For this a file nodejsapp.service in the directory /etc/systemd/system/ can be created:
[Unit]
Description=NodeJS Application
After=network-online.target
[Service]
Restart=on-failure
WorkingDirectory=/home/user/backend/site/
ExecStart=/usr/bin/node /home/user/backend/site/app.js
Environment=PATH=/usr/bin:/usr/local/bin
[Install]
WantedBy=multi-user.target