Really what your asking is a framework level query rather than what low level primitives the runtime provides.

Demo itself won't use all the cores for some JS you just slap in, that's not how JS works (it'll use the same as v8, which will be one main thread for the JS to be processed in and potentially several helper threads for IO etc)

If you want multi core you either build it yourself using the web workers or find a framework that does it for you.