It can be done, and has been in Rhino, Nashorn, and graal.js. The problem isn't in the language per se but in the libraries provided as standard (mutexes, threa

It can be done, and has been in Rhino, Nashorn, and graal.js. The problem isn't in the language per se but in the libraries provided as standard (mutexes, thread safe hashes/property access etc.) and in the implementation. It's much easier to build a JIT when you don't have to worry about concurrent threads of execution, and converting a single threaded JIT to a robust multithreaded one can take a lot of work.