I'm trying to setup a nextJS + ExpressJS site on Apache. I've got a reverse proxy which forwards any request to http://1.23.45.67 (my ip at port 80) to localhost:3000 where my nodejs server is running. When I pull up the site in a browser everything looks fine. Images/css loads properly and I can navigate around the site but whenever an ajax request is made to the backend (e.g. api call to register a user) it does this with the request url http://1.23.45.67/undefined/api/v1/users/register.

My virtual host apache config looks like

<VirtualHost *:80>
    ServerName www.myservername.com
    ServerAlias myservername.com
    DocumentRoot /var/www/myapp/html
    ErrorLog /var/www/myapp/log/error.log
    CustomLog /var/www/myapp/log/requests.log combined

    ProxyRequests on
    ProxyPass / http://localhost:3000/
    ProxyPassReverse / http://localhost:3000/
</VirtualHost>

I'm generating my ajax request url using a base url variable from a .env file. I've also tried http://localhost/ and http://localhost:3000/ as the BASE_URL

# simplified .env file
...
BASE_URL=http://1.23.45.67/
...

# simplified register page where we make the ajax request.
...
fetch(`${process.env.BASE_URL}/api/v1/users/register`, {method: 'GET'});
...