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Is there any extension that keeps local copies of popular JavaScript libraries such as jQuery and Mootools, and does a trick under the hood to make websites use them instead of downloading these files over and over again from different domains? I've posted this question on the mozillaZine forums but got no reply.

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There are many versions of both of these libraries around. And just because a site references jquery.js, the file could contain anything. It could be an entirely different set of javascript.

It could also contain a standard copy of the jquery library, but with custom additions.

So in order for an extension to know whether it was safe to use the local copy rather than the one to be downloaded, it would have to first download it and compare it to the local copy.

This would make the extension useless...

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This is valid for generic filenames, but what about for example self-hosted Wordpress blogs, which seem to host them with a pattern and version numbers: /wp-includes/js/jquery/jquery.js?ver=1.3.2 –  screener Oct 28 '11 at 4:38
This is precisely (one of) the issues that Google wants to solve with Dart - having consistent client side libaries that local code can access. Yeah, it looks like wordpress might do a special rewrite with the jquery includes to make sure a particular version is pulled in - but this would be limited to wordpress.org sites only. Non-wordpress.org wordpress installs don't pay attention to the ver= part. Look: ronsocialmedia.co.uk/wp-includes/js/jquery/jquery.js?ver=2.9.6 –  Paul Oct 28 '11 at 5:00

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