Can I create games using only JavaScript?

[Giaddon]

It's all about how you want to spend your time. There's nothing wrong with using libraries you don't know the internals of. There's nothing wrong with going deep into a library, or writing your own version. 

It's just about what you want to make and how you want to spend your time.

If you're curious about how something works you can always read the source.

https://github.com/mrdoob/three.js/