Nginx proxy_pass with Node and SSL - node.js

Question

I have managed to setup HTTPS using Nginx on a Node server at localhost:4000 by placing the cert and key paths in the Nginx conf file, however, I would also like to understand how to embed the ssl certificate inside the node index.js file and just pass the proxy'd address to the localhost:4000 with the certs inside the application rather than the nginx.conf file.
This nginx.conf file will work correctly with https/ssl once node isn't being served as ssl.
server {
listen 4500 ssl;
server_name example.com;
ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
access_log /var/log/nginx/example.com.access.log;
location / {
proxy_pass http://localhost:4000;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
}
}
However, if I comment out the following nginx.conf
# ssl_certificate /etc/letsencrypt/live/api.ballers.ie/fullchain.pem;
# ssl_certificate_key /etc/letsencrypt/live/api.ballers.ie/privkey.pem;
And then add the certs to my node server as follows, it tells me that the site can't be reached.
// ...
// dependencies & config
// ...
const privateKey = fs.readFileSync('/etc/letsencrypt/live/example.com.privkey.pem', 'utf8');
const certificate = fs.readFileSync('/etc/letsencrypt/live/example.com.cert.pem', 'utf8');
const ca = fs.readFileSync('/etc/letsencrypt/live/example.com.chain.pem', 'utf8');
const credentials = {
key: privateKey,
cert: certificate,
ca,
};
const httpsServer = https.createServer(credentials, app);
app.get('*', (req, res) => {
res.send('I am a HTTPS endpoint !');
});
httpsServer.listen(4000);
I have also tried changing proxy_pass to proxy_pass https://localhost:4000;
I'd be very interested to understand how to make this work, thanks for your assistance.