JavaScript does not support multi-threading because the JavaScript interpreter in the browser is a single thread (AFAIK). Even Google Chrome will not let a single web page’s JavaScript run concurrently because this would cause massive concurrency issues in existing web pages. All Chrome does is separate multiple components (different tabs, plug-ins, etcetera) into separate processes, but I can’t imagine a single page having more than one JavaScript thread.
You can however use, as was suggested, setTimeout to allow some sort of scheduling and “fake” concurrency. This causes the browser to regain control of the rendering thread, and start the JavaScript code supplied to setTimeout after the given number of milliseconds. This is very useful if you want to allow the viewport (what you see) to refresh while performing operations on it. Just looping through e.g. coordinates and updating an element accordingly will just let you see the start and end positions, and nothing in between.
We use an abstraction library in JavaScript that allows us to create processes and threads which are all managed by the same JavaScript interpreter. This allows us to run actions in the following manner:
Process A, Thread 1
Process A, Thread 2
Process B, Thread 1
Process A, Thread 3
Process A, Thread 4
Process B, Thread 2
Pause Process A
Process B, Thread 3
Process B, Thread 4
Process B, Thread 5
Start Process A
Process A, Thread 5
This allows some form of scheduling and fakes parallelism, starting and stopping of threads, etcetera, but it will not be true multi-threading. I don’t think it will ever be implemented in the language itself, since true multi-threading is only useful if the browser can run a single page multi-threaded (or even more than one core), and the difficulties there are way larger than the extra possibilities.
For the future of JavaScript, check this out:
https://developer.mozilla.org/presentations/xtech2006/javascript/
JavaScript multi-threading (with some limitations) is here. Google implemented workers for Gears, and workers are being included with HTML5. Most browsers have already added support for this feature.
Thread-safety of data is guaranteed because all data communicated to/from the worker is serialized/copied.
For more info, read:
http://www.whatwg.org/specs/web-workers/current-work/
http://ejohn.org/blog/web-workers/