I have an express server that has a button that issues a POST which activates a function in my node.js server.
I noticed that JSON values can be passed into the .ejs webpage when it's rendered, but I haven't been able to get it to work for functions.
You can use Socket.io to emit an event from the browser, and than listen for that event on the server to act on it.
Socket.io works flawlessly with express http://socket.io/#how-to-use
This recipe can be found in it's original form on Stack Over Flow.