For those already familiar with the client-side Strophe.js library then there is almost nothing to learn - the API is almost exactly the same. The only difference is that this time you can run your code on the server, and handle XMPP traffic from clients on behalf of a whole domain. It's like writing an XMPP server but with the hard parts handled for you.
xmpp.js works with any XEP-0114-compliant server (that's practically all of them), so you need not worry about your code being tied in to a particular server implementation.
XMPP components "bind" to a domain, usually a subdomain of the main XMPP service, such as pubsub.example.org, or conference.example.org. All incoming stanzas addressed to that domain (to='service.example.org') or to entities on that domain (email@example.com') will be routed to your xmpp.js-based code.
For outgoing stanzas your component is in full control. You can specify any 'from' address on your stanzas, many servers don't even enforce that the originating domain of the stanza is the component's domain, allowing you to send stanzas on behalf of any user on the server.
Firstly, you'll need Node installed if you haven't it already, this is fairly straightforward - instructions are here. xmpp.js is confirmed to work with version 0.1.30 (2010.02.22).
In the examples directory you will find an example component which echoes messages it receives back to the sender. If you have a local Prosody server installed then you can simply add these lines to your Prosody config to make this example work:
Component "echo.localhost" component_secret = "hellohello"
Ater restarting Prosody try running:
Log into your Prosody with a client and send a message to firstname.lastname@example.org - you should receive an instant response back - congratulations!