Javascript是一种单线程语言。这意味着它有一个调用堆栈和一个内存堆。正如预期的那样，它按顺序执行代码，并且必须在执行下一段代码之前完成。它是同步的，但有时是有害的。例如，如果一个函数需要一段时间才能执行，或者必须等待某项操作，那么它会同时冻结所有内容。

据我所知，谷歌Chrome将有多线程javascript，所以这是一个“当前实现”的问题。

你的意思是为什么语言不支持多线程，或者为什么浏览器中的JavaScript引擎不支持多线程?

第一个问题的答案是，浏览器中的JavaScript是在沙盒中运行的，以一种独立于机器/操作系统的方式运行，添加多线程支持会使语言变得复杂，并将语言与操作系统联系得太紧密。

目前一些浏览器确实支持多线程。如果你需要，你可以使用特定的库。例如，查看下面的材料:

https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers (支持后台线程); https://keithwhor.github.io/multithread.js/(图书馆)。

传统上，JS是为简短、快速运行的代码而设计的。如果你需要进行重要的计算，你就需要在服务器上进行——让一个JS+HTML应用程序长时间运行在你的浏览器中做一些重要的事情是荒谬的。

Of course, now we have that. But, it'll take a bit for browsers to catch up - most of them have been designed around a single-threaded model, and changing that is not easy. Google Gears side-steps a lot of potential problems by requiring that background execution is isolated - no changing the DOM (since that's not thread-safe), no accessing objects created by the main thread (ditto). While restrictive, this will likely be the most practical design for the near future, both because it simplifies the design of the browser, and because it reduces the risk involved in allowing inexperienced JS coders mess around with threads...

@marcio:

为什么不使用Javascript实现多线程呢?程序员可以用他们拥有的工具做任何他们想做的事情。

So then, let's not give them tools that are so easy to misuse that every other website i open ends up crashing my browser. A naive implementation of this would bring you straight into the territory that caused MS so many headaches during IE7 development: add-on authors played fast and loose with the threading model, resulting in hidden bugs that became evident when object lifecycles changed on the primary thread. BAD. If you're writing multi-threaded ActiveX add-ons for IE, i guess it comes with the territory; doesn't mean it needs to go any further than that.

JavaScript多线程(有一些限制)在这里。谷歌实现了Gears的worker, HTML5也包含了worker。大多数浏览器已经添加了对该特性的支持。

数据的线程安全得到了保证，因为所有与worker通信的数据都被序列化/复制。

欲了解更多信息，请阅读:

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

http://ejohn.org/blog/web-workers/