NGINX config to first check a server, e.g. webpack-dev-server, if it's down then local files.

nginx.conf

server {
    [... normal configs]
  
    # Try webpack server first, if error 502 then go try files
    location ~ ^/.+/static/js/.+\.js$ {
            proxy_pass http://127.0.0.1:8080;
            proxy_set_header X-Forwarded-Host 127.0.0.1:8080;
            error_page 502 = @localjs;
    }

    # Trying js locally after webpack failed
    location @localjs {
            try_files $uri =404;
    }
    
    [... other locations]
}

Couldn't find something like this anywhere. The "try_files" directive works the other way round, i.e. first try local files then fallback (named location or internal).