Or deamonize it.

Create file

/etc/systemd/system/yourservicename.service

With the following content

[Unit]
Description=nameofyourservice
After=network.target

[Service]
ExecStart=/usr/bin/npm start
WorkingDirectory=/opt/yoursoft
StandardOutput=inherit
StandardError=inherit
Restart=always
User=pi

[Install]
WantedBy=multi-user.target