How to Use next Function in Express: Simple Guide

In Express, the next function is used inside middleware to pass control to the next middleware or route handler. Calling next() continues the request-response cycle, while next(error) passes an error to Express's error handler.

Syntax

The next function is a parameter in Express middleware functions. It is called to pass control to the next middleware or route handler.

• next(): Moves to the next middleware.
• next(error): Passes an error to Express's error handling middleware.

javascript

app.use(function (req, res, next) {
  // Middleware logic here
  next(); // Pass control to next middleware
});

app.use(function (err, req, res, next) {
  // Error handling middleware
  res.status(500).send('Something broke!');
});

Example

This example shows two middleware functions. The first logs the request and calls next() to continue. The second sends a response. If an error occurs, it is passed to the error handler using next(error).

javascript

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

// Middleware to log request method and URL
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Continue to next middleware
});

// Middleware to handle route
app.get('/', (req, res) => {
  res.send('Hello from Express!');
});

// Error handling middleware
app.use((err, req, res, next) => {
  console.error('Error:', err.message);
  res.status(500).send('Internal Server Error');
});

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

Output

Server running on http://localhost:3000
GET /

Common Pitfalls

Common mistakes when using next include:

• Forgetting to call next(), which causes the request to hang.
• Calling next() multiple times in the same middleware.
• Not handling errors properly by passing them to next(error).

Always ensure next() is called exactly once unless ending the response.

javascript

/* Wrong: Missing next() causes request to hang */
app.use((req, res, next) => {
  console.log('This middleware does not call next()');
  // next() missing here
});

/* Right: Calls next() to continue */
app.use((req, res, next) => {
  console.log('This middleware calls next()');
  next();
});

Quick Reference

• next(): Pass control to next middleware.
• next(error): Pass error to error handler.
• Middleware signature: (req, res, next).
• Error middleware signature: (err, req, res, next).

Key Takeaways

Always call next() in middleware to continue the request cycle.

Use next(error) to pass errors to Express's error handlers.

Middleware functions receive next as the third parameter.

Error-handling middleware has four parameters: err, req, res, next.

Avoid calling next() multiple times or forgetting to call it.