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<h1>Canonicalize JavaScript Libraries</h1>
<h2>Configuration</h2>
<p>
The 'Canonicalize JavaScript Libraries' filter is enabled by specifying:

<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedEnableFilters canonicalize_javascript_libraries</pre>
  <dt>Nginx:<dd><pre class="prettyprint">
pagespeed EnableFilters canonicalize_javascript_libraries;

</pre>
</dl>
<p>
in the configuration file.
</p>

<h2>Description</h2>
<p>
This filter identifies popular JavaScript libraries that can be replaced with
ones hosted for free by a JavaScript library hosting service &mdash; by default
the <a href="/speed/libraries/">Google Hosted Libraries</a>. This has several
benefits:
<ul>
  <li>Most important, first-time site visitors can benefit from browser caching,
since they may have visited other sites making use of the same service to obtain
the libraries.</li>
  <li>The JavaScript hosting service acts as a content delivery network for the
hosted files, reducing load on the server and improving browser load times.</li>
  <li>There are no charges for the resulting use of bandwidth by site
visitors.</li>
  <li>The hosted versions of library code are generally optimized with
third-party minification tools. These optimizations can make use of
library-specific annotations or minification settings that aren't portable to
arbitrary JavaScript code, so the libraries benefit from more aggressive
optimization than can be provided by PageSpeed.</li>
</ul>
<p>
In Apache the default set of libraries can be found in
the <code><a href="https://github.com/pagespeed/mod_pagespeed/blob/master/net/instaweb/genfiles/conf/pagespeed_libraries.conf">pagespeed_libraries.conf</a></code>
file, which is loaded along with <code>pagespeed.conf</code> when Apache starts
up. It contains signatures for all the <a href="/speed/libraries/">Google Hosted
Libraries</a>.  In Nginx you need to
convert <code>pagespeed_libraries.conf</code> from Apache-format to Nginx
format:
</p>
<pre class="prettyprint lang-sh">
$ scripts/pagespeed_libraries_generator.sh > ~/pagespeed_libraries.conf
$ sudo mv ~/pagespeed_libraries.conf /path/to/nginx/configuration_files/
</pre>
<p>
You also need to include it in your Nginx configuration by reference:
</p>
<pre class="prettyprint lang-sh">
include pagespeed_libraries.conf;
</pre>
<p>
Don't edit <code>pagespeed_libraries.conf</code>.  Local edits will keep you
from being able to update it when you update PageSpeed.  Rather than editing it
you should add additional libraries to your main configuration file:
<dl>
  <dt>Apache:<dd><pre class="prettyprint">
ModPagespeedLibrary 43 1o978_K0_LNE5_ystNklf \
   //www.modpagespeed.com/rewrite_javascript.js</pre>
  <dt>Nginx:<dd><pre class="prettyprint">
pagespeed Library 43 1o978_K0_LNE5_ystNklf
   //www.modpagespeed.com/rewrite_javascript.js;</pre>
</dl>
<p>
The general format of these entries is:
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedLibrary bytes MD5 canonical_url</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed Library bytes MD5 canonical_url;</pre>
</dl>
<p>
Here <code>bytes</code> is the size in bytes of the library <em>after</em>
minification by PageSpeed, and <code>MD5</code> is the MD5 hash of the
library after minification. Minification controls for differences in whitespace
that may occur when the same script is obtained from different
sources. The <code>canonical_url</code> is the hosting service URL used to
replace occurrences of the script. Note that the canonical URL in the above
example is protocol-relative; this means the data will be fetched using the same
protocol (<code>http</code> or <code>https</code>) as the containing
page. Because older browsers don't handle protocol-relative URLs reliably,
PageSpeed resolves a protocol-relative library URL to an absolute URL based
on the protocol of the containing page. Do not use <code>http</code> canonical
URLs in configurations that may serve content over <code>https</code>, or the
rewritten pages will expose your site to attack and trigger a mixed-content
warning in the browser. Similarly, avoid using <code>https</code> URLs unless
you know that the resulting library will eventually be fetched from a secure
page, as SSL negotiation adds overhead to the initial request.
</p>
<p>
Additional library configuration metadata can be generated with the help of
the <code>pagespeed_js_minify</code> utility installed along with PageSpeed.
To use this utility, you will need a local copy of the JavaScript code that you
wish to replace.  If this is stored in <code>library.js</code>, you can generate
<code>bytes</code> and <code>MD5</code> as follows:
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >$ pagespeed_js_minify --print_size_and_hash library.js</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >$ cd /path/to/psol/lib/Release/linux/ia32/
      $ pagespeed_js_minify --print_size_and_hash library.js</pre>
</dl>
<p>
If you're using the <a href="filter-js-minify#new-minifier">new
javascript minifier</a>, add the <code>--use_experimental_minifier</code>
argument to <code>pagespeed_js_minify</code>. If you're using the old minifier,
add <code>--nouse_experimental_minifier</code>. (As of 1.10.33.0,
<code>--use_experimental_minifier</code> is default. Previously,
<code>--nouse_experimental_minifier</code> was.)
The default <code>pagespeed_libraries.conf</code> includes hashes for both
the old and new minifiers.
</p>
<p>
This filter is based on the best practices of
<a href="/speed/docs/best-practices/caching#LeverageBrowserCaching">
optimizing browser caching</a>
and <a href="/speed/docs/best-practices/payload#MinifyJS">reducing payload
size</a>.
</p>

