why Javascript SetTimeout() is not multithreaded

Question

I have a test:

Html:

Empty
Empty

​

js:

var s1 =         


        

Answer 1

Lots and lots of design decisions went into Javascript's implementation in a browser that assumed it had only single thread access to the browser DOM and to other global variables/properties. This makes programming with it a lot less likely to cause problems, but introduces some limitations that have to be dealt with.

The language itself is perfectly capable of being multi-threaded and we already see that in WebWorkers and in some server implementations of the language. But, any time you use multiple threads and try to read/write to variables or properties that are shared between multiple threads, one MUST use protective devices (like mutexes) to allow reliable access to those shared resources. That significantly complicates how to do this programming and Javascript in a browser decided NOT to require that level of understanding in order to program it reliably.

For anyone who has done multi-threaded programming, it can be powerful, but it is very, very easy to introduce difficult to find bugs. Those who are responsible for Javascript in a browser decided that that level of difficulty and the resulting types of bugs should be avoided entirely.

Even now with WebWorkers, there are no shared resources between a WebWorker and the main javascript thread. The two must communicate via a message passing system which is a foolproof way of forcing safety. And, the consequence is that one cannot access the DOM from a WebWorker. Instead, if you want the DOM to be changed, you have to post a message to the single main thread and ask IT to update the DOM. The main thread will recieve that message only when it's done doing other things (it's single threaded).

It is also likely that the DOM has now spent a zillion years as a structure that is only designed for single threaded access so it would be a gigantic task to design and implement a way to access it from multiple threads (and fix all the resuling bugs in that implementation).

Answer 2

Mozilla does support multi-threading in Javascript - as long as you don't want to do UI work from multiple threads. The early versions of my Remove Duplicate Messages (Alternatve) exentsion were multi-threaded.

See my own bug page regarding this issue in my extension, or better still, this page about using worker threads in Mozilla. eicto, you could very well implement your code using a background thread.