Re: JavaScript not being blocked/synched by Applet init()

Joshua Cranmer <Pidgeot18@verizon.invalid>
Mon, 11 May 2009 23:00:41 -0400
Mark Space wrote:

JavaScript IS NOT single threaded. It can receive events from outside
sources (hello XMLHttpRequest) and will spawn additional threads of
execution. Since there's no synchronization method in JavaScript, this
can be a huge hassle. There's an algorithm available to synchronize
threads using only shared memory. Read on:

I believe that, in the case of XMLHttpRequest, in Mozilla, everything is
proxied back to the display thread. The definition of XMLHttpRequest
also appears to not be thread-safe, which means that using the object
from multiple threads is more or less A Bad Thing?.

Although the SpiderMonkey engine is very much capable of doing
multithreaded execution, the DOM itself is not threadsafe and has to be
modified from a single thread, so must browser JS ends up running in one
thread. Hence the statement: "The JavaScript engines in all of today's
web browsers are conceptually single threaded with respect to the web page."

Yes, I am aware of DOM workers, but these do not allow concurrent access
to the DOM:
"The DOM APIs (Node objects, Document objects, etc) are not available
to workers in this version of this specification."

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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