Combining assets


How do you organize your assets like JavaScript or CSS? Does every request trigger several dozen additional requests needed for loading all of these resources? Do you often forget to run your static-CSS-builder script prior to viewing your local web application? Or do you hate to update something in the middle of your handcrafted jQuery-and-all-the-other-JS file?

Understanding the browser

When looking at the typical "waterfall" inside your favourite browser's developer tools you may have observed that JavaScript and CSS assets start loading after the HTML content got loaded to the point where the <script> or <link> tags reside. Some browsers even start loading these assets after the entire HTML markup has been fully loaded. Depending on the browser the loading uses a maximum number of parallel requests. In order to keep the loading time small one would like to minimize the amount of data to get transferred and the number of requests needed to load all the assets.

On the other hand, maintaining small well-named files each being responsible for a single task is much easier to handle. Specifying dependencies is very simple when looking at small files but can be hard to decide when facing a huge file.

These requirements quickly lead to the temptation to express dependencies in a data structure (all file names are randomly chosen):

    my %dependency_for = (
        'site.js'       => [ qw(uploader.js forms.js) ],
        'uploader.js'   => [ qw(jquery.js) ],
        'forms.js'      => [ qw(jquery_ui.js) ],
        'jquery_ui.js'  => [ qw(jquery.js) ],

It would be great if you could point your browser to http://yourdomain.tld/js/site.js and could get back a stream full of all JavaScript needed to run your site.js and have all dependencies resolved.

Combining assets

Fortunately, there is a helper module on CPAN: Catalyst::Controller::Combine

Ater installation visit your favourite shell and fire

    script/ controller Js Combine

for creating a "Js" controller (having the expected namespace "js") and consuming all URLs starting with /js/. By default, it expects its files to reside in root/static/js. Simply create this directory unless already present and populate it with some JavaScript files. CSS is very similar and just differs by the name and path chosen.

Now you have two choices:

using a long URL

If you have a countable number of JavaScript (or CSS) files, you may put them flat into the asset directory. Then, construct an URL listing all files you like to combine (with or without the .js extension) into one long pseudo-path. This will trigger the loading of every single file being listed in the URL and your browser will receive a stream with all your assets combined.

A URL might look like: http://yoursite.tld/js/jquery/uploader/forms/site.js

However, everybody can follow your effort, directly see your dependencies (or load the individual files directly) and you will have to modify this URL at a place different from your combining controller. So you might decide not to use this mode.

specifying dependencies

In order to keep your URLs simple you can specify dependencies like the hash listed above. Edit your "Js" controller and you will see a sample config entry you might modify to your needs.

        depend => {
            'site.js'       => [ qw(uploader.js forms.js) ],
            'uploader.js'   => [ qw(jquery.js) ],
            'forms.js'      => [ qw(jquery_ui.js) ],
            'jquery_ui.js'  => [ qw(jquery.js) ],

Now you may point your browser to http://yoursite.tld/js/site.js and you will receive the expected stream with all specified dependencies resolved.


There are two great modules on CPAN: JavaScript::Minifier::XS and CSS::Minifier::XS.

Both offer a sub named minify that returns the minified version of its argument. If you simply use one of both modules in your Controller, the combined asset will automatically get minified.

If you plan to use or create a minifier of your own, simply add a minify method that will automatically get invoked when present.


Usually, performance is not an issue if you just combine assets and minify them. If you encounter performance bottlenecks or plan to use a costly processor like sass for processing your asset, you may consider to generate your assets by loading the URLs immediately after deployment and save the generated asset files at a location where your web server may be able to deliver them statically.

A further improvement could be to gzip your response which is an option of your frontend web server or can be enabled with the right Plack middleware.

See also


With a simple helper controller you may keep your JavaScript or CSS URLs simple but enable delivering a complete resource with one request to your visitor's browsers while still keeping your assets maintainable. There is nothing you can forget during your development and no other tool you will have to invoke.


Wolfgang Kinkeldei <wolfgang [at] kinkeldei [dot] de>