| Version | Changes |
|---|---|
v12.0.9 | Enforce absolute URLs in Edge Runtime (PR) |
v12.0.0 | Middleware (Beta) added. |
Middleware enables you to use code over configuration. This gives you full flexibility in Next.js, because you can run code before a request is completed. Based on the user's incoming request, you can modify the response by rewriting, redirecting, adding headers, or even streaming HTML.
npm install next@latest
Then, create a _middleware.ts file under your /pages directory.
Finally, export a middleware function from the _middleware.ts file.
// pages/_middleware.ts
import type { NextFetchEvent, NextRequest } from 'next/server'
export function middleware(req: NextRequest, ev: NextFetchEvent) {
return new Response('Hello, world!')
}
In this example, we use the standard Web API Response (MDN).
Middleware is created by using a middleware function that lives inside a _middleware file. Its API is based upon the native FetchEvent, Response, and Request objects.
These native Web API objects are extended to give you more control over how you manipulate and configure a response, based on the incoming requests.
The function signature:
import type { NextFetchEvent } from 'next/server'
import type { NextRequest } from 'next/server'
export type Middleware = (
request: NextRequest,
event: NextFetchEvent
) => Promise<Response | undefined> | Response | undefined
The function can be a default export and as such, does not have to be named middleware. Though this is a convention. Also note that you only need to make the function async if you are running asynchronous code.
Read the full Middleware API reference.
Middleware can be used for anything that shares logic for a set of pages, including:
If your Middleware is created in /pages/_middleware.ts, it will run on all routes within the /pages directory. The below example assumes you have about.tsx and teams.tsx routes.
- package.json
- /pages
_middleware.ts # Will run on all routes under /pages
index.tsx
about.tsx
teams.tsx
If you do have sub-directories with nested routes, Middleware will run from the top down. For example, if you have /pages/about/_middleware.ts and /pages/about/team/_middleware.ts, /about will run first and then /about/team. The below example shows how this works with a nested routing structure.
- package.json
- /pages
index.tsx
- /about
_middleware.ts # Will run first
about.tsx
- /teams
_middleware.ts # Will run second
teams.tsx
Middleware runs directly after redirects and headers, before the first filesystem lookup. This excludes /_next files.
Middleware uses a strict runtime that supports standard Web APIs like fetch. This works out of the box using next start, as well as on Edge platforms like Vercel, which use Edge Functions.