Let me detail tachyonbeam's answer. All modern web browsers (I'll try not to troll about IE) support web workers. However, the semantics of web workers is mess

Let me detail tachyonbeam's answer. All modern web browsers (I'll try not to troll about IE) support web workers. However, the semantics of web workers is message-passing (by copy, most of the time), which is the semantics generally associated to multi-process rather than multi-threading. Most developers using threads expect some kind of memory sharing, which is impossible with web workers at the moment. Shared workers (not implemented in any browser atm, I believe) will take care of some usage scenarios. Other alternatives are being explored (e.g. [Parallel JS](http://smallcultfollowing.com/babysteps/blog/2012/01/09/parallel-javascript/) http://smallcultfollowing.com/babysteps/blog/2012/01/09/para...). However, for the time being, don't expect any support from JS to do shared memory concurrency. Note: I'm a Firefox dev.