output

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 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:

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:

cp -r public .next/standalone/ && cp -r .next/static .next/standalone/.next/

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

node .next/standalone/server.js

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 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.

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.

const path = require('path')
 
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 option accepts an object whose keys are route globs (matched with picomatch against the route path, e.g. /api/hello) and whose values are glob patterns resolved from the project root that specify files to include or exclude in the trace.

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

Using a src/ directory does not change how you write these options:

• Keys still match the route path ('/api/hello', '/products/[id]', etc.).
• Values can reference paths under src/ since they are resolved relative to the project root.

module.exports = {
  outputFileTracingIncludes: {
    '/products/*': ['src/lib/payments/**/*'],
    '/*': ['src/config/runtime/**/*.json'],
  },
  outputFileTracingExcludes: {
    '/api/*': ['src/temp/**/*', 'public/large-logs/**/*'],
  },
}

You can also target all routes using a global key like '/*':

module.exports = {
  outputFileTracingIncludes: {
    '/*': ['src/i18n/locales/**/*.json'],
  },
}

These options are applied to server traces and do not affect routes that do not produce a server trace file:

• Edge Runtime routes are not affected.
• Fully static pages are not affected.

In monorepos or when you need to include files outside the app folder, combine outputFileTracingRoot with includes:

const path = require('path')
 
module.exports = {
  // Trace from the monorepo root
  outputFileTracingRoot: path.join(__dirname, '../../'),
  outputFileTracingIncludes: {
    '/route1': ['../shared/assets/**/*'],
  },
}

Good to know:

• Prefer forward slashes (/) in patterns for cross-platform compatibility.
• Keep patterns as narrow as possible to avoid oversized traces (avoid **/* at the repo root).

Common include patterns for native/runtime assets:

module.exports = {
  outputFileTracingIncludes: {
    '/*': ['node_modules/sharp/**/*', 'node_modules/aws-crt/dist/bin/**/*'],
  },
}