Middleware

Middleware allows you to run code before a request is completed. Then, based on the incoming request, you can modify the response by rewriting, redirecting, modifying the request or response headers, or responding directly.

Middleware runs before cached content and routes are matched. See Matching Paths for more details.

Use Cases

Integrating Middleware into your application can lead to significant improvements in performance, security, and user experience. Some common scenarios where Middleware is particularly effective include:

Authentication and Authorization: Ensure user identity and check session cookies before granting access to specific pages or API routes.
Server-Side Redirects: Redirect users at the server level based on certain conditions (e.g., locale, user role).
Path Rewriting: Support A/B testing, feature rollouts, or legacy paths by dynamically rewriting paths to API routes or pages based on request properties.
Bot Detection: Protect your resources by detecting and blocking bot traffic.
Logging and Analytics: Capture and analyze request data for insights before processing by the page or API.
Feature Flagging: Enable or disable features dynamically for seamless feature rollouts or testing.

Recognizing situations where middleware may not be the optimal approach is just as crucial. Here are some scenarios to be mindful of:

Complex Data Fetching and Manipulation: Middleware is not designed for direct data fetching or manipulation, this should be done within Route Handlers or server-side utilities instead.
Heavy Computational Tasks: Middleware should be lightweight and respond quickly or it can cause delays in page load. Heavy computational tasks or long-running processes should be done within dedicated Route Handlers.
Extensive Session Management: While Middleware can manage basic session tasks, extensive session management should be managed by dedicated authentication services or within Route Handlers.
Direct Database Operations: Performing direct database operations within Middleware is not recommended. Database interactions should be done within Route Handlers or server-side utilities.

Convention

Use the file middleware.ts (or .js) in the root of your project to define Middleware. For example, at the same level as pages or app, or inside src if applicable.

Note: While only one middleware.ts file is supported per project, you can still organize your middleware logic modularly. Break out middleware functionalities into separate .ts or .js files and import them into your main middleware.ts file. This allows for cleaner management of route-specific middleware, aggregated in the middleware.ts for centralized control. By enforcing a single middleware file, it simplifies configuration, prevents potential conflicts, and optimizes performance by avoiding multiple middleware layers.

Example

middleware.ts

import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'
 
// This function can be marked `async` if using `await` inside
export function middleware(request: NextRequest) {
  return NextResponse.redirect(new URL('/home', request.url))
}
 
// See "Matching Paths" below to learn more
export const config = {
  matcher: '/about/:path*',
}

Matching Paths

Middleware will be invoked for every route in your project. Given this, it's crucial to use matchers to precisely target or exclude specific routes. The following is the execution order:

headers from next.config.js
redirects from next.config.js
Middleware (rewrites, redirects, etc.)
beforeFiles (rewrites) from next.config.js
Filesystem routes (public/, _next/static/, pages/, app/, etc.)
afterFiles (rewrites) from next.config.js
Dynamic Routes (/blog/[slug])
fallback (rewrites) from next.config.js

There are two ways to define which paths Middleware will run on:

Custom matcher config
Conditional statements

Matcher

matcher allows you to filter Middleware to run on specific paths.

middleware.js

export const config = {
  matcher: '/about/:path*',
}

You can match a single path or multiple paths with an array syntax:

middleware.js

export const config = {
  matcher: ['/about/:path*', '/dashboard/:path*'],
}

The matcher config allows full regex so matching like negative lookaheads or character matching is supported. An example of a negative lookahead to match all except specific paths can be seen here:

middleware.js

export const config = {
  matcher: [
    /*
     * Match all request paths except for the ones starting with:
     * - api (API routes)
     * - _next/static (static files)
     * - _next/image (image optimization files)
     * - favicon.ico, sitemap.xml, robots.txt (metadata files)
     */
    '/((?!api|_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)',
  ],
}

You can also bypass Middleware for certain requests by using the missing or has arrays, or a combination of both:

middleware.js

export const config = {
  matcher: [
    /*
     * Match all request paths except for the ones starting with:
     * - api (API routes)
     * - _next/static (static files)
     * - _next/image (image optimization files)
     * - favicon.ico, sitemap.xml, robots.txt (metadata files)
     */
    {
      source:
        '/((?!api|_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)',
      missing: [
        { type: 'header', key: 'next-router-prefetch' },
        { type: 'header', key: 'purpose', value: 'prefetch' },
      ],
    },
 
    {
      source:
        '/((?!api|_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)',
      has: [
        { type: 'header', key: 'next-router-prefetch' },
        { type: 'header', key: 'purpose', value: 'prefetch' },
      ],
    },
 
    {
      source:
        '/((?!api|_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt).*)',
      has: [{ type: 'header', key: 'x-present' }],
      missing: [{ type: 'header', key: 'x-missing', value: 'prefetch' }],
    },
  ],
}

Good to know: The matcher values need to be constants so they can be statically analyzed at build-time. Dynamic values such as variables will be ignored.

