Sans Express.js:

const https = require("https"),
          fs = require("fs");

const options = {
          key: fs.readFileSync("/etc/letsencrypt/live/example.com/privkey.pem"),
          cert: fs.readFileSync("/etc/letsencrypt/live/example.com/fullchain.pem")
};

https.createServer(options, function (req, res) {
          res.writeHead(200, {'Content-Type': 'text/html'});
          res.end('Hello World!');
}).listen(443);