This page has instructions on how to install forever on your Raspberry Pi.
Only install this if you are installing the HAP-NodeJS server
The first command you want to run is: cd
This command returns you to your home directory.
The second command you want to run is: sudo npm install forever -g
sudo npm install forever -g
This command starts the installation process. This can take 5-10 minutes. If it takes longer, just leave it.
That should complete the installation! Now, you can use the forever command wherever you need it. Ensure you do use sudo or else it won't work.
sudo forever start Core.js
Learn more about forever here.