Only implicitly: no threading model is documented in the ECMAScript Language Specification [1], and no widely-used interpreter implements threading. The new Web Workers specification adds something like threading to JavaScript in the context of the browser [2], and Node intends to add support for this on the server as well [3]. Have a look at [4] for a nice article on threading in JavaScript.

[1] http://www.ecma-international.org/publications/files/ECMA-ST...

[2] http://www.whatwg.org/specs/web-workers/current-work/

[3] http://nodejs.org/

[4] http://odetocode.com/blogs/scott/archive/2007/01/08/javascri...