Software that sits between requests and responses, processing, modifying, or validating data as it flows through your application.
Middleware is code that runs between receiving a request and sending a response. It processes requests before they reach your route handlers, and responses before they reach the client.
Think of middleware like airport security checkpoints. Every passenger (request) goes through security (middleware) before boarding (reaching your route).
In Express.js, middleware functions receive request, response, and next function. They can modify the request or response, then call next to pass control to the next middleware.
The middleware runs first, logs the request, then calls next to continue. Without next, the request hangs.
Authentication: Check if user is logged in before allowing access
Logging: Record every request for debugging and analytics
Parsing: Convert request body from JSON to JavaScript objects
CORS: Add headers allowing cross-origin requests
Compression: Compress responses to reduce bandwidth
Rate Limiting: Prevent abuse by limiting requests per user
Error Handling: Catch errors and return proper error responses
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Middleware executes in order. Each middleware can modify request or response, call next to continue, send a response and stop the chain, or throw an error.
Create a function that checks for authentication token in headers. If token is missing or invalid, return 401 error. If valid, add user to request object and call next to continue.
Most frameworks include common middleware:
Express.js:
Django:
ASP.NET:
Popular middleware packages handle common tasks like HTTP request logging (morgan), security headers (helmet), CORS handling, gzip compression, cookie parsing, file uploads, and rate limiting.
Apply middleware to specific routes only instead of all routes. Only admin routes require admin check, while public routes have no middleware.
Special middleware for catching errors must have four parameters. Put error middleware last to catch errors from all previous middleware and routes.
Next.js: Middleware runs at the edge before page renders
Laravel: Middleware for routes and route groups
Django: Middleware processes every request and response
Spring Boot: Filters and interceptors act as middleware
Every web framework has middleware concepts, though names and implementations vary.
Keep it Focused: Each middleware does one thing well
Call next: Always call next unless sending a response
Handle Errors: Catch errors and pass to error handler
Document: Explain what the middleware does and why
Order Matters: Place middleware in logical order (logging first, auth second, etc.)
Handle asynchronous operations properly with try-catch blocks and error passing.
Middleware is the backbone of web applications. It keeps your route handlers clean by extracting common logic into reusable pieces.
Authentication, logging, validation, error handling - middleware handles it all. Learn to write and use middleware effectively, and your code becomes cleaner, more maintainable, and more secure.
