NO
Multi threading is one of the hardest things in software to get right. There are way too many corner cases that are really hard to work out when your code is not deterministic. (I am talking about multi threading with locks etc). In addition all of the various JavaScript libraries are built on the assumption that it is not multi threaded.
That being said there are web workers which do give you an actor based framework for doing multi processing type of operations. You can create workers and pass data back and forth via events.
EDIT:
The other reason is that when JavaScript was created it was done so with the assumption it would get used for small tasks, so no concurrency was built in. To Retrofit it now would break a lot of code. By adding web workers it has been possible to have a system were there is concurrency without any shared memory, but by using actors, a model of concurrency that has shown itself to be very robust in a number of other languages like Erlang, Scala, Clojure etc.
(IF you can't tell I really dislike lock based concurrency)