If you know and love JavaScript (no accounting for taste ;) ) why not use a JavaScript application framework for the back end? Something like Meteor but there are many others. The benefit of using a framework is if you select a good one, it will really teach you how to structure an application as well as take care of many tedious tasks for you.
But since you are open to the idea of learning another language, could I suggest Ruby? The reason I chose it initially was that Ruby code looked almost like my pseudocode. It's strong object-oriented philosophy has helped me understand OO programming better. There are also two huge benefits for what you're trying to do:
Ruby on Rails: hugely poplular application framework, most famously used to build Twitter. Loved by millions, ported to many languages.
The Ruby on Rails Tutorial One of the best programming books I've read, it is free on-line and constantly, almost obsesivelly, updated by its author. This will take you through building a web-app on Rails from scratch. It will include a functioning authentication system, so you can see if what you have is really up to the job.