Core Functions of Middleware

Middleware functions run before the server processes cached content or matches routes, giving them a strategic position to influence responses based on the request context.

Key Use Cases for Middleware

• Authentication and Authorization: Verify user identity and session status before allowing access to specific pages or resources.
• Server-Side Redirects: Redirect users dynamically based on criteria like locale or user roles without reaching the client-side.
• Path Rewriting: Dynamically modify paths, which is useful for A/B testing, feature rollouts, or supporting legacy URLs.
• Bot Detection: Identify and block bot traffic to protect your resources.
• Logging and Analytics: Capture request data for analysis before it is processed further, enhancing insights into application usage.
• Feature Flagging: Dynamically toggle features on or off for users, facilitating seamless feature testing and rollout.

Limitations

While middleware offers extensive capabilities, it is not suited for:

• Complex Data Fetching: Operations that involve detailed database interactions should be handled in API routes or server-side utilities.
• Heavy Computational Tasks: Avoid using middleware for intensive computations to prevent delays in response times.
• Extensive Session Management: Complex session handling should be managed by dedicated services or deeper within the application's architecture.

Implementation

Middleware is defined in a middleware.ts or middleware.js file at the root of your project or inside the src directory if applicable.

Basic Example

import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

export function middleware(request: NextRequest) {
  return NextResponse.redirect(new URL('/home', request.url));
}

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

Matching Paths

Middleware can be configured to apply to specific routes using matchers, which can precisely target or exclude routes. Matchers can be simple paths or complex patterns using regular expressions.