What is middleware in web development?

Middleware is code that runs between an incoming request and the route handler that ultimately processes it, commonly used for authentication, logging, and request modification.

How does middleware differ from a route handler?

A route handler produces the final response for a specific route, while middleware runs before (and sometimes after) that handler, applying logic that often spans many routes.

Can middleware stop a request from reaching its handler?

Yes, middleware can short-circuit the chain — for example, returning a 401 response for an unauthenticated request instead of passing control further.

Where does middleware run in Next.js?

Next.js middleware runs at the edge, before a request reaches page or API route code, commonly deployed as an edge function.

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Middleware (web)

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In web development, middleware is code that runs between an incoming request and the final route handler, commonly used for tasks like authentication, logging, or request/response modification.

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Definition

In web development, middleware is code that runs between an incoming request and the final route handler, commonly used for tasks like authentication, logging, or request/response modification.

Overview

Middleware sits in the request-response pipeline of a web application, intercepting requests before they reach a route's main handler (and often intercepting the response on the way back out). This pattern is foundational across backend frameworks — Express.js popularized the idea of a chain of middleware functions in Node.js, and it appears similarly in frameworks like Django and Laravel — as well as in modern meta-frameworks like Next.js, where middleware can run at the Edge Functions layer before a request even reaches page or API code. Common middleware responsibilities include authentication and authorization checks, request logging, parsing request bodies, setting security headers, handling CORS, rate limiting, and redirecting or rewriting URLs. Because middleware runs on every matching request, it is the natural place to enforce cross-cutting concerns that would otherwise need to be duplicated in every individual route handler. Middleware functions are typically composed into an ordered chain, where each middleware can either pass control to the next one, short-circuit the chain (for example, to reject an unauthenticated request), or modify the request/response objects along the way — a pattern closely related to how an API Gateway applies cross-cutting policies at the edge of a system rather than inside individual services.

Key Concepts

Runs between an incoming request and its final route handler
Composable chain where each function can pass control or short-circuit
Common for authentication, logging, and request/response modification
Available in backend frameworks like Express.js, Django, and Laravel
Modern edge-deployed middleware in frameworks like Next.js
Handles cross-cutting concerns like CORS and security headers
Can rewrite, redirect, or block requests before they reach app logic
Reduces duplicated logic across many individual route handlers

Use Cases

Enforcing authentication and authorization before requests reach handlers

Logging and monitoring incoming requests across an application

Applying CORS, security headers, or rate limiting globally

Redirecting or rewriting URLs based on locale, auth state, or A/B tests

Parsing and validating request bodies before route logic runs

Frequently Asked Questions

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