Your app will have access to the current domain through the Host HTTP header.

With Express you can use a middleware like forcedomain—see the instructions in the README but it would look something like this:

app.use(forceDomain({
  hostname: 'www.mydomain.com'
}));

You could also skip the middleware and inspect the Host header manually but this one seems to be a minimal and well-tested package so it’s probably a good idea to just use it. Let me know if that works!

Also, you might have to add a condition to enable it only if you are in the production environment so it doesn’t affect your dev workflow, e.g.:

if (process.env.NODE_ENV == 'production') {
  app.use(forceDomain({
    hostname: 'www.mydomain.com'
  }));
}