Ruby on Rails
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The only reason why Ruby exists.
This web framework is pretty good as of 2020 compared to others, because it managed to gain a critical community size, and there's a lot of basic setup already done for you.
it is just big shame it wasn't written in Python or even better, Node.js, because learning Ruby is completely useless for anything else. As of 2020 for example, most Node.js web frameworks feel like crap compared to Rails, you just have to debug so much there.
Used in GitLab, which is why Ciro Santilli touched it.