JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

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Notice in the preceding script that we go to great lengths to accommodate the fact that the
image may take time to download (either because of slow server processing or a slow network
connection). Since the RPC (image download) may take time, we schedule the callback
function to be run every 50 milliseconds. Each time it runs, readResponse() checks the
complete property of the Image we‘re using for the request. As discussed in Chapter 15, this
property is set by the browser when the image has completed downloading. If complete is
true, the JavaScript reads the response encoded in the image‘s height. If complete is false,
the browser needs more time to fetch the image, so readResponse() is scheduled to run again
50 milliseconds in the future.
By far the most common mistake programmers make when implementing two-way
communication techniques in JavaScript is forgetting to allow for the possibility that the RPC
takes longer than expected. Paranoid coding is definitely called for in these situations, and
setTimeout() is a useful tool. We‘ll see a more sophisticated callback-based approach in a later
section.
One other noteworthy feature of the previous example is that we only allow for one outstanding
RPC at a time. If a new RPC comes in while we‘re still waiting for one to complete, we cancel
the first and issue the second. This policy simplifies coding a bit, but in truth accommodating
multiple outstanding requests at one time isn‘t much more work. All it takes is carefully
managing three things: references to the images executing each RPC, the functions that should
be called when each RPC completes, and the timers used to periodically check if an RPC has
completed.
Threading
A thread is an execution stream in the operating system. Code executing in a thread can do
exactly one thing at a time; to achieve multiprocessing, an application needs to be multithreaded
(i.e., be able to execute multiple streams of instructions at once).
Almost without exception, JavaScript interpreters are single-threaded, and they often share the
browser‘s UI thread. This means that when your JavaScript is doing something, no other
JavaScript can execute, nor can the browser react to user events such as mouse movement or
button clicks. For this reason, it is never a good idea to ―block‖ your JavaScript waiting for some
condition.
For example, instead of registering a timer to check whether the request in the previous
example had completed, we might have done away with the timers and written readResponse()
as
function readResponse()
{