Run a Node.js application
A Node.js application is a runtime a website can choose, like PHP or a static site. The panel starts it, restarts it if it exits, and puts your certificate and domain in front of it.
Choosing it
A runtime, set on the website.
Open the website, choose the Node.js application runtime, pick a Node.js version and press Apply runtime. The website keeps its domain, its certificate and its files; what changes is what answers a request. A site can move between runtimes later without being rebuilt.
The one rule
Listen on the port you are given.
Your application must listen on the port the panel assigns it, on loopback. Nothing is published directly: nginx holds the domain and the certificate and passes requests through, which is why you get HTTPS without your application knowing anything about certificates. An application hard-coded to port 3000 will not be reached — read the port from the environment, as every deployment guide already tells you to.
WebSockets work. The connection is passed through and kept open, so an application that upgrades a request behaves as it would anywhere else.
Staying up
The panel restarts it if it exits.
The application is supervised. If it exits — a crash, an unhandled rejection, an out-of-memory kill — it is started again, so a single bad request does not take the site down until somebody notices. That is not a substitute for fixing the crash: a process that exits on every request restarts on every request, and the site is still down. It buys time, not correctness.
Deploying new code means restarting the application so the new files are the ones running. The panel does that for you when you apply a change.
What it can reach
Its own website, as its own account.
A Node.js application runs as the website's own Unix account, inside the website's directory, exactly as a PHP site does. It can read and write its own files and reach its own database. It cannot read another website's files, and the account has no shell.
A database for it is made the same way as for any other website: create one with its own account and read the credentials from the panel. See the guide on how a website database works.
If you need more isolation
Containers are the other option.
A Node.js application shares the machine with every other website on it. If you need an image of your own — a language or a system library UHost does not install — a restricted container website runs an approved image instead. That is a different runtime with different trade-offs, and it needs an administrator to approve the image first.