Configured matchers:

MUST start with /
Can include named parameters: /about/:path matches /about/a and /about/b but not /about/a/c
Can have modifiers on named parameters (starting with :): /about/:path* matches /about/a/b/c because * is zero or more. ? is zero or one and + one or more
Can use regular expression enclosed in parenthesis: /about/(.*) is the same as /about/:path*

Read more details on path-to-regexp documentation.

Good to know: For backward compatibility, Next.js always considers /public as /public/index. Therefore, a matcher of /public/:path will match.

Conditional Statements

middleware.ts

import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'
 
export function middleware(request: NextRequest) {
  if (request.nextUrl.pathname.startsWith('/about')) {
    return NextResponse.rewrite(new URL('/about-2', request.url))
  }
 
  if (request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.rewrite(new URL('/dashboard/user', request.url))
  }
}

NextResponse

The NextResponse API allows you to:

redirect the incoming request to a different URL
rewrite the response by displaying a given URL
Set request headers for API Routes, getServerSideProps, and rewrite destinations
Set response cookies
Set response headers

To produce a response from Middleware, you can:

rewrite to a route (Page or Edge API Route) that produces a response
return a NextResponse directly. See Producing a Response

Using Cookies

Cookies are regular headers. On a Request, they are stored in the Cookie header. On a Response they are in the Set-Cookie header. Next.js provides a convenient way to access and manipulate these cookies through the cookies extension on NextRequest and NextResponse.

For incoming requests, cookies comes with the following methods: get, getAll, set, and delete cookies. You can check for the existence of a cookie with has or remove all cookies with clear.
For outgoing responses, cookies have the following methods get, getAll, set, and delete.

middleware.ts

import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'
 
export function middleware(request: NextRequest) {
  // Assume a "Cookie:nextjs=fast" header to be present on the incoming request
  // Getting cookies from the request using the `RequestCookies` API
  let cookie = request.cookies.get('nextjs')
  console.log(cookie) // => { name: 'nextjs', value: 'fast', Path: '/' }
  const allCookies = request.cookies.getAll()
  console.log(allCookies) // => [{ name: 'nextjs', value: 'fast' }]
 
  request.cookies.has('nextjs') // => true
  request.cookies.delete('nextjs')
  request.cookies.has('nextjs') // => false
 
  // Setting cookies on the response using the `ResponseCookies` API
  const response = NextResponse.next()
  response.cookies.set('vercel', 'fast')
  response.cookies.set({
    name: 'vercel',
    value: 'fast',
    path: '/',
  })
  cookie = response.cookies.get('vercel')
  console.log(cookie) // => { name: 'vercel', value: 'fast', Path: '/' }
  // The outgoing response will have a `Set-Cookie:vercel=fast;path=/` header.
 
  return response
}

Setting Headers

You can set request and response headers using the NextResponse API (setting request headers is available since Next.js v13.0.0).

middleware.ts

import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'
 
export function middleware(request: NextRequest) {
  // Clone the request headers and set a new header `x-hello-from-middleware1`
  const requestHeaders = new Headers(request.headers)
  requestHeaders.set('x-hello-from-middleware1', 'hello')
 
  // You can also set request headers in NextResponse.next
  const response = NextResponse.next({
    request: {
      // New request headers
      headers: requestHeaders,
    },
  })
 
  // Set a new response header `x-hello-from-middleware2`
  response.headers.set('x-hello-from-middleware2', 'hello')
  return response
}

Good to know: Avoid setting large headers as it might cause 431 Request Header Fields Too Large error depending on your backend web server configuration.