<h2>Operation</h2>
<p>
In order to identify a library and canonicalize its URL, PageSpeed must of
course be able to fetch the JavaScript code from the URL on the original page.
Because library canonicalization identifies libraries solely by their size and
hash signature, it is not necessary to authorize PageSpeed to fetch content
from the domain hosting the canonical content itself.  This means that it is
safe to use this filter behind a reverse proxy or in other situations where
network access by PageSpeed is deliberately restricted.  Browsers visiting
the site will fetch the content from the canonical URL, but PageSpeed itself
does not need to do so.
</p>

<h3>Examples</h3>
<p>
You can see the filter in action at <code>www.modpagespeed.com</code> on this
<a href="http://www.modpagespeed.com/canonicalize_javascript_libraries.html?ModPagespeed=on&amp;ModPagespeedFilters=canonicalize_javascript_libraries">example</a>.
</p>
<p>
If the HTML document looks like this:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
    &lt;script src="jquery_1_8.js"&gt;
    &lt;/script&gt;
    &lt;script src="a.js"&gt;
    &lt;/script&gt;
    &lt;script src="b.js"&gt;
    &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  ...
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>
Then, assuming <code>jquery_1_8.js</code> was an unminified copy of the jquery
library and <code>a.js</code> and <code>b.js</code> contained site-specific code
that made use of jquery, the page would be rewritten as follows:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
    &lt;script src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"&gt;
    &lt;/script&gt;
    &lt;script src="a.js"&gt;
    &lt;/script&gt;
    &lt;script src="b.js"&gt;
    &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  ...
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>
The library URL has been replaced by a reference to the canonical minified
version hosted on <code>ajax.googleapis.com</code>.  Note that canonical
libraries do not participate in most other JavaScript optimizations.  For
example, if <a href="filter-js-combine">Combine JavaScript</a> is also enabled,
the above page will be rewritten as follows:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
    &lt;script src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"&gt;
    &lt;/script&gt;
    &lt;script src="http://www.example.com/a.js+b.js.pagespeed.jc.zYiUaxFS8I.js"&gt;
    &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  ...
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>
The canonical library is <em>not</em> combined with the other two JavaScript
files, since this would lose the bandwidth and caching benefits of fetching it
from the canonical URL.
</p>

<h2>Requirements</h2>
<p>
Only complete, unmodified libraries referenced by <code>&lt;script&gt;</code>
tags in the HTML will be rewritten. Libraries that are loaded by other means
(for example by injecting a loader script) or that have been modified will not
be canonicalized.
</p>

<h2>Risks</h2>
<p>
You must ensure that you abide by the terms of service of the providers of the
canonical content before enabling canonicalization. The terms of service for the
default configuration can be found
at <a href="/speed/libraries/terms">https://developers.google.com/speed/libraries/terms</a>.
</p>
<p>
The canonical URL refers to a third-party domain; this can cause additional DNS
lookup latency the first time a library is loaded.  This is mitigated by the
fact that the canonical copy of the data is shared among multiple sites.
</p>
<p>
The initial request for a canonical URL will contain a <code>Referer:</code>
header with the URL of the referring page.  This permits the host of the
canonical content to see a subset of traffic to your site (the first load of a
page on your site that contains an identified library by a browser that does not
already have that library in its cache). The provider should describe how this
data is used in its terms of service. The terms of service for the default
configuration can be found at
<a href="/speed/libraries/terms">https://developers.google.com/speed/libraries/terms</a>.
Again, this risk is mitigated by the fact that canonical libraries are shared
among multiple sites; a popular library is likely to already reside in the
browser cache.
</p>
<p>
Sites serving content on both <code>http</code> and <code>https</code> URLs must
use protocol-relative canonical URLs as shown <a href="#sample">above</a>.
Fetching a library insecurely from a secure page exposes a site to attack.
Fetching a library securely from an ordinary page can increase load time due to
SSL overheads.
</p>
<p>
It is theoretically possible to craft a JavaScript file whose minified size and
hash exactly match that of a canonical library, but whose code behaves
differently.  In such a case the library will be replaced with the canonical
(widely-used) library.  This will break the page that contains the reference to
the crafted JavaScript.
</p>
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