Condenser

Condenser is a Ruby library for compiling and serving static web assets inspired by Sprockets. It features a powerful pipeline that allows you to write assets in languages like Sass and SCSS.

Installation

In your project's Gemfile with Bundler:

gem 'condenser'

Or via Ruby Gems

$ gem install condenser

If you are using Condenser with Rails, instead of the condenser gem use the condenser-rails gem.

Guides

For most people interested in Condenser, you will want to see the README below.

If you are a framework developer see Building an Asset Processing Framework.

If you are a library developer who is extending Condenser, see Extending Condenser.

If you want to work on Condenser or better understand how it works read How Condenser Works.

Overview

Since you are likely using Condenser through another framework, there will be configuration options you can toggle that will change behavior such as what directories or files get compiled. For that documentation you should see your framework's documentation.

Accessing Assets

Assets in Condenser are always referenced by their logical path.

The logical path is the path of the asset source file relative to its containing directory in the load path. For example, if your load path contains the directory app/assets/javascripts:

Logical path	Source file on disk
application.js	app/assets/javascripts/application.js
models/project.js	app/assets/javascripts/models/project.js
hello.js	app/assets/javascripts/hello.coffee

Note: For assets that are compiled or transpiled, you may want to specify the extension that you want, not the extension on disk. For example we specified hello.js even if the file on disk is a coffeescript file, since the asset it will generate is javascript.

File Order Processing

By default files are processed in alphabetical order. This behavior can impact your asset compilation when one asset needs to be loaded before another.

For example if you have an application.js and it loads another directory

import initializers from 'config/initializers/*';

initializers.forEach((i) => i());

The files in that directory will be loaded in alphabetical order. If the directory looks like this:

$ ls -1 config/initializers/

alpha.js
beta.js
gamma.js

Then alpha.js will be loaded before either of the other two. This can be a problem if gamma.js needs to be called before alpha.js. For files that are order dependent you can either rename the files or require individual files manually:

import alpha from 'config/initializers/alpha';
import beta from 'config/initializers/beta';
import gamma from 'config/initializers/gamma';

gamma();
alpha();
beta();

Cache

Compiling assets is slow. It requires a lot of disk use to pull assets off of hard drives, a lot of RAM to manipulate those files in memory, and a lot of CPU for compilation operations. Because of this Condenser has a cache to speed up asset compilation times. That's the good news. The bad news, is that Condenser has a cache and if you've found a bug it's likely going to involve the cache.

By default Condenser uses the file system to cache assets. It makes sense that Condenser does not want to generate assets that already exist on disk in public/assets, what might not be as intuitive is that Condenser needs to cache "partial" assets.

For example if you have an application.js and it is made up of a.js, b.js, all the way to z.js

import 'a';
import 'b';
// ...
import 'z';

The first time this file is compiled the application.js output will be written to disk, but also intermediary compiled files for a.js etc. will be written to the cache directory (usually tmp/cache/assets).

So, if b.js changes it will get recompiled. However instead of having to recompile the other files from a.js to z.js since they did not change, we can use the prior intermediary files stored in the cached values . If these files were expensive to generate, then this "partial" asset cache strategy can save a lot of time.

Directives such as import in Javascript and @import in SCSS tell Condenser what assets need to be re-compiled when a file changes. Files are considered "fresh" based on their inode number, mtime, size and a combination of cache keys.

In Rails you can force a "clean" install by clearing the public/assets and tmp/cache/assets directories.

Invoking Ruby with ERB

Condenser provides an ERB engine for preprocessing assets using embedded Ruby code. Append .erb to a CSS or JavaScript asset's filename to enable the ERB engine.

For example if you have an app/application/javascripts/app_name.js.erb you could have this in the template

var app_name = "<%= ENV['APP_NAME'] %>";

Generated files are cached. If you're using an ENV var then when you change then ENV var the asset will be forced to recompile. This behavior is only true for environment variables, if you are pulling a value from somewhere else, such as a database, must manually invalidate the cache to see the change.

If you're using Rails, there are helpers you can use such as asset_url that will cause a recompile if the value changes.

For example if you have this in your application.css.erb

.logo {
  background: url(<%= asset_url("logo.png") %>)
}

When you modify the logo.png on disk, it will force application.css to be recompiled so that the fingerprint will be correct in the generated asset.

Styling with Sass and SCSS

Sass is a language that compiles to CSS and adds features like nested rules, variables, mixins and selector inheritance.

If the sassc gem is available to your application, you can use Sass to write CSS assets in Condenser.

Condenser supports both Sass syntaxes. For the original whitespace-sensitive syntax, use the extension .sass. For the new SCSS syntax, use the extension .scss.

In Rails if you have app/application/stylesheets/foo.scss it can be referenced with <%= asset_path("foo.css") %>. When referencing an asset in Rails, always specify the extension you want. Condenser will convert foo.scss to foo.css.

Javascript, ES#, & ES Modules

Condenser transforms Javascript for the browser by transpiling all the files .js through babel and bundled together via rollup.js.

// app/assets/javascript/application.js

var square = (n) => n * n

console.log(square);

Start a Rails server in development mode and visit localhost:3000/assets/application.js, and this asset will be transpiled to JavaScript:

var square = function square(n) {
  return n * n;
};

console.log(square);

JavaScript Templating with EJS

Condenser supports JavaScript templates for client-side rendering of strings or markup. JavaScript templates have the special format extension .jst and are compiled to JavaScript functions.

The templates can then be imported. When invoked they will render the template as a string that can be inserted into the DOM.

<!-- templates/hello.jst.ejs -->
<div>Hello, <span><%= name %></span>!</div>

import hello from 'templates/hello';

$("#hello").html(hello({ name: "Sam" }));

If the ejs gem is available to your application, you can use EJS templates in Condenser. EJS templates have the extension .jst.ejs.

Minifying Assets

Several JavaScript and CSS minifiers are available through shorthand.

In Rails you will specify them with:

config.assets.js_minifier  = :uglify
config.assets.css_minifier = :scss

If you're not using Rails, configure this directly on the "environment".

environment.register_minifier 'text/css', Condenser::SassMinifier
environment.register_minifier 'application/javascript', Condenser::UglifyMinifier

If you are using Condenser directly with a Rack app, don't forget to add the dependencies (the sassc gem in the example above) to your Gemfile.

Gzip

By default when Condenser generates a compiled asset file it will also produce a gzipped copy of that file. Condenser only gzips non-binary files such as CSS, javascript, and SVG files.

For example if Condenser is generating

application-12345.css

Then it will also generate a compressed copy in

application-12345.css.gz

This behavior can be disabled, refer to your framework specific documentation.

Serving Assets

In production you should generate your assets to a directory on disk and serve them either via Nginx or a feature like Rail's config.public_file_server.enabled = true.

On Rails you can generate assets by running:

$ RAILS_ENV=production rails assets:precompile

In development Rails will serve assets from Condenser::Server.

Version History

Please see the CHANGELOG

License

Condenser is released under the MIT License.