Run on a client PC/website, or backend only?

Can the javascript run on the client's computer like in a website or is it backend only?(the code only runs in the server and then displays to the client).
And if it can run on the client, can it use more than one thread?

Not a dumb question at all. The javascript runs on the server only not the client (this is a server-side plugin).

Not a great title for a GitHub issue, though... :-)