Concurrent JavaScript

This seems really interesting, I would say I'm a bit concerned of the complexity it may add to JS when most developers are fairly familiar with the single-threaded model (or at least don't need to think about it).

Where is concurrency beneficial? Are there other languages that are sometimes concurrent, and sometimes not?

Will a JS developer who is jumping between front-end and back-end get confused by the two paradigms?

Warning: this project is GPL licensed