How do I daemonize a node.js script?

Let's use forever.
Monitor the life and death of the node.js script that you started, and restart it automatically if it stops.

npm install forever-g
forever start spp.js

Stop or restart a running script

forever stop
forever stopall
forever restart

How about creating a unit file for systemd (although I'm not familiar with node.js)?

I think it's better to manage it using pm2.The reason is that Node.js supports clustering mode or not at the language level, so I think it would be appropriate to use pm2 as a framework to absorb it.

I don't think the pm2 process itself is likely to die if left alone, but if we do our best, I think it would be better to daemonize pm2 itself at the OS level with systemd, as Hayao Yamada said.