Next.js Middleware

Introduction

Middleware in Next.js is a powerful feature that allows you to run custom logic before a request is completed. This can be useful for various functionalities such as authentication, logging, and modifying request/response objects.

Key Concepts

**Middleware Execution**: Middleware runs before the request is processed by the route handler.
**Edge Functions**: Middleware operates at the edge, enabling fast, serverless execution.
**Request and Response**: Middleware can modify both the request and response objects.

Setup

To set up middleware in a Next.js project, follow these steps:

**Create a Middleware file**: In the root directory, create a file called middleware.js.
**Define Middleware Logic**: Use the following template to define your middleware logic.


import { NextResponse } from 'next/server';

export function middleware(request) {
    // Add custom logic here
    const { pathname } = request.nextUrl;

    // Example: Redirect unauthorized users
    if (pathname.startsWith('/protected')) {
        return NextResponse.redirect(new URL('/login', request.url));
    }

    return NextResponse.next();
}

Use Cases

Middleware can be applied in various scenarios:

**Authentication**: Protect routes by checking user authentication status.
**Localization**: Set user-specific language preferences.
**Analytics**: Log user actions or requests for analytics purposes.

Best Practices

**Keep it Lightweight**: Avoid heavy logic in middleware to maintain performance.
**Use Edge Functions**: Leverage edge functions for faster execution.
**Test Thoroughly**: Ensure middleware is well-tested to prevent unintentional redirects or modifications.

FAQ

What is middleware in Next.js?

Middleware is a function that runs before a request is completed, allowing you to modify the request or response.

Can middleware access request headers?

Yes, middleware can access and modify request headers.

Is middleware executed on the server or client?

Middleware executes on the server, specifically at the edge, enhancing performance.