Ask HN: What new or hot technology do you recommend learning?

I feel that I've been procrastinating a lot in my free time and it's time to learn something new. I see the hype around Go and Rust and I believe I'm missing a lot in my professional development. I'm involved with my start-up and I am facing a lot of challenging data analytics and software development problems during the day. I am using Java/JavaScript/R on a daily basis, but these are mature languages that have been around for a while and I have a hard time learning new frameworks that are beyond my work scope. So what (hot) languages/frameworks are you learnin/using now, that give you the warm feeling that you get a lot of useful knowledge out of them, that's widely applicable?