Could you build a fully functional programming language like Java, in a scripting language like JavaScript?

https://github.com/YaroslavGaponov/node-jvm And same question on StackOverflow: http://stackoverflow.com/questions/12316557/javascript-jvm-implementation

This is a Python Interpreter -> http://www.skulpt.org Built using Javascript. So yeah! its possible! But building a language via Javascript and taking it to production level performance and speed is not easy.

Given that Javascript is turing-complete, yes absolutely. Now of course, if you want it to be efficient, that's another story.

http://pypy.org PyPy is exactly this. It is Python implemented in Python and it outperforms CPython which is implemented in C. PyPy has also been used as a basis to implement other languages in Python. http://docs.topazruby.com/en/latest/ Topaz is Ruby implemented in Python (using PyPy code) also with the aim of outperforming. http://coffeescript.org CoffeScript is implemented in JavaScript but I wouldn't count it as a full implementation because no VM or machine code output. It just uses JavaScript itself as its output. If I started implementing a performance-oriented language today, my platform of choice would be LLVM (C++) but if performance weren't critical PyPy would be a great place to start. Regardless I would do the front-end (Lex, Parse, Lower) in some high-level language because it is just a ton of work.

Yes and has already been done recently. That is a JVM that runs in JavaScript and doesn't need a plugin. I forget the name of the project.. Answer was collapsed, adding another redundant sentence.