Here's a great blog post from Doug Crockford, one of the gods of Javascript on the subject:
"""" Many people prefer to use it synchronously. When used this way, the JavaScript engine is blocked until the interaction with the server is complete. Because it blocks, the flow of control looks a lot like an ordinary function invocation. Temporal complexity is abstracted away, leaving a very familiar and comfortable programming pattern. It works particularly well when the server is on the same machine, or nearby on the LAN. Unfortunately, it can perform very badly if the server is under heavy load, or if the browser is connected to the server over a slow link. Because the JavaScript engine is blocked until the request completes, the browser will be frozen. The user cannot cancel the request, cannot click away, cannot go to another tab. This is extremely bad behavior. Fortunately, XMLHttpRequest provides an option for asynchronous operation. When you set the asyncFlag flag to true, the JavaScript engine does not block. Instead the request returns immediately, with a potential action that will be triggered later when the result on the request is known. """" Callbacks are your friend, and give you the same functionality as synchronous calls without the downside :) Chris
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