Brainfuck? JavaScript is one of the most popular programming language (https://www.stackscale.com/blog/most-popular-programming-lan...) You're comparing apples to oranges. By the way I mostly work in Java, but I do not look down on JavaScript, it's shows ignorance.
As others have already said, person who wrote the library may not be familiar with your favorite language, moreover certain things are easier to do in some languages. This is not the attitude that we should be showing when receiving free work from someone. If you don't want to port it to your favorite language when needed that's ok, I'm just glad that it exists.
> Java Nashhorn was passing all ECMAScript 5.1 tests in 2012
Half supported means it is some Mozilla side project which likely is not funded.
> JavaScript is one of the most popular programming language
it is popular on UI yes, for backend I am not sure about that.
> person who wrote the library may not be familiar with your favorite language
he/she could create it as a throwaway weekend toy project, without interest in wide adaptation and high impact, yes. Also, my critics was directed not to him/her specifically, but to the fact that all other do the same.
> moreover certain things are easier to do in some languages.
this is not about implementing prover n javascript
> This is not the attitude that we should be showing when receiving free work from someone.
My attitude is that I am entitled to express opinion if it is well justified.
Javascripts can be different, there is no guarantee that some half supported engine will run that specific code.
> JavaScript has been running on server side for 10 years now since NodeJs came out.
Same as brainfuck: https://github.com/EvanHahn/brainfuck-web-app
But the point is that potential adaptation could be much higher if library is implemented in C/C++ with bindings to Java/Py/C# and NodeJS too.