CORS

You can set CORS headers in Middleware to allow cross-origin requests, including simple and preflighted requests.

middleware.ts

import { NextRequest, NextResponse } from 'next/server'
 
const allowedOrigins = ['https://acme.com', 'https://my-app.org']
 
const corsOptions = {
  'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
  'Access-Control-Allow-Headers': 'Content-Type, Authorization',
}
 
export function middleware(request: NextRequest) {
  // Check the origin from the request
  const origin = request.headers.get('origin') ?? ''
  const isAllowedOrigin = allowedOrigins.includes(origin)
 
  // Handle preflighted requests
  const isPreflight = request.method === 'OPTIONS'
 
  if (isPreflight) {
    const preflightHeaders = {
      ...(isAllowedOrigin && { 'Access-Control-Allow-Origin': origin }),
      ...corsOptions,
    }
    return NextResponse.json({}, { headers: preflightHeaders })
  }
 
  // Handle simple requests
  const response = NextResponse.next()
 
  if (isAllowedOrigin) {
    response.headers.set('Access-Control-Allow-Origin', origin)
  }
 
  Object.entries(corsOptions).forEach(([key, value]) => {
    response.headers.set(key, value)
  })
 
  return response
}
 
export const config = {
  matcher: '/api/:path*',
}

Producing a Response

You can respond from Middleware directly by returning a Response or NextResponse instance. (This is available since Next.js v13.1.0)

middleware.ts

import type { NextRequest } from 'next/server'
import { isAuthenticated } from '@lib/auth'
 
// Limit the middleware to paths starting with `/api/`
export const config = {
  matcher: '/api/:function*',
}
 
export function middleware(request: NextRequest) {
  // Call our authentication function to check the request
  if (!isAuthenticated(request)) {
    // Respond with JSON indicating an error message
    return Response.json(
      { success: false, message: 'authentication failed' },
      { status: 401 }
    )
  }
}

`waitUntil` and `NextFetchEvent`

The NextFetchEvent object extends the native FetchEvent object, and includes the waitUntil() method.

The waitUntil() method takes a promise as an argument, and extends the lifetime of the Middleware until the promise settles. This is useful for performing work in the background.

middleware.ts

import { NextResponse } from 'next/server'
import type { NextFetchEvent, NextRequest } from 'next/server'
 
export function middleware(req: NextRequest, event: NextFetchEvent) {
  event.waitUntil(
    fetch('https://my-analytics-platform.com', {
      method: 'POST',
      body: JSON.stringify({ pathname: req.nextUrl.pathname }),
    })
  )
 
  return NextResponse.next()
}

Advanced Middleware Flags

In v13.1 of Next.js two additional flags were introduced for middleware, skipMiddlewareUrlNormalize and skipTrailingSlashRedirect to handle advanced use cases.

skipTrailingSlashRedirect disables Next.js redirects for adding or removing trailing slashes. This allows custom handling inside middleware to maintain the trailing slash for some paths but not others, which can make incremental migrations easier.

next.config.js

module.exports = {
  skipTrailingSlashRedirect: true,
}

middleware.js

const legacyPrefixes = ['/docs', '/blog']
 
export default async function middleware(req) {
  const { pathname } = req.nextUrl
 
  if (legacyPrefixes.some((prefix) => pathname.startsWith(prefix))) {
    return NextResponse.next()
  }
 
  // apply trailing slash handling
  if (
    !pathname.endsWith('/') &&
    !pathname.match(/((?!\.well-known(?:\/.*)?)(?:[^/]+\/)*[^/]+\.\w+)/)
  ) {
    return NextResponse.redirect(
      new URL(`${req.nextUrl.pathname}/`, req.nextUrl)
    )
  }
}

skipMiddlewareUrlNormalize allows for disabling the URL normalization in Next.js to make handling direct visits and client-transitions the same. In some advanced cases, this option provides full control by using the original URL.

next.config.js

module.exports = {
  skipMiddlewareUrlNormalize: true,
}

middleware.js

export default async function middleware(req) {
  const { pathname } = req.nextUrl
 
  // GET /_next/data/build-id/hello.json
 
  console.log(pathname)
  // with the flag this now /_next/data/build-id/hello.json
  // without the flag this would be normalized to /hello
}

Runtime

Middleware currently only supports APIs compatible with the Edge runtime. APIs exclusive to Node.js are unsupported.

Version History

Version	Changes
`v13.1.0`	Advanced Middleware flags added
`v13.0.0`	Middleware can modify request headers, response headers, and send responses
`v12.2.0`	Middleware is stable, please see the upgrade guide
`v12.0.9`	Enforce absolute URLs in Edge Runtime (PR)
`v12.0.0`	Middleware (Beta) added