I'm trying to profile the performance of a web site that I'm fairly confident is being slowed down by the loading of JavaScript files on the page.
The same JavaScript files are included several times on the page, and <script /> tags are scattered throughout the page instead of being included at the bottom.
As I suspected, when looking at FireBug's "Net" tab, most of the time (not all) when JavaScript is being loaded, no other files are requested. The browser waits for the JavaScript to complete loading.
There are a few exceptions however. There are a few occasions where JavaScript is loaded, but then at the same time, other resources appear to get loaded, such as other JavaScript files and images.
I always thought that JavaScript blocks the loading of other resources on the page. Am I incorrect in thinking this, or does this behavior vary depending on the browser or browser version?
UPDATE:
To those who have explained how loading a script blocks the loading of other resources, I'm already aware of this. My question is why a script wouldn't block the loading of other resources. Firebug is showing that some JavaScript files do not block loading other resources. I want to know why this would happen.
Javascript resource requests are indeed blocking, but there are ways around this (to wit: DOM injected script tags in the head, and AJAX requests) which without seeing the page myself is likely to be what's happening here.
Including multiple copies of the same JS resource is extremely bad but not necessarily fatal, and is typical of larger sites which might have been accreted from the work of separate teams, or just plain old bad coding, planning, or maintenance.
As far as yahoo's recommendation to place scripts at the bottom of the body, this improves percieved response times, and can improve actual loading times to a degree (because all the previous resources are allowed to async first), but it will never be as effective as non-blocking requests (though they come with a high barrier of technical capability).
Pretty decent discussion of non-blocking JS here.