How to Create Middleware Chain in Express: Simple Guide

In Express, you create a middleware chain by passing multiple middleware functions in sequence to app.use() or route handlers. Each middleware calls next() to pass control to the next function, forming a chain that processes requests step-by-step.

Syntax

Middleware functions in Express have the signature (req, res, next). You chain them by listing multiple middleware functions in app.use() or route handlers. Each middleware must call next() to continue the chain.

• req: The request object.
• res: The response object.
• next: Function to call the next middleware.

javascript

app.use(middleware1, middleware2, middleware3);

// or for a route
app.get('/path', middleware1, middleware2, (req, res) => {
  res.send('Done');
});

Example

This example shows three middleware functions chained together. Each logs a message and calls next(). The final handler sends a response. This demonstrates how requests flow through the middleware chain.

javascript

import express from 'express';
const app = express();

// First middleware
function mw1(req, res, next) {
  console.log('Middleware 1');
  next();
}

// Second middleware
function mw2(req, res, next) {
  console.log('Middleware 2');
  next();
}

// Third middleware
function mw3(req, res, next) {
  console.log('Middleware 3');
  next();
}

app.use(mw1, mw2, mw3);

app.get('/', (req, res) => {
  res.send('Hello from final handler');
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Output

Server running on http://localhost:3000
Middleware 1
Middleware 2
Middleware 3

Common Pitfalls

Common mistakes include forgetting to call next() in middleware, which stops the chain and leaves requests hanging. Another is sending multiple responses by calling res.send() in more than one middleware. Always call next() unless you end the response.

javascript

import express from 'express';
const app = express();

// Wrong: missing next(), request hangs
function badMiddleware(req, res, next) {
  console.log('This middleware forgets next()');
  // next() missing here
}

// Right: calls next() to continue
function goodMiddleware(req, res, next) {
  console.log('This middleware calls next()');
  next();
}

app.use(badMiddleware); // This will cause requests to hang
app.use(goodMiddleware);

app.get('/', (req, res) => {
  res.send('Hello');
});

Quick Reference

• Middleware signature: (req, res, next)
• Call next() to pass control to next middleware
• Chain middleware by listing them in app.use() or route handlers
• End response with res.send(), res.json(), or similar
• Do not call next() after sending a response

Key Takeaways

Chain middleware by passing multiple functions to app.use() or route handlers.

Always call next() in middleware to continue the chain unless ending the response.

Middleware functions receive req, res, and next parameters.

Avoid sending multiple responses or forgetting next() to prevent hanging requests.

Middleware chain processes requests step-by-step in the order they are declared.