I'm comfortable in Javascript, but not a ninja with it or anything. How do you effectively use this to keep what would be client side code on the server side?

I'm comfortable in Javascript, but not a ninja with it or anything. How do you effectively use this to keep what would be client side code on the server side? You can already write code in server side frameworks and only ship as much code to the client side as actually needs to be there, but surely this isn't what you're talking about. What am I missing?