What are middleware best practices in Express.js?

Question

Accepted Answer

**Middleware** — functions that execute during request-response cycle.

**Best practices:**
- 1. **Order matters** — auth before routes.
- 2. **Keep middleware small** — single responsibility.
- 3. **Always call next()** or send response.
- 4. **Error middleware** — 4 params (err, req, res, next).

```javascript
// Logger middleware
function logger(req, res, next) {
    console.log(`${req.method} ${req.url}`);
    next();
}

// Auth middleware
function auth(req, res, next) {
    const token = req.headers.authorization;
    if (!token) return res.status(401).end();
    req.user = verifyToken(token);
    next();
}

// Error handler
function errorHandler(err, req, res, next) {
    console.error(err.stack);
    res.status(err.status || 500).json({
        error: err.message,
    });
}

app.use(logger);
app.use(auth);
app.use(routes);
app.use(errorHandler);
```

**Organization:**
- Group by feature (auth, validation, logging).
- Use express.Router for modular routes.

Node.js: What are middleware best practices in Express.js?