It's single threaded. Any compute-heavy task will block that single thread until it's over.
In Node JS you can spawn child processes. It's also designed to be asynchronous. But what I like about JS is that it doesn't force me into paradigms.
And when you run something heavy in Node JS the VM can use many threads. So I think it's weird to say it's single threaded. Aren't all programming languages single threaded then?
I'm developing an MMO server in Node JS. So this is welcome in for example Vector calculations.
The reason I choose JS is that I can write "dumbed down" code, that just anyone with JS experience can manage.
Hopefully, JS will be just as fast as optimized C++ in the near future ... First you will ignore it, then laughs at it ...