You need to know the trend of the programming world to learn efficiently
Some criteria
- How active it is
- Active community & lots of resources/documentation
- RedMonk statistics - programming language rankings
- Widely respected
- A tool/stack used in a big company is chosen because of the senior devs chose it, so it must be of good choice
- Stackshare - a website that lists the languages/framework/libraries/tools companies use
- Potential
- see which tools are likely to grow in the future
- some languages/stacks grow out of favor over the years
- need to keep track of the IT industry