You’ve probably already heard about Meteor, it’s been around for a few years now and has a stable release (1.2) available. But if not…
Apart from allowing developers to write apps entirely in JS (well, through in some HTML and CSS for good measure), it also promotes a publish-subscribe pattern which means code changes can automatically be synchronized between server and all clients in real-time.
Check out the official Meteor website – it has great getting started guides. You could be running your first full app in about ten minutes!