Just-In-Time learning means you learn a thing because you need to know it right then. Just-In-Case learning means you learn a thing because it may come in handy at some point in your life. This separation is already well documented, even on Wikipedia.
It gets more interesting when you add in expiring skills vs. permanent skills. Expiring skills will stop being useful someday. Permanent skills will be always be as useful as they are today. Permanent skills compound over time, and expiring skills don’t.
So what happens when you learn an expiring skill and don’t end up using it right away? That skill creeps towards obsolescence. You wasted your time. If you know a skill is going to expire, then you should only learn it if you’re sure you can use it.
You should only learn expiring skills Just-In-Time. Permanent skills are worthwhile to learn Just-In-Case.
But here comes the tricky bit. The only skills we can call permanent are soft skills: attitude, discipline, charisma, etc. Hard skills like Typescript and Kubernetes will expire, so learning them Just-In-Case is a waste of time.
But then how can I ever become a well rounded technologist, you ask? You work freaking hard to get on projects with technology you haven’t used before. If you get an opportunity to hop onto a project using some tech that’s new to you, then do it.
When someone asks for some help with a build system you’ve never used before, raise your hand. When a project needs realtime streaming data and you have no idea how to do it, raise your hand. When a client gets excited about creating an Alexa skill, raise your hand.
Raising your hand gets you the best of both worlds. You become a well rounded technologist and avoid wasting your time learning technology you won’t use.
So raise your hand to learn the hard skills when they’re needed. Spend your Just-In-Case learning on the stuff that will stick around. Put down the Python book (unless you’re working on a Python project). Pick up the book on habit forming, or dealing with difficult people, or taking ownership.
That’s the stuff that will compound for the rest of your life.