NodeJS
NodeJS can be used to run programs on the server that change what the user sees immediately. The Net Ninja has a great course on NodeJS, where you will learn the basics of NodeJS, before jumping into any of the 1000 APIs for it, all which I have never seen be useful.
I, for one, am not a big fan of this all and consider it stupid writing 1000 APIs, when the original is great. If you need a reason to justify something. If you need to spend time using something to like it, and it doesn't make sense from the beginning. That's not something great. It is like crime, where we at first abhor it, and only after coming into contact with it a lot, we tolerate it, and finally embrace it, I propose.