How can I protect my API endpoints from unauthorized access?

<!-- BEGIN COPY / PASTE -->
    // Example of setting up a secure API endpoint with Express.js
    const express = require('express');
    const app = express();
    const jwt = require('jsonwebtoken');

    // Middleware to check JWT token
    function authenticateToken(req, res, next) {
      const token = req.headers['authorization'];
      if (!token) return res.sendStatus(401);

      jwt.verify(token, process.env.ACCESS_TOKEN_SECRET, (err, user) => {
        if (err) return res.sendStatus(403);
        req.user = user;
        next();
      });
    }

    app.get('/api/protected', authenticateToken, (req, res) => {
      res.json({ message: 'This is a protected endpoint!' });
    });

    app.listen(3000);
    <!-- END COPY / PASTE -->