracingExcludes: {
    '/api/hello': ['./un-necessary-folder/**/*'],
  },
  outputFileTracingIncludes: {
    '/api/another': ['./necessary-folder/**/*'],
    '/api/login/\\[\\[\\.\\.\\.slug\\]\\]': [
      './node_modules/aws-crt/dist/bin/**/*',
    ],
  },
}
```

**Note:** The key of `outputFileTracingIncludes`/`outputFileTracingExcludes` is a [glob](https://www.npmjs.com/package/picomatch#basic-globbing), so special characters need to be escaped.