Interests or career building?

I’ve been playing with Prolog in my spare time for a while now. It’s good fun, makes me think differently about programming and sometimes hurts my brain thinking about the way it works.

But does this actually help me for my career?

Should it matter that it helps me for my career?

Talking to colleagues and friends, I hear a lot of different opinions.

"Free time is for learning new frameworks and libraries so you can do better in your next job."

Or also:

"Free time is for making sure you stay current with the latest trends in programming."

And me:

"Free time is for doing stuff that is interesting."

Unfortunately, the latest trends aren’t always interesting. They are mostly not interesting at all, to be honest. Javascript never appealed to me and all the latest fads seem to be in that space anyway.

You could say that learning a 40+ year old programming language is a waste of time, because it’s not used in the industry. You could also say that learning the next JS framework is useless because it’s going to be obsolete in the next 2 years (if that) anyway.

So what do you think? Should spare time be used to prepare you for the next job opportunity, or should it be used for doing/learning things that are just interesting to you?