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How to skip Next.js middleware for static and public files

Category
Engineering
Published

Stop your Next.js middleware from running on static assets like images and Next.js internals

Next.js middleware is incredibly powerful, but by default it runs on more requests than is normally necessary. To see this in action, you can add a simple logger:

middleware.js
export default function middleware(req) {
  console.log(req.url)
}

Here's the log for loading http://localhost:3000/ on a brand new Next.js application:

http://localhost:3000/
http://localhost:3000/_next/static/chunks/webpack.js?ts=1665116452838
http://localhost:3000/_next/static/chunks/main.js?ts=1665116452838
http://localhost:3000/_next/static/chunks/react-refresh.js?ts=1665116452838
http://localhost:3000/_next/static/chunks/pages/_app.js?ts=1665116452838
http://localhost:3000/_next/static/development/_buildManifest.js?ts=1665116452838
http://localhost:3000/_next/static/development/_ssgManifest.js?ts=1665116452838
http://localhost:3000/_next/static/chunks/pages/index.js?ts=1665116452838
http://localhost:3000/favicon.ico
http://localhost:3000/_next/static/development/_devMiddlewareManifest.json
http://localhost:3000/_next/static/development/_devPagesManifest.json
http://localhost:3000/vercel.svg

The log shows several static assets that you probably don't want to process in your middleware, and doing so could break your application. To solve this, you can configure a matcher, which tells your middleware to only run on requests that match a certain pattern.

To prevent middleware from running on static files, you can leverage a simple trick: static files have a . in the path, while dynamic files do not.

With a regular expression, you can configure middleware to run on every file that does not contain a .:

middleware.js
export default function middleware(req) {
  console.log(req.url)
}

export const config = { matcher: '/((?!.*\\.).*)' }

With this matcher, the log only shows one request:

http://localhost:3000/

This matcher is a great starting point that minimizes middleware invocation. As your application grows, though, you may need to tweak it. Below are the two most common revisions.

Including some static assets

The easiest way to include some static assets in middleware is to retain the exclusionary regular expression, but define an additional matcher. Here's a sample that excludes all static assets from middleware except favicon.ico:

middleware.js
export const config = {
  matcher: ['/((?!.*\\.).*)', '/favicon.ico'],
}

Excluding more paths from middleware

Sometimes you will want to filter more than just static files. This can be done in two ways:

1. Expanding the regular expression

Expanding the regular expression prevents additional middleware invocations, but regular expressions are also harder to read and write. Here's an example that excludes API routes in addition to static assets:

middleware.js
export const config = { matcher: '/((?!.*\\.|api\\).*)' }

2. Adding a conditional statement

A more readable approach is to create a conditional statement, but this will waste compute resources since your middleware will be unnecessarily invoked. Here's an example:

middleware.js
import { NextResponse } from 'next/server'

export default function middleware(req) {
  if (req.nextUrl.pathname.startsWith('/api/')) {
    return NextResponse.next()
  }
  // ... your middleware
}

export const config = { matcher: '/((?!.*\\.).*)' }

This should be everything you need to get started on the right foot with middleware!

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Author
Colin Sidoti