Node.js — Middleware

What is Middleware?

Middleware functions have access to request, response, and next.

app.use((req, res, next) => {
    console.log('Middleware executed');
    next();
});

Built-in Middleware

app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(express.static('public'));

Custom Middleware

// Logger
const logger = (req, res, next) => {
    console.log(`${req.method} ${req.url} - ${Date.now()}`);
    next();
};

// Auth
const auth = (req, res, next) => {
    if (!req.headers.authorization) {
        return res.status(401).json({ error: 'Unauthorized' });
    }
    next();
};

app.use(logger);
app.use('/api', auth);

Route-Level Middleware

app.get('/admin', auth, adminHandler);
app.post('/api', [auth, validate], apiHandler);

Error Handling Middleware

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).json({ error: 'Something went wrong!' });
});

Third-Party Middleware

const cors = require('cors');
const helmet = require('helmet');
const morgan = require('morgan');

app.use(cors());
app.use(helmet());
app.use(morgan('combined'));

Mini Practice

1. Create a logger middleware
2. Add authentication middleware
3. Handle errors with middleware
4. Use third-party middleware

Up Next

Continue with Authentication — user authentication.